The following post describes a simple low battery indicator circuit by using just two inexpensive NPN transistors. The main feature of this circuit is its very low stand by current consumption.
The Circuit Concept
We have so far seen how to make a low battery indicator circuits using a 741 IC and a 555 IC, which are no doubt outstanding with their abilities of detecting and indicating low battery voltage thresholds.
However the following post relates yet another similar circuit which is much cheaper and employs just a couple of NPN transistors for producing the required low battery indications.
Advantage of Transistor over IC
The main advantage of the proposed two transistor low battery indicator circuit is its very low current consumption compared to the IC counterparts which consume relatively higher currents.
A IC 555 would consume around 5mA, a IC741 around 3 mA, while the present circuit would just consume around 1.5mA current.
Thus the present circuit becomes more efficient especially in cases where stand by current consumption tend to become an issue, example suppose in units which depend on low current battery supplies such as a 9V PP3 battery.
Circuit can Operate at 1.5V
Another advantage of this circuit is it's ability to work even at voltages around 1.5V which gives it a clear edge over the IC based circuits.
As shown in the following circuit diagram, the two transistors are configured as voltage sensor and inverter.
The first transistor on the left senses the threshold voltage level as per the setting of the 47K preset. As long as this transistor conducts, the second transistor on the right is held switched OFF, which also keeps the LED switched OFF.
As soon as the battery voltage falls below the set threshold level, the left transistor is no longer able to conduct.
This situation instantly triggers the right hand side transistor, switching ON the LED.
The LED switches ON and provides the required indications of a low battery warning.
Circuit Diagram
Video Demonstration:
The above circuit was successfully built and installed by Mr. Allan in his paranormal depletion detector unit. The following video presents the implementation results:
Upgrading the above Transistorized Low Battery circuit into a Low Battery Cut-off Circuit
Referring to the above diagram, the low battery indicator is formed by the two NPN transistors, while the additional BC557 and the relay are used for cutting OFF the battery from the load when it reaches the lower threshold, in this state the relay connects the battery to the available charging input.
However when the battery is in its normal state the relay connects the battery with the load and allows the load to operate through battery power.
scott cacciamani says
sorry i’m having so much trouble with this. the center lug of 547 near the 47 k resister is soldered exactly where? The center lug of the 47k trim pot goes under which base, the base of itself or the 547. also where can i find a diagram on how to hook up a on off switch to this? You can tell i have no experience.
thanks again
Swag says
What is so complicated? Please make your assembly layout exactly as shown in the diagram, place the components on the PCB exactly as presented in the diagram, then you have any problems.
The 47k preset will have 3 leads, the center one connects with the base…out of the other two, one goes to the ground, the other to the 33K.
If you are making it over a breadboard then it can a lot confusing for you.
And make sure to check the base, emitter, collector pins of the BC547 correctly
scott cacciamani says
sorry, forgot the last one. Is the 47k resester center lug soderd directly to the center lug of the bc-547?
Swag says
it is directly connected to the base, because we already have a series resistor with the positive arm of the preset.
scott cacciamani says
Hi, again, when using a power supply to set the low voltage on the 47k trim pot. i assume i hook up the leads positive to positive and neg to neg, but what voltage should i set the power supply at before turning the screw on the trim pot. This would be for a low voltage indicator of a 9v battery
Swag says
apply 8.25V to the circuit, and adjust the preset such that the LED just begins illuminating, that’s all.
scott cacciamani says
Hi, in the 9v depletion detector circuit the 47k variable resister is a preset for what voltage? With this circuit there is not pot to set correct? Also, my battery positive wire has a diode to prevent the battery from charging dc converter is plugged in, can I still use this circuit? Thanks Scott
Swag says
the 47k preset can be used for adjusting the threshold at which the LED needs to light up and indicate the situation.
The diode will not have any impact of the the above circuit, you can still use it…
jindro says
Sir I’m just a newbie hobbyist, I’m not yet well versed using Analog Multimeter and I’m really having a hard time using it. When you said series a bulb meaning, using 1 wire of the primary, breaking it then link it with both wires of the bulb?
Swag says
Jindro, connecting in series means, break one of the input wures, and join the cut ends with the bulb terminals.
