trouble with controlling a 12V Motor with an LDR

Hi I've been using a circuit that controls the speed of a 4.5V DC (23g/CM) motor through an LDR and I want to use it with a higher torque 12V DC (180g/CM) Motor but can not get it to work.

I dont have a subscription to the circuit program this site uses so I'm going to have to best describe it myself. the Circuit consists of an LDR that is connected to an NPN Transistor (2N3904) through a 1K Ohm Resistor and also has a 10K Ohm resistor connected from the LDR to the GND, the emitter of the NPN is also connected to the GND. The collector of the NPN is connected to the + of the DC motor and the - is connected to the other side of the LDR and the + of the 9V battery.

This all works but I need there to be more torque as I'm using it to turn a 20g wooden arm and it is just not moving with the arm connected. I'm trying to upgrade to a 12V DC (180g/CM) motor. I have tried replacing the old motor with this one but it will not work. I've tried halving the resistance to a 510 Ohm resistor and a 5K Ohm resistor which was better but wont work with the arm attached. I even went down to a 10 Ohm resistor and a 100 Ohm resistor but that just seemed to not work at all.

Im reasonably new to electronics so I'm sorry if my explanation didn't cover or over covered certain areas but any help would be greatly appreciated as I need to get this to work in the next couple of days. I'm not sure if this circuit simply just won't work with a 12V motor so if anyone knows of a circuit that does work it would be greatly appreciated.

regards Julien

by JulienC-K
August 27, 2018

1 Answer

Answer by mikerogerswsm

Try replacing your 2N3904 transistor with a BU806 darlington pair (cost about £1.50). This sounds the simplest way to do the job.

+1 vote
by mikerogerswsm
August 27, 2018

Thanks, I'll give that a try.

by JulienC-K
August 27, 2018

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