Question about similar 555 timer circuit- LED Flasher and Fader

Thread Starter


Joined Jun 9, 2018
I recently built a fading LED circuit according to the schematic below:

It worked fine, but then, I came across this schematic, for an LED flasher, not a fader:

My question is, what makes the difference between whether or not the LED fades, or flashes? The only thing I could figure is that the addition of the connected 7 pin would allow the capacitor to discharge to ground through it instead of going through the LED to give it the fading effect in the first circuit.

The only other main difference I see is the addition of the transistor in the first schematic, but I don’t know what difference that would make. The question is kind of pointless, but I’m hoping getting an answer will help me get some more intuition for working with the 555.


Joined Nov 18, 2016
On the first circuit the capacitor will gradually discharge after receiving a charge from pin 3, which in turn will decrease the voltage on the base of the transistor and the transistor will gradually shut off.

The second circuit is a normal astable circuit, where the capacitor is only used to set the time at which 555 should work. The 555 will output an impulse and turn on the led, then off.

The first circuit is professionally made. The capacitor is not only used to set the time constant with the resistor on pin 3, but for the fading effect with the transistor also.


Joined Sep 7, 2010
I was just going to build a fader for my LED lights ....

I have some spare supercapitors and thought I might use these 10F , 2.7V ... connected across the lights.

five in serries give 2F @ 13.5 V ............. energy stored 1/2 CV2 ...= 182J ....

lights consume 3A @12V = 36W .... but the capacitor won't be fully charged or discharged each time ....perhaps a 3 sec fade period.


Joined Feb 20, 2016
I would recomend you do not use an analog fade method, but go with PWM instead. It is a lot more efficient and controlable.
Have a look for Arduino PWM examples for instance.


Joined Nov 18, 2016
I do not know. Super capacitors for a light? That seems like an overkill to me, also the losses will be big.

You need to charge the capacitors fully to achieve the voltage you need. You will need a large current drain to achieve 3 sec. The transistor may need a heat sink.

The idea normally is to use a small 100uF capacitors in order to just control the transistor, which in turn ampifies the current and just gradually turns off.


Joined Aug 1, 2013
In the first post, the two 555 circuits are the two most common variations for an astable oscillator. The second one is the classic, and the first one uses 1 fewer resistor and guarantees an almost perfect 50/50 square wave. I you modify the 2nc circuit by decreasing R2 to 33K and increasing C1 to 100 uF, and attach the base of Q1 from the first circuit to the top of C1, the performance will be very similar.

The LED fading in and out comes from the charge-discharge waveform across C1. It is an exponentially-curved triangle wave, sometimes called a "shark fin" waveform. Very little current is available in-out of C1, which is why Q1 acts as a current amplifier to drive the LED. A 555 output is good for a few hundred mA, which is why it can drive the LED directly in the 2nd circuit.