I'm trying to build a automated toggle circuit which toggles in a certain frequency determined by an RC time constant. I made a circuit which i planned to toggle between 5V and transistor saturation voltage ( ground ). It does the toggle but only just once. I want the circuit to do it repeatedly like switching two outputs in a certain frequency (Not too high, like with a period of half a second or near that).


simulate this circuit – Schematic created using CircuitLab

I want when Vo1 = 5V while Vo2 is grounded and switch the states for example in 1 seconds repeatedly.

If you say that i can do it with integrated circuits, logic gates or with a 555 timer, i know i can but i want to bulid it with electrical components by myself for educational purposes, to gain more understanding.

Is my circuit design totally wrong ? How can i improve it or rebuild to manipulate those two outputs (Vo1, Vo2) with transistors and capacitors in order to make them automaticly toggle between their states in a certain frequency ?

Note : I Forgot to put the voltage source on top of the circuit. It's 5V on the top.

  • 1
    \$\begingroup\$ nice idea, small nitpick: a 555 is just as much an "electrical component" as a single transistor, but the idea is clear (you want to build this from discrete semiconductors). \$\endgroup\$ Jun 16, 2021 at 14:29

1 Answer 1


The circuit you are looking for is called an "astable multivibrator" (astable because it runs continuously). There is a very classic two transistor circuit that works by cross-coupling the collectors and bases of two transistors, such that when one switches on, it starts charging a capacitor which will eventually do the same to the other one.

Your circuit needs to somehow implement the same idea - or of course you you could build and experiment with the one that is already well known.

  • 1
    \$\begingroup\$ Yes, this is excatly what i was trying to build. \$\endgroup\$
    – jwdomain
    Jun 16, 2021 at 14:43

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