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Need help testing this circuit

hevans1944

Hop - AC8NS
Jun 21, 2012
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In a conversation, Mark said that he wants blocking and unblocking the light from the phototransistor to turn the load LED on and off and if replacing the SCR with a transistor will do it (like I explained in my last post here) and I said yes, but I also explained all the missing details that are important to do the actual design.
I didn't realize you and Mark were having a private conversation. He started one with me, too, but it hasn't gone anywhere.

If this is, indeed, a high-speed photo-flash application, then the SCR provides the inherent positive feedback necessary for fast triggering of the flash tube. Later, on the same website page, it is mentioned that a Schmitt trigger is necessary for "slow moving" objects interrupting the light beam. The authors suggest using the 555 timer IC to obtain the fast trigger transition for the SCR. But, if fast triggering is NOT an objective, then replacing the SCR with a bi-polar junction transistor (BJT) is a viable option.
 

Mark34

Sep 23, 2016
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Sep 23, 2016
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Well my variable resistor was broken I replaced it with a 22k rezistor and OMG and the light led is turning off and on depending on the photogate status.
I don't know how but its working even with the SCR or without it. With no SCR diagram looks like this.

Original
 
Last edited:

Audioguru

Sep 24, 2016
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Why did you connect the indicator LED to the very low current phototransistor? Then the 2N2222 transistor does nothing.
Do it like this:
 

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Mark34

Sep 23, 2016
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Thanks guys and especially Audioguru, the circuit you draw does the job.
I will be back ...:) because this was just phase 1 .
 
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