When Astable Muvibs don't want to oscillate when the voltage is slowly applied. Why? And what to do?
Please read the description first. In many electronics books/publications no attention is payed to this problem: the AStable Multivibrator does not want to start (=oscillate).
The problem happens especially when the supply Voltage slowly comes up (rises) to the necessary voltage to supply the circuit.
In that case the Astable Muvib often starts to “hang” and does not want to oscillate.
But, when you “tip” your Astable Muvib to the voltage/current source it always starts! Strange!! It suddenly gets its supply voltage/current in that case! Say it gets that voltage via a pulse!
But why does it not start when the voltage supplied to it slowly goes from 0 to (say) 12 Volt?
There are some reasons: especially this: the Astable Muvib must be brought “out of balance” in the first milliseconds to be started properly, thus: starts to oscillate.
Here that is realized with a 12 Volt relay that “suddenly” (when its contact closes) biases the first transistor in the circuit.
Via that way, it always starts, even when the supply voltage slowly raises from (say) 0 Volt to 12 Volt. I used here a 12 Volt relay. Other relays & supply voltages are of course possible, say in the 5 Volt range.
Frequencies: with electrolytics in the order of 1 uF: flashes per second. Higher electrolytic capacitor value (say 10 uF) much more delay. With 2 capacitors of 4N7 we are in the 700 Hertz or so range. Or higher....
Experiment with these capacitor values, they set the puls-pause ratio. That ratio differs when their values differ. It can work to high frequencies. Align the 1 M potentiometer to change the pulse-pauze and the frequency.
With the help of that relay it always starts, but of course, that potmeter of 1 MOhm must be set to a position where the circuit is able to (!) oscillate. Not difficult!
Correction to the schematic the 12 K base resistor (to transistor 2) was not 12 K but 82 K. Both values work, by the way.
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