To trafo wire < ------(bulb)---------> 220V < --------------> To other trafo wire
jindro says
Sir can you help me to identify a certain scavenge transformer? sorry for asking here because it is not the topic here, but I don’t know where to go or ask, it is ok if you cant , tnx sir
Swag says
you can feed 220V from the primary side, and check the AC voltage on the secondary side to verify the specs
jindro says
tnx again Sir! for replying patiently, the transfo sir has 4 wires in the primary, the number from the first 2 wires from left is erased and the last 2 is 240 240
Swag says
Jindro, first check the resistances across the wires, the pair which shows maximum resistance can be assumed to be the safest pair for the mains input.
Still, for 100% safety, use a 100 watt bulb in series while connecting it to 220V input, and make sure the 100 watt bulb remains almost shut, if it is illuminating brightly then the wires may not be specified for the 220V input, then you may have to investigate the other sets of wires in the same manner.
jindro says
Sir I just scavenge the batts. from broken lap top batts. in junkshop, can I trust it to charge equally? just cut the charging using your fullcharge indicator?
Swag says
if the two cells are not uniform with their charging specifications, then it can be a problem, and the results will be inefficient.
you can try the following arrangement if possible:
https://homemade-circuits.com/parallel-battery-chargerchangeover/
the diodes will need to be Schottky type not the normal rectifier diodes.
jindro says
My problem Sir is, if I charge it individually and it is also wired in parallel as a powersource will it not charge also in parallel?
Swag says
Jindro, ideally you must remove them and connect them separately while charging, however this may not be critical, you can charge them together in parallel, just make sure the supply voltage is strictly cut-off at 4.2V
jindro says
Sir Good Evening! I will build this for my power bank circuit, I’m using 2 li ion bat. Can I use 2n2222 as alternative Sir?
Swag says
Good Evening Jindro,
2N2222 can be used instead of BC547, no problem.
jindro says
Good Morning Sir! Is ok to wire 2 li ion bat. in parallel for power source of phone charger then wire it individually to charge the battery individually? because I’m not sure if its ok to charge the battery in parallel.
jindro says
Sir can I ask again what is the alternative for Bc 557? T.Y. very much Sir for patiently helping lots of people like me 🙂
Swag says
you can try 2N2907, it will be compatible with your 2N2222
Swag says
Good Morning Jindro,
“2 Li-ion batts in parallel for power source of phone charger” I did not understand this statement.
Are you trying to make a power bank circuit?
Please clarify?
jindro says
tnx again Sir for replying! yes sir I’m trying to build a powerbank and give it to my friend, there’s no electricity in there place, my idea is to make your lm7805 phone charger then use solar panel to charge the battery.
Swag says
Hi Jindro, in that case it is OK to use two cells in parallel as the source for the mobile phone, and once discharged you can charge them separately.
Pierre says
Hi,
Thank you for sharing this schematic.
I have few questions if you can help :
– Why using 2 transistors and not only one?
– I am trying to find a SMD sot-23 transistor but looking at the datasheets they all have a threshold voltage that has values like “min 0.6V, Typ 1V, max 1.4V” with such difference between min and max voltage threshold how is it possible to set correctly the (3.2V) low battery voltage?
Thanks !
Swag says
Hi, two transistors are used to achieve better sensitivity and higher accuracy margin between the cut off points.
which specification are you referring to, is it the base/emitter voltage….??
Fajar says
Thank You
Anonymous says
can this be use in 24V batteries?
Swagatam says
yes it can be used
Anonymous says
Thank you
Anonymous says
I couldn't find bc547 and bc557 in my area,but they gave their equivalent PN2222A and BC327 so those components will work perfectly
Swagatam says
the mentioned numbers will also work…..you can use them
Unknown says
Tip:
Instead of using the potentiometer as a potentiometer, use it as a variable resistor by only using one side.
From the 33k ohm resistor tie into the middle pin of potentiometer along with the base of the transistor that was already tied into it, and leave the top pin of potentiometer unhooked. This makes no difference as far as the workings of the circuit but provides a huge benefit I will explain.
If you do this you will more easily be able to convert this to a more permanent solution with all fixed resistors. If you have any type of device you are hooking this to that can be dropped, shaken, or simply have a flaky pot this can cause problems later and the set point could be off. I would only use the potentiometer as a test to begin with to help find the appropriate resistor value(s) to use so the potentiometer could be replaced with fixed resistors.
If you leave it as a potentiometer with both sides instead of variable resistor(only one side) then you have two changing values of resistance as you turn the knob, one on top in series with the 33k ohm resistor that is on top and the one below. There is no need for the extra resistance in series with the 33k ohm resistor.
