How to chain one transistor's output with another circuit (multi-vibrator for example, but may be some other sound generating circuit on discrete transistors)?
The R1-Q1 with addition of a 1k resistor and LED works fine from collector: LED is ON when I connect R1 and OFF, when I disconnect it.
Now, I want to produce some sound with a multivibrator, when R1 resistance drops to 1-2m. The latter just does not work from collector!
(Of course, multivibrator works fine when Q2 and Q3 emitters are connected directly to ground).
Maybe, there is a better way to control circuits like multivibrator from collector?
UPDATE: R1 is MegaOhm, sorry for typo.
UPDATE 2: Thanks to hints from JIm Dearden, the circuit following started to (somehow) work:
However, I am going to implement better (and more general solution): Schematics for device, which reacts to water?
Disclaimer: any mistakes in the circuit are solely mine.
Answer
I find the question very confusing but if you want to turn the multivibrator on and off with another transistor (Q1). All you need to do is pull the base of one of transistors (Q2 or Q3) down to ground - preferably Q3 as it has the largest load and so switching Q3 off would save current consumption.
To turn the transistor (Q3) OFF its base-emitter voltage needs to be pulled below 0.6V.
Q1 will be subject to a negative voltage on its collector due to the switching action on the capacitor C1 which could cause it to breakdown. A Schottky diode (low forward voltage drop) is used to protect the transistor Q1 but still allow it to pull the base voltage below 0.6V.
As you don't give values its difficult to calculate the size of R4 but 1M0 would give a base current of 4.4uA and a collector current (assuming gain = 250) of about 1mA. If R3 is greater than 5k (5V/1mA) this should put Q1 into saturation.
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