Some time ago i uploaded a breathing LED circuit with the 555 timer chip. It became very popular and i received many comments and emails with people that made this circuit and worked fine, as well as comments with people that had troubles converting it to operate at 12 volts supply. It was designed to operate with 5 volts, because i plan to use it for a future PC mod. Since the PC power supply has 5 volts output, and since the LEDs that i plan to use require 3.8 volts to operate, choosing 5 volts for supply was the best choice to minimize power dissipation on the transistor.
Converting the original circuit to operate at 12 volts is not a big deal, but it requires some transistor knowledge. The only parts that should be changed were the biasing resistors R4 and R5. But i decided to take it one step further. I decided to make a more flexible breathing circuit. The new version as 2 more functions: It has an adjustable voltage oscillation amplitude, and an adjustable output DC offset. What this means is that it can be easily adjusted to operate with different LEDs. If for example the load is a 12V LED strip with operating voltage range 8 to 12 volts, the circuit can be adjusted to provide exactly this: 8 to 12 volts output. Similarly, it can be adjusted to operate with a high brightness LED that has voltage range from 2.4 to 3.8 volts.
Here is the schematic circuit for the new version:
The circuit (click to enlarge)
You can set the fade-in and fade-out durations with these potentiometers
You can set the output voltage amplitude and the output DC offset with these potentiometers.
You may need to add a heatsink to the power transistor.
As you may notice, the left portion of the circuit, the 555 astable multivariate - has not been changed. The functionality is exactly the same as described in the previous article. The two variable resistors R2 and R3 are used to set the fade-in and fade out time.
From the same position (across C1) i get the capacitor's voltage. This is the waveform that we will work with. I like the charging characteristic of the capacitor because it provides a more "natural" breathing effect. Instead, you may wanna use a triangular waveform as an input, but I've test it already and rejected it immediately.
Instead of using a diode to couple the 2 circuit parts, i use a large electrolytic capacitor. That is because i want to have a pure AC waveform to feed the first transistor amplifier. So, C2 is a coupling capacitor which cuts-off all DC currents form the 555 timer. A large resistor (R4) is used to minimize the effect of the transistor amplifier to the 555 operation.
The potentiometer R7 is the first new feature added to the circuit. This is the amplitude control potentiometer. This potentiometer changes the voltage amplitude that will be fed into the base of the transistor amplifier, and thus it controls the output signal amplitude. It works exactly as the volume control on an audio amplifier: the bigger the input signal, the bigger the output signal.
The first amplifier (T1) is a common emitter with fixed bias. R8 is the base resistor which sets the operation point, and R5 is the collector resistor which sets the amplification gain. The output of the amplifier is again coupled though a large electrolytic capacitor (C3). At the output of this coupling capacitor we have an AC signal oscillating around gnd, with the final amplitude that will be fed into the base of the second amplifier. Since the LEDs operate with voltages above 0V, we need to shift this oscillation to the proper position.
Suppose for example that we want to power an LED that operates from 2.4 to 3.8 volts. This means that the output oscillation amplitude should be 1.4 volts (3.3-2.4). At the output of C3 we will have an oscillation from -0.7 to +0.7 volts. So we need to shift this oscillation 3.1 volts. This is achieved by adding a DC offset. The potentiometer R6 does exactly this. It is connected as a voltage divider, and provides a DC voltage from 0 to 12 volts. This voltage is then injected at the AC voltage. The result is the required oscillation.
Finally, the signal is fed into an emitter follower amplifier (T2) to increase the current. I chose a power darlington pair transistor for this stage, so that it can provide sufficient current for the LEDs, and also to be able to dissipate enough power.
There is something that you should notice here: An emitter follower amplifier can provide maximum current, but the maximum voltage will be about 0.7 volts less than the supply voltage. What this means is that, if you plan to control a 12V LED strip (this is what most people told me that want to use), you will need at least 12.7 volts as supply to have full brightness.
ONE LAST THING!!! BE CAREFUL! As you see, i have no limiting resistor for the LEDs. You need to add a limiting resistor if required, otherwise bot the LEDs and the transistor may be damaged. I chose not to add a resistor, since the circuit can provide exactly the voltage required for the LEDs. If you set the circuit correctly, then you do not need to add any resistor. This is one good advantaged if you plan to use high power LEDs, since a high power LED would require a high wattage resistor. But make sure that you have set the circuit properly before connecting your expensive LED!
Adjusting the circuit
It is very important to correctly adjust the circuit before you connect the LED. The adjustment has to do with the output voltage amplitude and DC offset. The maximum voltage of the oscillation must NOT exceed the maximum forward voltage (Vf) of the LED. As for the minimum oscillation voltage, this can be adjusted to any value.
If the LED forward voltage is the same as the circuit's supply then you can adjust the circuit more freely. But if the supply is much higher than the LED's voltage, you must follow these steps. First, you need to have a volt meter. It will be easier if the voltmeter has analog display.
Connect the voltmeter at the output of the circuit (pads LED+ and LED-).
