Which transistor do I need?

Thread Starter

robotallien

Joined Aug 22, 2018
10
Hello everyone I want to make a light detection circuit but I don't know which transistor I need, I need a circuit with an LDR sensor 9V comes in and 9V comes out when it's dark.

Thanks in advance :)

Btw, I know electronics just not how to select right transistor
 

Dodgydave

Joined Jun 22, 2012
8,465
Any general pnp or npn transistor will do, A circuit like this is probably what you want using a pnp upto 100mA, if you need more current then a BC327 will do.

LDR-circuit-dark-detector.png
 
Last edited:

BobTPH

Joined Jun 5, 2013
2,047
No, the output would be high impedance to 9V when lit and low impedance to 9V when dark.

We cannot tell you whether this would work or not without knowing what you want to power with that output.

Bob
 

wayneh

Joined Sep 9, 2010
16,114
9V led light strip and Arduino (controlling LEDs)
I think I'd do this differently, for one reason: I think you want the Arduino to be either fully off or fully on, and never somewhere in between. This is hard to achieve with a single transistor. With just an LED as a load, it might flicker a bit as it transitions from one state to the other. No big deal. But that period of flicker might be rough on the Arduino. That's just speculation on my part, maybe it would be fine.

The solution to the problem that may not exist is a comparator with hysteresis. The downside is that it would be operating all the time and using a small amount of current.
 

Bernard

Joined Aug 7, 2008
5,283
" 9 V light strip ", not much help. Post 2, Q 1 might be getting around 300 uA base current , not much output so might use a NPN to drive the PNP ?
 

SamR

Joined Mar 19, 2019
1,293
Do you have any transistors available to chose from?

Do you have a basic schematic of what you want to do?
 

Thread Starter

robotallien

Joined Aug 22, 2018
10
Thank you all for your help but I found the solution and my light strip was 9V 2A but it's plugged in with a adaptor into the wall so it's able to supply this Voltage and Current :)
 

Thread Starter

robotallien

Joined Aug 22, 2018
10
I think I'd do this differently, for one reason: I think you want the Arduino to be either fully off or fully on, and never somewhere in between. This is hard to achieve with a single transistor. With just an LED as a load, it might flicker a bit as it transitions from one state to the other. No big deal. But that period of flicker might be rough on the Arduino. That's just speculation on my part, maybe it would be fine.

The solution to the problem that may not exist is a comparator with hysteresis. The downside is that it would be operating all the time and using a small amount of current.
Thanks but in my case this isn't really a problem since it will be in a room where the light is always On/Off
But thank you very much for your time and help
 

Dodgydave

Joined Jun 22, 2012
8,465
Thanks but in my case this isn't really a problem since it will be in a room where the light is always On/Off
But thank you very much for your time and help

If you're switching a led strip on then a 555 comparator will do, take the output from pin 3, to a relay to put your leds on....


713d747a4aad4baf30795a106c101463.png
 

BobTPH

Joined Jun 5, 2013
2,047
Have you got it working? Because I don't believe that circuit will drive a 2A amp load even if you pick a BJT capable of that power. You would need 200mA to the base to turn the transistor on fully.

A circuit with a MOFSET on the low side would be much better.

Bob
 

Thread Starter

robotallien

Joined Aug 22, 2018
10
Have you got it working? Because I don't believe that circuit will drive a 2A amp load even if you pick a BJT capable of that power. You would need 200mA to the base to turn the transistor on fully.

A circuit with a MOFSET on the low side would be much better.

Bob
I'm only using a short part of the strip, so it worked
But thanks!
 
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