So instead just take the 33k ohm from top and put it in middle pin of potentiometer leaving everything else the same. Then adjust the knob and voltage until it the LED comes on when you want it to. Then afterwards use a multimeter and check the resistance of the side of the potentiometer that you used(disconnect one or both of the pins used first before checking resistance so you aren't also getting resistance reading from anything else). This will give you the resistance that you need to put in place of the potentiometer. You may not find an exact value but may be able to get close by putting some you have in series or parallel depending on what you have and what value you need but this makes it a lot easier by not using both sides of the potentiometer.
For example I wanted my LED to come on at 5.2V so I did as I described and the resistance after testing the side of potentiometer with multimeter was 1430 ohms. I wound up replacing potentiometer with a 1.2k ohm and a 220 ohm resistor in series for 1420 ohms and it is working perfectly.
I also used 10k ohms instead of the 33k and used 330 ohm for LED.
Swagatam says
thanks for the interesting suggestions!!
Charles Wright says
Your welcome. I was curious though about the resistor showing in series with the LED which is why I chose my own value. I made circuit for use with 4AA batteries in series for 6 volts. And I figured in the voltage drop of LED 2V and and came to the conclusion that with 4volts across the 10k ohm resistor that it would be 0.4mA or 400uA. That was without figuring in any voltage drop across the transistor so it would actually even be less than that amount of current flowing to light the LED. Will an LED even light with that little current, I try to give mine around 15 to 20 milliamps and I came up with only 0.4 milliamps? I'm assuming if it does light with 10k(with 6V as I used) it would be extremely dim. Was that a typo that was supposed to read 1k ohm instead of 10k ohm? Or was this circuit originally designed to test much higher voltage? Either way I used smaller value 330 ohm, I was just wondering out of curiosity.
Swagatam says
Thanks again! I appreciate your keen interest!!The 10K is not a typo, yes, the circuit was initially built and tested with a 12V supply, and the 10K was intentionally selected to make sure that the LED did not respond to the transitions, rather lit up only when the threshold was actually close to the preferred point
Abdullah bin wahid says
Sir, i have to present project tomorrow and i have selected this circuit for that purpose. Please tell me that with exactly this circuit at which voltage the bulb will glow off. And at which voltage it will be off. Please give me reply soon.
Swagatam says
It will depend on the low voltage level that you may have selected for the circuit.
This low voltage level trigger must be adjusted with the help of the preset, until the LED just lights up.
Rohit Singh says
Sir,
I had made a Light sensor lamp that works when it goes dark and turns off when lights falls in it, with using minimum parts to conserve battery power, but the light output is very loq but light sensor works accurate.
i Had used a smartphone battery that i had an extra peice of 4.2v, a white 1-watt led, 1- 220k resistance, 1-LDR, 1- BC547 Transistor only..
my problem is when i give power from 4.2v battery directly to 1-watt white led it give maximum huge light, so do i was expecting to give that much light from the light sensor i created but when i connect the led from the light sensor circuit its power out becomes low, but sensor response is awesam it quickly gets on when dark and turns off when lights fall on it.
i want the same maximum light output from the sensir, is there is anyway to increase the light without increasing the battery power??
Swagatam says
Rohit, please show me the schematic that you have built or you can comment under the same article which you are referring to, it will help me to view the diagram and provide an appropriate solution to your problem….
Rohit Singh says
its working .. awesm..
thanx..
sir,
I had saw a screw driver looking type a dc tester, the body of screw driver was made of tranparent plastic just like ac tester, but this tester was dc., inside that i saw a red led with three button cells battery connected with a tiny kit., there were mainy three parts in that kit, two transistors one was BC557 and other was BC547 and resistance, from that tiny kit two wires where comming out ,one wire goes to the top and screwed to a small metal so that we can touch that with out finger and the other wire goes down to the tester connected with the metaltalic type screw.. when ever a person touch the top metal with the finger and touch any electronic compenent part and now touch the bottom part of the tester to the component, then the led glows as to check the continuty of that component is ok.. i saw that many years ago but i cant make that..
sir can u make that circuit.
Swagatam says
Rohit, you can refer to the last circuit presented in this article:
https://homemade-circuits.com/2012/05/make-this-simplest-continuity-tester.html
It works on a similar concept
Rohit Singh says
sir,
i had maked battery full circuit by replacing the transistors with BC557 and changing the led and power supply polarity. it works fine !
but my
problem is … when ever i connect the battery with the charger when the battery is low, the battery full indicator turns ON… what should i do to make led turn ON, only after battery is full..?