Adjust the 555 to maximum fade-in and fade out duration, by turning the potentiometers R2 and R3 to maximum resistance
Turn the potentiometers R6 and R7 to the middle. The volt meter must now show readings. Notice that the readings are not stable, the voltage must increase and decrease slowly.
Now you need to decide what will be the voltage oscillation. You may wanna start with a typical 2 volts oscillation for a simple 3.2 or 3.8 Volts LED, or 4 volts oscillation for a 12 Volts LED strip. Turn the potentiometer R7 to adjust this oscillation. The maximum voltage shown on your volt-meter minus the minimum voltage is the oscillation amplitude. So, if for example you want to have 4 volts oscillation and the minimum voltage shown on the voltmeter is 4 volts, then turn the R7 slowly until the maximum voltage becomes 8 volts (8-4=4).
Now you need to adjust the DC offset. Slowly turn the potentiometer R6 until the maximum voltage of the oscillation becomes equal to the forward voltage of the LED. Do not rush finishing this step. Watch the multimeter for one minute or so and make sure that the voltage does NOT exceed the forward voltage of the LED.
If you finish all the previous steps successfully, you can connect the LED. Since the transistor has an internal collector resistance, you may wanna repeat step 5. That is because if the LED draws too much current, there will be a high voltage drop across VCE.
This is the output when the circuit operates as a flasher
The fade-in is faster than the fade out
The fade in is much slower than the fade out.
No DC offset added - The oscillation is around the zero line.
Added 4 volts DC offset - The oscillation is from 2 to 6 Volts
The amplitude is decreased. It is less than 2 volts
The amplitude is increased. It is about 11 volts
Bill Of Materials [BOM]
Resistor 100 Ohm 1/4 Watt 5% Carbon Film
Potentiometer 10 KOhm Linear Rotary 1/2W
Potentiometer 10 KOhm Linear Rotary 1/2W
Resistor 82 KOhm 1/4 Watt 5% Carbon Film
Resistor 1 KOhm 1/4 Watt 5% Carbon Film
Potentiometer 100 KOhm Linear Rotary 1/2W
Potentiometer 470 KOhm Linear Rotary 1/2W
Resistor 470 KOhm 1/4 Watt 5% Carbon Film
Resistor 1 KOhm 1/4 Watt 5% Carbon Film
Resistor 100 KOhm 1/4 Watt 5% Carbon Film
Electrolytic Capacitor 470 uF 16 Volts
Electrolytic Capacitor 470 uF 16 Volts
Electrolytic Capacitor 470 uF 16 Volts
1N4148 Switching Diode
BC338 NPN Silicon AF Transistors
TIP142 Complementary Silicon Power Darlington Transistor
@Juan Romero Good day to you, Juan. I just want to ask do you have any modifications on the schematic diagram and components of this circuit? After double checking all the connections, transistor polarization and wiring, still it doesn't work like on the demo video during circuit operation.
@Ricks Tamayo 1. current depends on the LEDs. You need like 100mA for the circuit and then the rest goes to the leds. I suppose 2A is enough.
2. Decoupling capacitor goes across the power supply. 100uF us ok
I built this circuit and got some problems like the LEDs glow at first but not breathing. After 3 or 4 seconds gradually dimming as the LEDs breathing. I am using 12 volts, 2 Ampere power supply unit.
My questions are:
1. What is the minimum current of power supply to be used in order to breathe smoothly the LEDs?
2. Where do I connect the decoupling capacitor and what is the value of capacitor, what type of capacitor also.
got the circuit all built on my bread board. powered it up and the led's came on... so far so good. i just can't seem to the the flashing to work right.. the fade out is adjustable but the fade in is just really quick.. any ideas where i went wrong?
I build this circuit for learning purposes, I have no knowlodge on transistors, and didn't check for pinout of equivalent transistor for the bc338. but after the correct placement, the circuit worked flawlessly. I used a 12v led strip with a 12v transformer source. Great work Giorgos, and thanks!
Hi Giorgos, I am having trouble locating a 470K trimpot 0.5 Watt, would a different value affect the operation of the circuit a great deal? Can you please suggest an alternative value that I can use. Many thanks, Nick W
What is the max input voltage of this circuit? I am planning on using this circuit in a car so will be running at high as 14.5 volts while the alternator is charging, would this be ok or would I have to regulate the input to 12v somehow?
@rajat joshi Measure the current through this resistor. It must be small, not enough to dissipate enough heat. So either the 555 has a problem or the connection you made is wrong. It happens some times that chips seem to work but the input current of them is huge.
hi i made this project it worked as expected in the first attempt but the only problem is the "R1-100 Ohm 1/4 Watt 5% Carbon Film Resistor" as it becomes burning hot during operation and burns away.plz help
Thank you for providing such a reliable circuit combination. It works like charm for me with very smooth fading effect (on 12v 7Ah battery).
Just one question - what value decoupling capacitor do you suggest/recommend for power supply.(I am using computer SMPS for supply and the fading effect is not as smooth as i get it by connecting 12v 7Ah battery).
