This LED light bar with dimmable feature will allow the user to adjust the brightness of the lamp in 4 steps, with100%, 50%, 10% and 0% illumination control at each subsequent steps.
The idea was requested by one of the avid readers of this blog. Here's the required working concept.
Hi Swag !
I stumbled across your web page whilst searching for a solution to make an effective touch led lamp.
Actually the one my dad uses at night has gone kaput. So I thought why not make him one - being an engineer in the past.
NTE’s touch dimmable led light bar. I love how it’s built and I’d like to use this as a ref to make a night lamp for my dad whose old and needs it at night….
I intend to may be use a dimmable Led bulb or a led strip to make the lamp.
If you can help me with the circuit and maybe a tutorial sort as I’ve been out of this for a long time, it would really mean a lot.
Monish
The Design
The basic design of the proposed touch dimmable LED light bar circuit can be seen in the block diagram below:
The touch sensor converts the tiny finger touch signal into amplified electrical pulses. The next decade counter stage converts these pulses into shifting logic levels across its outputs. These shifting logic pulses are fed to the corresponding LED drivers which convert these signals into a sequentially varying voltages for the LED stage.
The varying voltages from the transistor stage which are set at specified levels cause the LEDs to illuminate with different light levels or brightness., accomplishing the dimmable effect on the LEDs.
How the Circuit Works
Referring to the circuit diagram above, the basic circuit functioning could be understood with the help of the following points:
The two BC557 transistors at the left side of the diagram form the touch sensor stage.
Tiny electrical pulses from the finger are amplified to the supply level and applied to the clock input of the IC 4017.
The IC 4017 is a 10 stage divide by 10 Johnson decade counter, which responds to these input signals and converts them into a shifting HIGH logic across its output pins from 3 to 4.
Initially when the circuit is powered, the 1uF at pin15 of the IC resets the IC so that the HIGH logic is set at its first pin out #3.
Due to this the corresponding BD670 transistor stage conducts and illuminates the LED array brightly. The BD670 being a Darlington device illuminates the LEDs with high brightness. At this stage the brightness of the LEDs is maximum also because the BD670 has no potential divider at its base configuration. This allows it to deliver an optimum 11 V from the 12 V supply to the LEDs at full current, illuminating the array with full brightness.
When the touch sensor is touched, the decade counter responds and causes its output logic to shift from pin#3 to pin#2.
This shuts off the BD670 stage and powers the pin2 transistor stage which is also wired like an emitter follower. Therefore, now the 2N2222 transistor becomes responsible for illuminating the LED array. However, since the base of the 2N2222 emitter follower is rigged with a potential divider that creates around 10 V at its base, causes the emitter of the 2N2222 to have a decreased emitter voltage, at around 10 V.
The 1 V reduction of the supply to the LEDs, decreases the illumination and dims the LED brightness to 50% less than the original level.
Next, when the touch sensor is touched again, shifts the HIGH logic from pin#2 to pin#4 of the IC. Likewise, now the BC547 driver stage activates and takes over the job of illuminating the LEDs. But again, due to a potential divider at its base set to generate approximately 9 V output at the emitter, causes the LEDs to further dim at the lowest 10% of its original full level.
After this when the touch pad is touched, the clock signal at pin#14 of the IC shifts the HIGH logic from pin#4 to the next subsequent pin which is the pin#7.
However, since the pin#7 is attched with the reset pin#15, cause the IC output reset back to pin#3. This enables the LEDs to illuminate again with full brightness.
Thus means the dimmable tube light bar does not have a switch OFF step across the touch sensitive range.
If you wish to have the switch OFF function at the last stage, after the pin#4 step, you can achieve this simply by replacing the pin#7 with pin#10. Meaning, pin#15 now connects with pin#10 via the 10K resistor. This will allow the 3rd touch action to switch OFF the entire LED bar, and the next subsequent touch will yet again restore the LEDs to its full brightness level.
Yasitha Navodya says
What’s the max amount of LEDs per row?
Swagatam says
As shown in the diagram.
Yasitha Navodya says
15? What if need more than 15? Changing the serial resisters to LEDs is enough?
Thanks!
Swagatam says
You can add more LEDs in parallel with individual series resistor. But if the there are too many of them then the transistor and their base resistor will also need to be modified.
Nicholas Chelule says
Very smart and updating electronic devices and circuits. Thanks alot
Bharat says
Sir ,thanks a lot for this circuit , I made it and it worked well with 12v battery but when I use 12 v adapter it loses touch l.e touch does not work at all.Kindly guide me.
Swagatam says
Bharat, try this: Connect a diode at each of the outputs of the 4017 IC such that their anode is connected to the IC output pin and the cathode is connected to the transistor base resistor.
After this, connect a 1uF between the cathode and ground of each diode.
Also try replacing the 1uF at pin14 with a 10uF.
Additionally make sure that your adapter has a high value filter capacitor in the order of 2200uF
Tim says
Hi, one more question.
I have added a p-mosfet before the circuit, between the v= of the battery, and the Vcc main power line of the circuit. So that the battery stays disconnected and doesn’t drain until I am ready to use it.
