please explain this IR circuit

Thread Starter

jack hammer

Joined Jan 25, 2016
3
Hi
I have 2 questions about it.
1. I understood reason why emitter of TR2 and base of TR3 was connected.
but I can't understand reason why base of TR3 and emitter of TR4 was connected with R11.

2. how was c5, c6 capacitor value selected?

please explain this reason!:)
 

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Dodgydave

Joined Jun 22, 2012
10,027
Tr3/4 are wired as a schmidt trigger, tr3 is turned off by R11 when Tr4 is on. This is to give a snap action on the relay.
 

Thread Starter

jack hammer

Joined Jan 25, 2016
3
thanks !
i tested this circuit without R11 connection.
but circuit worked well.
i think emitter of TR2 and base of TR3 connection sufficiently play a role as TR4 trigger.
 
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