Simple blinking LED circuit

For reasons I will not go into here, I wanted to build a simple electrical circuit that:

  • Has a blinking LED
  • Is driven by 5V
  • Does not require any IC or Microcontrollers

I found a few options, but this one seemed best.

(image linked from other site)

Well, it was not 5V, and I did not have the right components so I replaced:

  1. 470 Ohm resistors with 220 Ohm
  2. 100k Ohm resistors with 22k Ohm
  3. 10uF capacitors with 100uF
  4. The 2N3904 NPN transistors with BC547B

(Picture with 1uF and 10k Ohm/100k Ohm)

That worked fine! But I wanted faster blinking speeds and I wanted to try different blinking speeds. I am quite sure the transistors and the 220 Ohm (470 Ohm) resistors do not significantly affect blinking speed. But I tried different combinations of the capacitors and high resistors.

(Hz)      100uF   22uF    1uF
1000 Ohm  10      47      
2200 Ohm  4.1     19      480
5100 Ohm  1.5     7.0     153
10k Ohm   0.87    4.0     88
100k Ohm                  10

It seems that replacing the capacitors give a very near-linear change in frequency (and my capacitors are quite accurate). However, changing the resistors is not giving a completely linear change in frequency (or the resistors are of lower quality with more error tolerance). Nevertheless, with the table above, I think it should be quite possible to generate quite the frequency you need. To save energy, you should use small capacitors and high resistors.

Mixing resistors
Mixing resistors gives what it should. I mixed 10k with 100k and got a frequency of 15Hz (basically, half of them are 100k-slow, and the other ones are much faster). The “side” with the “higher” resistor is the side that is off most of the time.

Measuring method
I built an Arduino tool that measures IR blink speed (to measure for example rotation speed). I built my circuit as a test circuit. Clearly, it would have been easier to use a second Arduino with the simple Blink program than to build a circuit like this.

  1. Hi,

    I built this and it worked. Is there any chance you can explain what is going on in this circuit.


  2. If course it works 😛

    I have to admit… when I tried to simply explain it to myself now I am not really sure.
    There are many ways to rationalise about how it works, but not all of them are strictly correct, and I dont want to give you the wrong answer. I will try to get back some day!

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