The following article explains a simple circuit which can be used for alternately switching LEDs with gradual brightening and fading effects.
Circuit Operation
The circuit can be effectively used for generating spooky effects in idols, for example it can be used for illuminating the eyes of a Jack-o'-lantern during haloween celebrations.
The circuit is overly simple and requires just a couple of opamps and a few other passive components for implementing the proposed brightening and fading actions in the LEDs.
The opamps can be a couple of separate IC 741 or a single IC with dual opamp such as IC 1458, 4558 or a TL072.
The opamp A1 is used for generating a gradual rising and sinking voltage, which ranges from 3 to 6 volts.
The opamp A2 is simply configured as a comparator for supplying an alternately varying voltage between 2 and 7 volts in order to charge and discharge C1 and C2 through a constant current input.
Thus the above operations become responsible for generating a linear peak to peak ramping signal at pin#1 of A1.
This signal is amplified with a couple of transistors wired as emitter followers to pin#2 of A1. Here the LEDs become the emitter loads of the transistors.
R4, together with C1 and C2 determines the rise and fall frequency of the connected LEDs.
R4 can be replaced with a 100K pot for making the fading rate manually adjustable.
The circuit should be operated from a 12V DC power supply for supporting at two LEDs on each channel.
For accommodating more LeDs, the collector of T1 and the upper end of R7 should be connected to a separate high voltage supply may be to a 30V supply which would then allow the connection of 6 LEDs on each channel.
Parts List for alternate brightening and fading LED circuit
- R1, R2, R3 = 56K,
- R4, R5 = 120K,
- R6, R7 = 150 OHMS
- C1, C2 = 33uF/25V
- T1 = BC547,
- T2 = BC557
- LEDS = 5mm, 20mA,
About the Author
I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!
i am using an led light strip.. the circuit is correctly connected but the fading effect is not shown.. what should i do now?
did you connect the 741 pinouts correctly? use a single LED or 2 LEDs as shown in the diagram to test the effect, don't use strip.
yes i have used all connections correctly also i have tested the circuit for single led but the result is the same… should i connect the +ve terminal of power supply to pin #7 and negative to pin #4?
how can you say you did everything correctly when you have not connected the pin4/7 to the supply rails?
obviously you must connect them to the supply, otherwise how will the IC work…?
have i used the correct opamps?? there is no #9 and #11 pin on opamp A1
for IC 741, pin#7 is the positive supply, and pin#4 is the negative supply pins.
pin#3 is the (+) input pin
Pin#2 is the (-) input pin,
pin#6 is the output.
you must modify the connections according to this
but in the circuit given there is no connection of power directly to the IC through pin #t and #4
the diagram shows a dual opamp IC, that means two opams in one package.
What is A1 and A2?? I made the whole circuit,connected everything but it's not working.. i used IC 741 as A1 and A2
A1 and A2 are opamps, please check all the connections once again and also the polarity of the capacitors…you can also try reducing the voltage to test the response
Good morning sir,
THANK YOU VERY MUCH…. 🙂 🙂 🙂
K.Kausik
Sir,
I'm really sorry. Apologizing for any kind of mistake.
I didn't maintain the name with any kind of intention.
Actually I always try to gather information before asking you anything,just to provide you more free time,that you may help others, all over the world. Not only me but the visitors out of India also appreciate your attitude to help and responsibility to answer as soon as possible. And that adds some extra colour in your website and it's the only green tree in the desert.
I really feel proud to you and like to pray for your prosperity and happiness.
Again I'm really sorry for any kind of mistake.
Thanking you,
K.Kausik
Kaushik, please do not apologize, I know that you innocently included the external link without knowing that such links were not acceptable in most websites….
You are one of the avid and dedicated readers of this blog and I appreciate and value your involvement very much.
So please keep up the good work and feel free to comment whenever you have a query or doubt regarding anything in electronics.
Sir,
Sorry for reposting …..
Thanks for kind and quick response….
k.Kausik
No problems kuashik…keep up the good work
Good morning sir,
There were a request,that I posted here yesterday but somehow it didn't appear here ! ! ! !….
Actually I want to control a 5Mts led strip(China made,with its own power supply;12v, .75amp ).
Is it possible with 555ic and/or what will be the transistor value.
I have done with another page that explains different types of fader circuit with 555ic. And there are 139 no of comments and I'm not so wise to find out the definite solution. So I'm here to disturb you. Any suggestion….
Thanking you,
K. Kausik
Good Morning kaushik,
comments once posted will 100% reach me, it might get slightly delayed but all will be answered appropriately.
If you want to use a 555 circuit, you can build any standard IC 555 astable circuit and use the transistor stage which is explained in the above article along with C1/C2 with its pin#3 for the same results.
Simply by connecting the C1/C2 end with pin#3 of the IC 555 astabe you should be able to get the identical fading effect from a 555 circuit also.
Kaushik, I did not publish your last comment since you had an added an external electronic site's link in it (sorry that's not acceptable here)
I 'll surely try to answer your queries in my free time…
Sir,
I want to control a 5mts(China made) led strip available in the market with its own power supply(12v,.75amp). Then which ckt is most useful;this one or any 555 one,explained in another post(actually there are 139 comments and I'm not so wise 🙁 !!! ). Please inform with desired transistor values.
Thanking you,
K. Kausik
Hi Swagatham,
Sorry, I forget to tell you this important things.
My IC is LM358, LED…..1 blue led with series resistance 330ohms. Its ok now with c945 transistor. But I would like to control many leds using N mosfet.
Hi Anil, If the circuit is working well with BJTs as recommended in the design then I think it's better to use BJTs only instead of mosfets which are not giving proper results.
you can try 2N2222/2N2907 or 8050/8550 in place of BC547/BC557 and use many number of LEDs in series/parallel combinations as per your requirement.
also you can try replacing R2, and R4 with 100K and 1M pots for getting an adjustable feature from the design
Hi Swagatham
Thanks in advance.
I assembled this circuit. R1 & R2 47k and R4 150k, T1 2sc945. Supply 12VDC. Circuit did not worked. LED always ON.
I reduced supply voltage to 9v, R2 to 33k. Then the fading effect started. But rising and falling were not perfect. I replaced R4 with 1megaohms. Now rising-falling timing ok as I desired.
I replaced c945 with N-mosfet 50n06. Again I lost fading effect. LED always ON. I changed supply 9v to12v. But no benifit.
I have many 50n06 N channel mosfets stock. I want to control 2 channel LEDs, 1 fade ON then the 2nd fade OFF with 2 N-Mosfets. Supply must be 12v.
Hiii this project is very useful sir.. I have a doubt how to connect audio jack with a mobile phone …
Hi, thanks, why do you want to connect it with a cell phone audio?
this is not a music level indicator circuit……