Electronics beginners frequency generator (audio generator) that can generate frequencies between 40 Hertz and 22 KHz with a minimum of components.
Used transistors (left to right in the schematic): TS1 = BC547 TS2= BC547 TS3=BD139
Waveforms differ very (!!) substantially due to the way that the astabile multivibrator was made: 1) it had to work under all circumstances 2) in a broad frequency band 3) with a minimum amount of components 4) with a differentiator circuit.
So I had to realize the maximum of incongruency between transistor 1 and transistor 2 (Darlington) to fulfill these needs. Watch the oscilloscope views for more info.
When it stops during use, press the “restart” (=start) switch, that disconnects the power supply and starts the multivibrator again.
Only seldom necessary to use that “restart” switch, only when using the differentiator potentiometer the generator sometimes switches off on (say) 20 KHz.
When you do NOT want the generator to stop when you use the “differentiator” switch (S3) and potentiometer (P2) on high frequencies, say 20 KHz, change the 12 Ohm resistor into a resistor of 22 Ohm or 33 Ohm or 47 Ohm or 100 Ohm.
Of course the “differentiator effect” with such a higher value resistor will be less present when turning P2 (1K). Position switch is possible.
By the way: the circuit generates good even squarewaves with identical (..identical) capacitors (transistor 1 – Darlington 2) between 470 N and 22 N. But that was not the aim of this circuit.
Of course: you can buy a perfect frequency generator nowadays 2019 for (say) 10 Euro’s or Dollars in China, say Banggood etc. But doing experiments with this circuit leads to more practical knowledge about electronics.
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Be aware of that, I saw on the internet my circuits reproduced in a poor or even not proper way. I can not help that, sorry. Upload 3 August 2019.