In this video, you can see that the circuit produces two blinking LEDs. These are all the components to make it work.


  1. BC 547 Transistor
  2. LED
  3. Breadboard
  4. Battery 9V
  5. 10uF Capacitor
  6. 47 komh resistor
  7. 100 omh resistor
  8. Wires

My question is, how do you know which transistor/capacitor to use in order to produce the blinking LEDs? How do you know to select BC547 transistor and use it with 10uF capacitor? I just don't understand how to come up with this? Do you need a formula? make

I want to understand this so that I can make my own in the future without relying on a tutorial video. Thanks!

  • 3
    \$\begingroup\$ I haven't checked the circuit connection for connection, but most likely it's a transistor astable multivibrator. You should study the many pages on the web about this circuit and then come ask here if you have a specific question you still need answered. \$\endgroup\$
    – The Photon
    Commented Sep 14, 2020 at 19:18
  • \$\begingroup\$ I think this circuit can be imaged. \$\endgroup\$
    – nick_n_a
    Commented Sep 14, 2020 at 19:30

2 Answers 2


47k and 10uF determine the flashing frequency and 100 Ohm determines the current of the LEDs. ast1

There is a small problem with it. The reverse B-E voltage of transistors is too high due to the 9V supply voltage. With two plus diodes, this can be reduced to a safe value. ast2

  • \$\begingroup\$ Thank you very much. I have a follow up question. I tried switching 47k to 10k and 220k to see if it would change the flashing frequency. However, it didn’t and the lights were shining without blinking. Do you know why that is? \$\endgroup\$
    – Mac_79
    Commented Sep 17, 2020 at 11:02
  • \$\begingroup\$ 10k is too small, 220k is too big. :) Due to the small collector (LED) resistors, the usable range is relatively small (33k ... 100k). 47k is an optimal value for this. Can be switched over a larger range by replacing 10uF capacitors. \$\endgroup\$
    – csabahu
    Commented Sep 17, 2020 at 20:56

I didn't look at the video, but that looks like a standard astable multivibrator circuit. Perhaps it should have an electrolytic capacitor across the battery.

The BC547 (and many other parts such as 2SC1815, S8050, 2N4401, 2N3904 etc.) are standard cheap "jellybean" NPN transistors that are common in the world. If those transistors work you don't need to look further. This is a circuit that does not place great demands on the transistors.

If you understand how the circuit works including some basic characteristics of the components you can calculate the formula for frequency of oscillation, or look it up given the "astable multivibrator" search term. Only simple math is required.


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