Swagatam says
Rohit, please show me the schematic which you have made.
It should be done as shown in the following article:
https://homemade-circuits.com/2016/08/battery-full-charge-indicator-circuit.html
Jason Pierce says
I am interested in using this circuit with a micro-controller that has a super low power sleep mode around 35uA. While 1.5mA is a small draw, it is large by comparison. I would only need to periodically test the battery level. To conserve power, can the battery indicator circuit be toggled on/off by a digital IO from my micro-controller?
Swagatam says
yes that's very much possible, the above low batt circuit can be toggled ON/OFF intermittently for monitoring the batt status whenever required, the circuit does not need to be continuously ON.
Unknown says
Thanks so much for this circuit. I am testing with a bench top power supply and have adjusted the potentiometer so that the LED goes on at around 3.2V. It works wonderfully, except when power is turned on and off at a voltage higher than the 3.2V threshold – the led flashes briefly. Is there a way to prevent this from happening? Thank you.
Swagatam says
It could be due to switch ON current surge….you can try adding a capacitor at the base/ground of the right side transistor and see if this checks the issue.
the capacitor value can be anything between 10uF and 33uF
Bryan B says
Thanks for posting. I am also trying to use this circuit as a low voltage indication for 3.2V. I'm trying to work out the resisitance values I will need especially for the 47k adjustable potentiometer. Do you have a formula or do you just adjust the potentiometer? I'm guessing the other resistance values don't have to match yours exactly, but the ones that are the same, need to remain the same? (As in, instead of two 33k's, I could use two 10k's if I had those?)
Swagatam says
For a 3.2V input you can use any resistor between 1K and 33K, same goes for the preset…. the values are not critical.
I can suggest formulas but that might require long explanation and anyway it's not important for the above simple design, so we can skip it.
the preset needs to be set manually, by applying the intended minimum threshold voltage to the circuit and adjusting the preset until the LED just begins glowing
Aminu Yahaya Ibrahim says
Morning Sir,
Thank for your effort to help us hobbiests and engeneers.
Sir, I want use this circuit in my inverter inorder to monitor the battery level.
1) I want use S8050 inplace of BC546 for the low battery and S8850 instead of BC557 for the battery full, is this possible, and is there any difference in battery power consumption?
2) My design will include 4 LED look as follow;
>LED 1 will show fully charged level 100%;
>LED 2 will show 75%;
>LED 3 will show 50%
>LED 4 will show up If the battery level is down to 11v
Based on this, my questions are:
a. What level of the 12v battery is 100%, 75%, 50% and Low battery, inorder to save the battery life?
b. Instead of using VR or Pots can I use zener diod of required volt?
Thank you once again Sir.
Am very much proud of you!
Swagatam says
Hi Aminu,
you cannot get all 4 indications with the above circuit, you will need the following circuit to implement this
https://homemade-circuits.com/2013/06/3-step-dc-voltage-level-monitor.html
14.4V can be considered as the 100% charging level for a 12V battery….just multiply the other percentage levels with it for getting the equivalents in volts.
11V is the lowest discharge level for 12V batt. that's 77% of 14.4V
Alex Baruth says
Hi, thanks for the schematic. I was looking for a way to have the light turn off when the battery gets to a certain low voltage, or, actually interrupt a portion of a circuit, thereby disabling it. Would I be able to rearrange this circuit to do this?
Thanks.
Swagatam says
Hi, yes it's possible, you will need to add a third BC547 in the circuit with its base connected to the collector of the right side BC547…the collector or the emitter of this transistor can then be appropriately integrated with the intended external circuit, depending on whether the disabling needs a "sink" or a "source" action from this BC547
Alex Baruth says
Thanks for your reply Swagatam, you and your contributions have been very helpful. I was not able to get the NPN BC547 to work, but when I thought about it, I realized it was a PNP transistor that was needed. I used a 2N3906, and that seemed to do it. I was also thinking that using other transistors, or another arrangement, a two transistor setup might work. Thanks for the help.
Cheers!
Swagatam says
Both will work actually…BC547 will work better as a "sink" while the BC557 will work better as a "source"
the emitter/collector can be swapped for achieving the opposite from the both counterparts for triggering the external circuit as may be required.
Swagatam says
It can be enhanced to any level, but we want to keep it as compact as possible so probably a single transistor improvement should be just enough.
Alex Baruth says
Hi Swagatam, I have been working on this circuit trying to get it to work for some time now without success. I'm not sure what the problem is. I can get the original to work just fine, but I basically want the reverse of it. I've tried the load on either side of the additional BC547 (sink/source)? to no avail. Maybe I'll work on some schematics in case one can spot the fault.
