Regulate motor speed with photoresistor circuit

Hello all,

I am attempting to power a simple ~12V motor with a battery, and regulate its speed using a photoresistor, such that under high light conditions the motor turns quickly, but under low light conditions the motor turns slowly or not at all.



I was able create a circuit using batteries, a resistor, and a photo-resistor similar to whats above. The output would be high (around 12V) in light and low (< 3V) in dark.

The problem arose when hooking up the motor inputs to this voltage output. Simply put, the motor didn't turn. The voltage at the motor input dropped to around 4V or 5V when the motor was connected.

I tried using an op-amp to buffer the output of the photo-resistor circuit, but the motor still won't turn. I'm not sure if it unable to draw enough current or what.

The motor turns perfectly when connected directly to the battery.

If there is a way to fix this, or an easier way to accomplish this, please let me know.

Thank You

How much current does your motor draw? The voltage divider formed by the fixed resistor and photoresistor will have too high an output impedance to drive the motor directly.

You're right about needing a buffer, but an op-amp also cannot supply much output current. Maybe the simplest way would be to use an NPN transistor configured as an emitter follower. Just make sure it can handle the motor current, and watch out for the temperature rise. The transistor can get hot!

Regards,
Chris