The gate is connected to the source of the mosfet with a 470k resistor. I would expect the Vgs to be 0v until I pull it down with an extra transistor and an ‘on’ signal. However, without doing anything, the Vgs seems to settle at -0.3v
https://ibb.co/0ts16qs
here is the circuit. any idea what is causing the leakage? or is it a high frequency on/off oscillation? And how do I fix it?
much appreciated!
Swagatam says
Hi, the image shows an N channel MOSFET?
Swagatam says
But the N channel looks correct, so the diagram has no problems….I think you can try changing the MOSFET or try a BJT instead.
Tim says
the n channel mosfet is the one that pulls down the gate of the p-channel mosfet (i incorrectly wrote that a transistor pulls down the gate)
so the top chip, with 8 pins, is the p-channel mosfet (dmp2022)that is giving me the problems…
Tim says
Hi, never mind – I finally fixed it. The really confusing part was that I was still getting the Vgs voltage drop even when everything else was removed. i.e. leaving just the input voltage Vbatt, the resistor across G-S, and the p mosfet…
I don’t know if I fully understand why, but I think when the resistor across G-S gets very large, the own internal capacitance of the mosfet can cause a small current pulse into the resistor and cause a voltage that turns on the mosfet. I assume with 470k it was oscillating to a steady state Vgs of -.3, and with 100k to a Vgs of -0.05v. with 24k I couldn’t measure a Vgs, but the output still had .02 and was passing an initial peak that latched Vcc With 1k everything works perfect! I will try 5 or 10k tomorrow, just to reduce power loss a bit when on. But otherwise, I will live with the 1k.
Swagatam says
Yes that was the exact problem, glad it is solved now…
Swagatam says
you can replace 470k with 10k and check the results.
for the N channel also, you can replace the 470k with 10k, and R98 with 100 ohms
mustakh says
Hi can we use IR remote sensor instead of touch sensor
Swagatam says
for IR remote you will have to modify the IC 4017 in the following manner:
https://www.homemade-circuits.com/how-to-make-simple-infra-red-remote/
Tim says
Hi, one further question: in my case, I put the lowest power LED level on pin3. Now I want to use a comparator so that in a particular situation, the comparator (an lm358, since I am using those already) would reset the counter & keep it in reset mode. Effectively, from then on, resetting any other power modes, and only allowing the low power light mode.
Can I run the output from the lm358 directly to the pin 15? Or would this cause problems with the reset function of pin 7? I can imagine since the output of the lm358 is either active on, or active grounded, the active grounded condition would keep the pin 7 from resetting the counter. Would a simple diode on the output of the lm358 be sufficient to prevent the grounding?
Swagatam says
Hi, yes that will work.
You can use a diode at the op amp output and connect its cathode to pin#15 of IC 4017, for the intended results!
Tim says
Hi, I am looking for a touch sensor that will work underwater. (to add in front of a dimming circuit such as this one.) So far, the most promising seems to be am infrared touch sensor, though a small window. when you cover the small window with a finger, it pulses the dimming circuit above. do you have any advice on particular infrared emitters or detectors to use? or a filtering circuit to eliminate stray reflected light from the lens or small particles in the water? or is there a better touch sensor method you can think of?
Thanks, & compliments on a great website!
Swagatam says
Hi, you can try the following arrangement, hopefully this should fulfill your requirement:
Tim says
Thanks for the quick response on a Sunday! And thanks for the circuit suggestion.
So do you think an infrared sensor is the best way to go here, or is there another technology better suited?
For the filtering of the IR sensor, I had thought to use a basic lm358 circuit, using a potentiometer to set the comparison voltage high enough such that anything below 40% or 50% relative collector current gets filtered out. And feeding the output of the lm358 directly into the pin 14 of the 4017. Is the circuit with the double 557 transistors more stable? Would I replace the 10k resistor with a potentiometer to change the sensitivity?
With the LM358, I had also thought to add a capacitor in parallel with the sensor resistor, so that the greater than 50% collector current needs to flow for at least half a second before it would switch the count up. (in order to filter out something else floating by briefly, or a brief brush against the arm or leg.) where would I place the capacitor with the double 557 circuit?
Swagatam says
You are welcome!
Since it is an underwater application, infrared will suit better.
Sensitivity of the circuit suggested cannot be changed, since it is configured as a digital output
If you want to control the sensitivity of your optocoupler output then your op amp idea will be more appropriate.
More information regarding optocoupler interfacing can be learned from the following article:
Optocouplers – Working, Characteristics, Interfacing, Application Circuits
The capacitor could be placed at the input LED side. Two resistors could be connected in series with the LED positive line, and a suitably rated capacitor connected between the junction the resistors, and the negative line or the LED cathode line.
Philip Smith says
Hi, I think this circuit is close to what I am looking for.
I need to cycle between three states as the touch point is activated – Off – Red, Warm White, Bright White – Off. Using three separate Single Color LED strips
Do you have any suggestions for this modification, or an alternative using a single RGB strip?
Swagatam says
Hi, you can use the same circuit, with a few modifications:
remove the 56k, 33k resistors.
remove the LEDs from the emitter side, and connect the strings in series with the collectors of the transistors.
Connect the emitters with the ground line directly.
Spiros says
Really nice article and very informative, thank you for the time and effort for explaining everything!