Swagatam says
Hi Alex, If you can specify your exact requirement then I can suggest the correct solution, because it depends on the type of the load you are using, if it's a logic input then it should easily work using the earlier idea but if it's a heavy analogue load then the circuit would probably need to be upgraded much more in order to make it work
Alex Baruth says
Thanks Swagatam, I see what you mean.
AngryHelder says
Great circuit and very useful.
I have a question though could a second LED be used maybe even one that has 2 colors so it would be one colored when battery is high then switch colors to indicate when the battery is low?
Maybe something like a reverse able polarity Bi-Color LED or a 3 lead Dual Color LED?
Thanks.
Swagatam says
thank you very much,
yes it could be done by introducing an additional transistor stage, as shown in the second diagram of this link:
https://homemade-circuits.com/2014/06/flashing-led-battery-status-indicator.html
the shown connections could be replaced with a single common-anode dual color LED, by connecting its common lead with the positive and the cathodes to the respective collectors of the BJTs via limiting resistors.
Jean says
Hello, I'm going to use your circuit in my self switching off machine, but I have a problem. My machine runs on a 9v pp3 battery and I don't have a 47k pot. If you could give us a formula to calculate the resistance of the preset or give us some values for commonly used voltages, it would be nice. Thank you.
Swagatam says
If you are intending to use a relay in place of the LED, and if the relay coil resistance is above 200 ohms, in that case you can simply replace the 33k resistors with 10k, and the presets with 1k presets.
however a 9v pp3 will never control any relay, so the above assumption cannot be true…but since the resistor values are so critical you can replace it with any nearby value, anything above 4k7 for the fixed resistors and also the presets would do.
A relevant article can be learned here:
https://homemade-circuits.com/2012/01/how-to-make-relay-driver-stage-in.html
Swagatam says
correction:
…sorry I meant….."since the resistor values are NOT so critical you can replace it with any nearby value…..
B. J. Sutrisno says
Hi there, hope you can answer my question for this old post…
1) Using your circuit, how to switch on LED if the battery is 3v? My battery's fully charged is around 4.5v…
2) Is this circuit possible to run without getting off that Red LED. I mean it's still bright until minimum forward volts that LED has?
Thanks…
Regard,
~ Joko
Swagatam says
Hi,
you'll need to adjust the given 47k preset such that the LED illuminates at around 3V, once it's illuminated it'll stay illuminated until the battery voltage has fallen below the forward voltage spec of the LED.
Keyur Patel says
Will this ckt work for 1.5 volt battery ?
To indicate its low voltage?
Plz give proper details of ckt if not?
Swagatam says
yes it can be used for 1.5V also….reduce the 33k resistors to 1k that's all.
Ram kumar says
Sir please suggest for 14.4 volt lithium ion battery .preset value resistor and normal resistor.please explain
Swagatam says
you can use the same set up as given above
Adithya K says
can u please send me the design of the above circuit
Adithya K says
Can u please send the design of the above circuit.
monkeypi says
can i use a different resistor than a 33k one??
Swagatam says
yes any nearby value will do +/- 20%
monkeypi says
Can i use a 68k one?
Swagatam says
yes will do.
MUJAHID SHAH says
Ic 4017 is great but you are greater than it hence you made it easier for us to understand.thank you
Swagatam says
You are welcome!
MUJAHID SHAH says
Dear sir You mentioned about the ic 4017 that it produces sequential output what does it mean kindly explain a bit.thank you.
Swagatam says
Dear Mujahid,
please read this article for more details:
https://homemade-circuits.com/2011/12/how-to-understand-ic-4017-pin-outs.html
DJ Tube says
How would i make this work for a 36v battery with an alarm at 30v please?
Swagatam says
you can use the same circuit for 36V also, simply adjust the preset for 30v LED illumination
MUJAHID SHAH says
Sir Swagatam circuit you recommended are great. The problem is that I cannot use an Ldr in open surroundings where it can be affected by sun light and other lights as well. I need circuit where there is specific light and its detector so when any object interrupt them means coming between the receiver and detector it should give alarm. I can configure the remaining circuit with scr application circuit.
