I found this simple circuit of controlling the speed of a DC motor using a potentiometer. I'm wondering if this could still work if I replace the potentiometer with an LDR. I have a variable voltage supply up to about 15V.

 

Hello,

The LDR would probably burn.
It can most likely not handle the current needed for the motor.
You could try to make a PWM circuit that reacts on the LDR.

Bertus

 

Oh I see.. But why would the potentiometer work?

 

Hello,

They are using a rheostat, that is an high power potentiometer.
It is also strongly dependent on the current drawn by the DC motor.

Bertus

 

How about this circuit? Would a regular potentiometer (10k or 100k) work?

 

Hello,

A 10K potentiometer might work.
A 100K potentiometer will not work.
The transistor needs a certain current as input to control the motor speed.
The 10K might already be on the border of a correct working of the circuit.

On the following page you will find some circuits for motor control:
Motor control

Bertus

 

I only have some simple components such as resistors, capacitors, potentiometer, voltage supply, transistors, etc.. I don't have ICs so I can't use PWM. Could you suggest other ways of doing it aside from the Darlington pair?

 

Hello,

Can't you get hold of an 555?
If you can get one, you might want to look at this thread:
http://forum.allaboutcircuits.com/showthread.php?t=24028

Bertus

 

Using a straight resistance based control scheme will waste lots and lots of power creating HEAT.

The circuit above uses transistors as a variable resistance to lower voltage. The designer used a 5 inch square chunk of aluminum heatsink material, with fins 1.7 inches tall. The fan was included to keep temperature down. Avg current for the device in use is 10-20 amps and that will result in a 160 degree avg. temp rise. at the transistor mounting points. It was used to provide a slow ramp up of power for bench testing of RC plane motors.(paraphrased from the original descriptions)

Using a PWM scheme with a 555 IC might prove to be easier to do than what you see above, AND it would save lots of power that can be used by the intended load.