  Flexible 555 LED Pulsing (Breathing) Circuit AuthorGiorgos LazaridisDecember 28, 2011

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.

The Circuit
Here is the schematic circuit for the new version: The circuit (click to enlarge)  You can set the output voltage amplitude and the output DC offset with these potentiometers.
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!

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.

Step 1:

Connect the voltmeter at the output of the circuit (pads LED+ and LED-).

Step 2:

Adjust the 555 to maximum fade-in and fade out duration, by turning the potentiometers R2 and R3 to maximum resistance

Step 3:

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.

Step 4:

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).

Step 5:

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.

Good Luck.   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]
 Resistors R1 Resistor 100 Ohm 1/4 Watt 5% Carbon Film R2 Potentiometer 10 KOhm Linear Rotary 1/2W R3 Potentiometer 10 KOhm Linear Rotary 1/2W R4 Resistor 82 KOhm 1/4 Watt 5% Carbon Film R5 Resistor 1 KOhm 1/4 Watt 5% Carbon Film R6 Potentiometer 100 KOhm Linear Rotary 1/2W R7 Potentiometer 470 KOhm Linear Rotary 1/2W R8 Resistor 470 KOhm 1/4 Watt 5% Carbon Film R9 Resistor 1 KOhm 1/4 Watt 5% Carbon Film R10 Resistor 100 KOhm 1/4 Watt 5% Carbon Film Capacitors C1 Electrolytic Capacitor 470 uF 16 Volts C2 Electrolytic Capacitor 470 uF 16 Volts C3 Electrolytic Capacitor 470 uF 16 Volts Diodes D1 1N4148 Switching Diode Transistors T1 BC338 NPN Silicon AF Transistors T2 TIP142 Complementary Silicon Power Darlington Transistor Integrated Circuits IC1 555 Timer Name  Email (shall not be published)  Website Notify me of new posts via emailWrite your comments below:BEFORE you post a comment:You are welcome to comment for corrections and suggestions on this page. But if you have questions please use the forum instead to post it. Thank you.  HOT in heaven!  NEW in heaven! New Theory: AC electric motor working principle