Swagatam says
Mujahid, you can keep the LDR enclosed inside a pipe like enclosure and align the laser beam such that only this beam enters the pipe, no other light is able to enter.
as shown in the last diagram in this ink:
https://homemade-circuits.com/2013/02/make-these-simple-cheap-home-burglar.html
otherwise you will have to go for an IR proximity detector circuit as given here
https://homemade-circuits.com/2013/10/accurate-infrared-motion-detector-or.html
MUJAHID SHAH says
Sir I would like to make a circuit which includes a laser light and laser detector where they both can face each other when any human body or any other object comes in between the laser light and detector it should give alarm. I don.t have good idea please help me in making the diagram and description of components needed.thank you..
Swagatam says
Mujahid, you can try the first circuit shown the following link:
https://homemade-circuits.com/2012/03/how-to-make-simple-scr-application.html
replace the "sensor" points with an LDR
MUJAHID SHAH says
I made the above circuit it.s working marvelously now I need a battery full indicator circuit as simple as the above one.
Swagatam says
Thanks!
To make it work as an over charge indicator you will need to alter the polarity of the transistors.
You will need to replace the transistors with BC557.
Its orientation will be exactly as given in the above diagram, just the supply will need to be reversed, and also the LED polarity.
ally prieto says
sir can i use it for 12V supply? what changes do i need to do? thank you 🙂
Swagatam says
yes you can use it for 12V also.
achilles hector says
Good day sir!!! another question from your no. 1 fan!! :-)…
1.Do you remember the the Ultrasonic weapon circuit of yours? You mentioned about the power supply which is 12v to 18v. Can I use 2 x 6f22 9v battery which would become 18v? And in case yes, I will put the above circuit. What modification will I make so that the led will glow at 12v?
I will always credit you in my projects…
My teacher once asked me in one of my project like the transformer less emergency light, she said "who is this Swagatam Majumdar in your acknoledgement" I said "He is an Engineer from India."
Swagatam says
Thanks very much Achiless I appreciate your enthusiasm!
9V PP3 battery will not work for too long, because the ultrasonic weapon circuit could consume considerable amount of power at a go…..therefore you will have to either go for mini SMF batt or arrange many 1.5V pencil cells in series…you would require 12 nos of them in series for optimum results.
Sham says
Hi Swagatam,
I tried with 4th ckt. I am not getting Battery cutoff. When emitter is connector to base of bd140 indicator is not lighting up.
Swagatam says
Hi Sham,
Remove the LED, connect the collector directly to positive and then check.
Sham says
Yes Swagatham, tried that too before posting. Changed the resistor values to lower ones too.. 🙁
Swagatam says
according to me it should have worked, anyway there's only way left now… revert the connections back to its actual form as shown in the above article.
Connect an additional BC557 transistor base via a 10k resistor to the collector arm of BC547 where the LED is present.
connect the emitter of BC557 to positive and collector to base of BD140.
adjust the 47k preset such that at 5V all the white leds just shut off
This is 100% work, otherwise the issue could be something else.
Sham says
Hi Swagatham,
I tried the ckt you suggested. But no response. I did some rnd and found that the biasing of ldr ckt is taking over the 557 one.
When pos supply from transformer is applied to base of bd140, ckt switches off. That means pos supply from battery is not sufficient to turn off bd140. If i connect pos of transformer to 557 emitter, my requirement is not met, bcos in absence of power supply battery may get drained.
I know its straight forward solution. but its not happening. Hope to get better solution to this.
Swagatam says
Hi Sham,
the suggested circuit is perfect, it's the ultimate way of doing it and should work.
When the low voltage LED lights up, check the voltage at the junction of collector of the BC557 transistor and base of BD140, it should be equal to the battery voltage.
check by keeping the meter prods across positive and base of BD140
with potential at base of BD140 equal to the battery voltage, there's no way it can conduct.
you may also try increasing the base/ground resistor of BD140 for better and instant response.
Sham says
Hi Swagatham,
Hats off to your contribution. I tried this circuit https://homemade-circuits.com/2011/12/how-to-make-efficient-led-emergency.html, the second one with LDR, through another transistor bd140. It works like a charm. Initially i had set up the ldr ckt without low bat indicator for 440ma. R1 220E and p1 10k. I know one more stage of pnp is required to connect low bat ckt to LDR ckt to cutoff the battery power to LEDs. But i dono what to be used and where to connect to maintain the same 440 ma as output to LEDs. Plz suggest the circuit to be used. Thanks. Sham.
Swagatam says
Hi Sham,
thanks!
For adding a low voltage cut off in the circuit you can repeat another stage identical to the LDR stage, with the following changes.
