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Forum Index : Electronics : bc557 transistor

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irishron40
Senior Member

Joined: 22/09/2014
Location: Ireland
Posts: 254

Posted: 03:46pm 29 Oct 2017

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an anyone tell me what I can do to change this ciruit so I wont need the bc557

its for halloween ,and i need a 12 volt laser circuit which does switch a relay.
Thanks .

Trevorc
Newbie

Joined: 26/02/2017
Location: United Kingdom
Posts: 22

Posted: 11:09pm 29 Oct 2017

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Hi irishron40

An interesting one. I think the transistor is there to ensure that the circuit only operates with a very bright light and will not trigger for ambient light changes.

If this does not matter, then I would leave out the 0.22uf capacitor, the transistor, and 10k resistor and connect the LDR to the remaining 10k resistor and ground.

This should work though you may have to experiment with the 10k resistor value maybe use a 47k pot.

Of course the circuit will operate with ANY light, you will have to test it in the dark.

Incidently, you did not say why you don't want to use the BC557.

Hope this helps.

hotwater
Senior Member

Joined: 29/08/2017
Location: United States
Posts: 120

Posted: 04:34am 30 Oct 2017

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Seconds remaining. What do you have as a photo detector? Many don't know you can use about any LED to detect light. You could connect that directly to the trigger pin. The transistor just provides a high impedance input almost any PNP would work or an op amp in unity gain.

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