DC Motor Speed Controller

[cenation]

Hello all,

I have been given this circuit to implement. It is a DC Motor speed controller, with the LM2907 used as a tachometer. However, the LM2907 is not available in my country and would take too much time to be delivered. Could you suggest other ways to achieve the same results, i.e controlling the speed of a small dc motor.

Thanks.

 

[chuckey]

If your light detector triggers a 555 pulse generator. The pulse must be less the 1/maximum number of pulses/second. So when the motor is running flat out you have a series of pulses with a very small gap between them. If your pulses go from 0 -> 5V, then the average voltage is , 4.5 V. As the motor slows down the gaps between the pulses increase, so at a low speed there might be a pulse for 2 % of the time, in which case the average voltage will fall from the 4.5 volts (fast) to 5 X .02 = .1V.
So you now need an op amp to change the pulses into a DC level (large capacitor between input and output), alter the change in voltage to something convenient (gain of op amp) and perhaps, add in a DC level for the next circuit to operate with (DC offset pt on positive input).
Frank

 

[cenation]

If your light detector triggers a 555 pulse generator. The pulse must be less the 1/maximum number of pulses/second. So when the motor is running flat out you have a series of pulses with a very small gap between them. If your pulses go from 0 -> 5V, then the average voltage is , 4.5 V. As the motor slows down the gaps between the pulses increase, so at a low speed there might be a pulse for 2 % of the time, in which case the average voltage will fall from the 4.5 volts (fast) to 5 X .02 = .1V.
So you now need an op amp to change the pulses into a DC level (large capacitor between input and output), alter the change in voltage to something convenient (gain of op amp) and perhaps, add in a DC level for the next circuit to operate with (DC offset pt on positive input).
Frank

Good idea... I'll try that... Thanks!