Use a 10k preset instead of 100k, remove the LDR and use a series resistor equal to 10k for the ground link. Adjust the preset such that the LEds just shut off when the battery voltages reaches 5V
Sham says
I tried connecting the circuit as per your suggestion, but i could not get the desired result. So changed the values as given. I connected one more bc557 in the same manner as ldr ckt. Emitter to vc+ via 1k pot and other end to base of 2nd trans. Again base to grnd via 1k resis. Collector connected to base of bd140. Now ckt switches off with 5v battery when preset is calibrated but when replaced with 6v battery it continues in the same off state unless i have to recalibrate to new setting. Actually it should be ON since the voltage is more.
When checked at 2nd trans collector during LED off state at 5v it was reading 3.4 volts. When 6 volts battery was connected, it was showing 4.4 volts which keeps the bd140 base at high and hence it was still off.
Plz suggest me what to do.
Sham says
Ho… BTW addition to my previous reply, in ldr ckt, i hv used 10k pot instead of 100k at emitter and 470E for bd140 base and grnd. I hv connected ldr to grnd without 1k.
plz suggest where the 2nd bc557 collector to be connected?
Swagatam says
Sham, please try the 4rth circuit as shown in this article:
https://homemade-circuits.com/2011/12/how-to-make-efficient-led-emergency.html
Sham says
Sure Swagatham, will try this and give feedback.
Anonymous says
Yes I understand, but what I meant was, is it somehow possible to skip the dim glowing and go from an OFF led straight to a bright ON led?
Thanks for your time!
Swagatam says
Probably not possible through this simple design, you will have use an opamp circuit for that, as shown here:
https://homemade-circuits.com/2011/12/how-to-make-simple-low-battery-voltage.html
Anonymous says
Dear Swagatam, I was wondering if you could draw a simple 'fully charged' circuit based on two transistors, if that is possible. I build a charger for 3xaa batteries and i want to light a green LED when voltage of the combined batteries reach 4.2 volts, without the LED slowly getting brighter but just stays off and then goes on suddenly.
Thanks for your great circuits!
John
Swagatam says
Dear John,
Thanks!
To make it work as an over charge indicator you will need to alter the polarity of the transistors.
You will need to replace the transistors with BC557.
Its orientation will be exactly as given in the above diagram, just the supply will need to be reversed, and also the LED polarity.
However it won't give a sudden glow, a dim glow could be witnessed when the level is within 0.5V near the set threshold.
Anonymous says
Thanks for your swift answer! Do you have any idea which component(s) to use to not have a dim glow? Some kind of relay of some sort?
Swagatam says
I meant that the LED won't produce sharp turn ONs rather grow brighter slowly as the threshold is reached, but at the actual threshold the brightness would be optimal.
dheeraj says
hai sir, i want to use this circuit with 12V 4.5Ah battery but i dnt have 47k preset but am having 10K preset.. what to do now ??
Swagatam says
10K preset will also work, you can use it.
Anonymous says
I think I understand, sorry for my previous question about the 47k resistor, but if I'm not mistaken the 47k is a potentiometer 🙂
So the 33k are not to be changed with other resistors at different voltages?
Swagatam says
47K is a preset to be precise,
yes you got it right about the 33k resistors, anyway nothing is crucial here.
Anonymous says
Just for reference, I was wondering for which voltage your diagram is, with the 47K resistor?
Anonymous says
so that means that the emiiter of bc557 will now recieve a postive suppky while negative supply goes to the resistors .
Swagatam says
That's correct!
Anonymous says
if a bc557 is used instead of the bc547 it becomes a full charge indicator but my question is the bc557 is a pnp so will the emmiter of bc557 be connected to ground just like the bc547 is grounded.
Swagatam says
The transistor orientation will be exactly as given in the above diagram, just the supply will need to be reversed, and also the LED polarity.
Chris Ault says
This circuit is going on my little single cell micro helicopter to light a super bright led when battery goes from 4.20volts down to 4.0 v. Currently I'm shrinking it down & taking it off the breadboard.
Swagatam says
Wow! that's great….go on
Yuva Rajan says
hi sir,
i am not good in electronics please guide me how second transistor is off while the voltage level is high. And how second transistor is on while low voltage. thanks
Swagatam says
At higher voltages the left transistor conducts and grounds the base of the right transistor so that it gets switched OFF, at low voltages the left transistor doesn't get enough voltage at its base to conduct and switches OFF itself which initiates and switches On the right transistor.
jonnie walson says
hello sir can i use this circuit for 12v battery?