Hi and thanks for sharing this circuit with us! For whatever reason, regardless of how I set the potentiometers, I cannot get it to pulse or fade, but the 6 leds that I have attached stay at a constant light. I have rebuilt it 3 times, and I am confused as to what is causing the problem. Can you please take a quick look at these photos and tell me if you see something wrong? I really want to build this right, and this is the second electronics project I have ever done.
I've used C3 - 470uf/16v as in figure, but it getting -ve value and capacitor exploding, so i've changes polarity of C3 but its not properly breathing, kind of getting high seps, like 3V -> 4V -> 5v ->11v -> 12V and same reverse
is there any solutions for that?
@Tim Brant The output power depends on the output transistors. What you wanna do is add another power transistor (darlington pair) as an emitter follower to amplify the current. Then you can have as many LEDs as you like.
@Joep This is normal because at startup C1 and C2 are discharged, so the T1 base voltage=0 and T2 base voltage is max. You need to make some sort of a start-up reset circuit, something to quickly charge the capacitors at startup or simply disconnect the T2 for a period of time.
Wonderful circuitry you provide here!! Would this circuit be usable for up to 66 LED's? They would be in groups of 2 and 3 (16 groups altogether), green LED, and some groups would be on some of the time, and maybe all groups on at the same time. In other words, there will be times where 3 LED's would be powered, and times when maybe all 66 would be. Would another power transistor (maybe like a BD243 like in another circuit you have) be required? Thanks.
Buid your setup and everything seems to work except for the fact that at the moment i turn on the 12 v power i get a (around) 5V peak (very short) before the volatage cycles starts. I just fried my high power led,... after this verty short peak the confiugred voltage cycle between 0.7 and 2.7 V starts. did i do anything wrong ? or can i prevent this first very short 5 v peak ? As soon as i turn on the power i measure between 4 and 5 v on the base of tip142.
@ Giorgos Lazaridis
Yes, when I connect an LED at pin 3, it does flash. However, on your schematic, there is nothing connected at pin 3 in the circuit. I ordered 3 of the BC338 transistors, and all 3 do the same thing. When I try to make similar circuits using that transistor, I can get 7V out of it, which is sufficient to make 2 LED's breathe, but I need the 12V for my application, which is what attracted me to this setup. I'm using a 555p, I've seen some saying 555n's in other setups.
Thanks for your circuits , I'm looking for it for my home external IP cam.
Just one thing : I would like the led brightness is automaticaly adjusted according to the ambiant light : very bright if sunny, medium if cloudy, low (but not 0) during nighttime. I think about add a lightCellResistor near R7, but since I didn't play with electronics since 25 years, could you help me please, before I build the circuit.
Thank you very much.
First, thanks for sharing your circuit designs, these make for some cool projects. I'm having trouble with both of your breathing LED circuits. I believe I've followed your specs exactly, but both oscillators decay quickly (within ~1.5 cycles) to a steady voltage. So typically the voltage will increase over the course of 3-4sec, then decrease slowly for ~2min and stabilize. Adjusting any of the pots perturbs the voltage, but then it settles back to a steady value following the same pattern. The only pot that seems to do anything else is R6, which also adjusts the resting voltage as stated.
@Raman 2 is the middle one. 1 and 3 are the sides. does not really matter how you connect 1 and 3. if the circuit works the wrong way (e turning CW the potentiometer goes slower) then change 1 with 3 and it will reverse operation (turning CW it will go faster).
Thanks Giorgos for reply..
wana ask one more question,
in schematic u show the potentiometer pins as 1 2 3 so how to determine that which pin is 1 and which pin is 2 and which is 3...i am very new in circuits and components so please help me...
Finally got down to building the circuit, again thank you for this! I have also built your previous fader but find this one seems to fade the led's in and out much faster, is there any way to slow the effect? R2 & 3 are already set to max.
Is it possible to make this to control 4 different LED's with different breathing rates? Or do I just need to make 4 of these one for each led? I would like to control 4 different led's and have all four of them breath at different rates.
Also if you made a board to make these and supply the parts you could probably make so good money off them:)
I a sculptor who would like to incorporate the breathing led effect in some sculptures. However, being I have very little experience with electronics and schematics readings, I was wondering if you could share step by step directions in assembling a unit that controls 3 or more led's.
@BARIS DOGAN This circuit is flexible so the output can vary by the potentiometer. You do not need to go strictly 82 Ohms, because you can simply supply less voltage to the LEDs. I would say 47 Ohms is pretty much fine, the reason for this resistor is for balancing, not really limiting.
@BARIS DOGAN Hello! The best solution is to connect 3 LEDs in series and then connect 8 of these series in parallel. You need to use also a small limiting resistor in series to each 3-series LEDs (so you will need 8 resistors). These are balancing resistors in case that something goes wrong. Also, to connect LEDs in parallel you need to be sure that all LEDs are exactly the same, otherwise one or more branches will not work. There is a relative article for this:
really great job! I will try out and look if I manage to fade away (i.e. my LEDs...;)
Do you think it's possible to trigger a normal 3W LED-bulb with the circuit (probably after changing the resistors in the bulb which are for 240V)?