Transistor Circuit Help

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GAHorton
 
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Transistor Circuit Help

Post by GAHorton »

Hello All

I have a board that reacts when resistance gets high with a photoresistor. I want to build a circuit that will make it do the opposite.

Basically it would have a phototransistor and a regular transistor, when light is seen, trigger transistor that completes the circuit with the necessary ohms.

Can anyone turn me in the right direction?

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stinkbutt
 
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Re: Transistor Circuit Help

Post by stinkbutt »

Would something like this work?
phototransistor.darlington.png
phototransistor.darlington.png (18.43 KiB) Viewed 1014 times
The idea here is the phototransistor will refuse to conduct until light is incident on it, at which point it conducts & pulls the base of the 2N3904 (a very very common NPN transistor) high, which turns the rest of the circuit on. The R1 in the 2N3904-controlled half of the circuit is there because you said you wanted the transistor to conduct at a certain resistance. You set it for whatever you need that resistance to be.

Also, I'm not certain but it might be advisable to pull the base of the phototransistor weakly low to ensure it doesn't conduct under normal circumstances. I don't expect it would, but that also would allow you to tune the sensitivity of the phototransistor with the pull-down resistor value.

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zener
 
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Re: Transistor Circuit Help

Post by zener »

I think I would also connect the emmiter of the 2N3904 to ground and make it a low side switch.

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stinkbutt
 
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Re: Transistor Circuit Help

Post by stinkbutt »

He doesn't appear to want that:
GAHorton wrote:...when light is seen, trigger transistor that completes the circuit with the necessary ohms...
That's why I left the connections of the collector & the emitter of the 2N3904 unbound. That part is his problem. Though, to be fair I've not the first clue what the phrase "necessary ohms" means. It might just be a lost in translation.

eventhorizon
 
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Re: Transistor Circuit Help

Post by eventhorizon »

Though, to be fair I've not the first clue what the phrase "necessary ohms" means.
Prolly it's ohms from the resistance of the photoresistor.

Anyway, using NPN with a photoresistor:
a) to turn it on when light is seen, is just to connect the photoresistor to the transistor's base and to Vcc. When you want something pulled-down, you connect the emitter to ground, and the collector to whatever you want pulled-down. If you need to switch something, you connect the collector to Vcc, and the emitter to whatever needs turning on.

b) to turn it off when light is seen. This time, you connect a pull-up resistor to the transistor's base and to Vcc. You then connect the photoresistor in-series with the base and GND. The resistor value should be high enough that the photo-resistor's resistance would be lower than the resistor when illuminated, but low enough that it can fully turn on the transistor. Small photoresistor may not be enough for this, since 3904 are used with 1k to 10k resistors, you could try using photoresistors in parallel. If your photoresistor can counter that (sometimes it just forms a voltage divider with the resistor and "dims" or partially turns on the transistor) then you are good.

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