Swagatam says
yes it can be used with 12V supply also.
Rajagilse says
Sir, what happens if i use BC 557 instead of BC 547 in the above circuit? Whether it works as indicator for battery high indicator? Or what else alteration should i do , to make it as battery high voltage indicator?
Swagatam says
yes that's correct! it'll become a full charge indicator.
UK esop says
Hi Swagatam. Thank you for this circuit. I have built it for use in paranormal investigations. A YouTube video of the completed unit is here:
youtu.be/Nuv6iuhyReY
I have credited you in the video description and posted your url.
Thank you very much for sharing your knowledge.
Allan
Swagatam says
Hi Alan,
That's great! I am really pleased to hear this.
Thanks!
Anonymous says
Sir thanks very much for this wonderfuk circuit, I appreciate. Pls i don't knnw how to set the low battery refrence voltage. I mean the voltage at which the LED will glow, indicating low battery. Could you pls tell me how? I want to use the circuit to monitor a 12volts battery i am using for my inverter to avoid over discharging the battery. Thanks a lot. Awaiting your reply.
Swagatam says
You will need a variable power supply unit for setting up the circuit.
adjust the variable supply to 11.2V and connect it to the above circuit.
Then carefully adjust the preset such that the LED just illuminates.
The circuit is all set.
Now you can remove the power supply and connect the battery supply to it, when the battery voltage reaches around 11.2V the LED will light up.
Muhammad Junaid says
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Please tell how this circuit can be adjusted to indicate for 12V Battery with a threshold of about 10.5 DC Volts and can we use beeper instead of LED, if so, how??
Swagatam says
The preset can be adjusted right from 1.5V to 12V….so it can be set for 10.5V also.
you can replace the LED with a piezo buzzer directly for getting audio indication.
Muhammad Junaid says
Thanks for your swift response…!
Anonymous says
thank u for your replay sir(faysal)
Anonymous says
Sir,im faysal.
Thank u for giving me reply.
Sir i want to know another thing that
if i want to use a normal resistor in the place of variable, then how to connect them?
Swagatam says
see the above comment.
Anonymous says
Sir,if i use a normal resistor after setting the value, using variable resistor then how to connect ?
Can u plz help me sir?
Swagatam says
after setting up the preset, measure the resistance between the center and the upper arm of the preset and similarly across the center and the lower arm of the preset.
then solder these two value resistance across the transistor base and 33k…. and across base to ground of the circuit
Anonymous says
hi what I want is that the flashing or flasher LED when the battery is low.
thank you very much
Luis from Argentina.
Swagatam says
you can try using a flashing multicolor LED in place of the ordinary LED.
Anonymous says
Hello sir my name is faysal,im a student of eee.
Im not experienced.
Sir,im not understand where to connect the base of left side transistor, after 47k resistor or before?
Plz help me.
Swagatam says
Helo Faisal,
47k is a preset, or a variable resistor. it will have three leads, the center will go to the base of the transistor while the other two leads will go to the respective points of the circuit as shown.
Anonymous says
Hi how do I do the LED flashes when the battery is low … Thank you very much. Luis Argentina
Swagatam says
let it flash….. it will happen for a few minutes then stabilize to a constant glow.
Rajib.digi says
This is superb because simple,what will be the operating battery volt&AH in between,Preset 47K threshold control process,Thanks Swagatam
Swagatam says
thanks, the above circuit is just an indicator which will read the battery voltage status…it will work with all types batteries.
Anonymous says
thanks am going to try it how long the battery will be full charged?
Swagatam says
battery charging has no connection with the above circuit, the above circuit is just an indicator device.
the battery charging parameters will depend on the rating of the transformer used.
Anonymous says
hi sir way i have a simple charge control circuit for charging 50ah battery within a few hours by using 30w solar panel? regard
Swagatam says
try this circuit:
https://homemade-circuits.com/2012/04/how-to-make-solar-battery-charger.html
Sayad Hadia says
Hi Swagatam
This is agood circuit for warning of minimum DC.
Is it possible to Drive a Relay instead of a LED for monitoring and centralized Alarm
thank you very much
Swagatam says
Thanks Sayad!
For driving a relay you will have to introduce another transistor stage, using a PNP BC557 transistor.
Anonymous says
Thank u sir, i will make this circuit and send feedback soon.
Anonymous says
Can i use this circuit for indicating 3v ni-cad battery
Swagatam says
yes can be done by slightly modifying the resistor values.