I dont think a tach would be so expensive, 1 led, 1 phototransistor, 1
tiny piece of plastic on the motor shaft. total cost 98 cents and no
special circutry to translate to PIC compatable signals, just measure the
frequency.


On Mon, 22 Jun 1998, Ron Fial wrote:

> Another (perhaps old fashiond) way to measure DC permanent magnet motor speed
is to measure the back EMF from the motor when the voltage to the motor is momen
tarally off.  If you are doing PWM, then when the current is off, if you read th
e back EMF voltage from the motor, it is directly related to speed.  I have used
 this and the speed control is quite accurate -- You are using the motor as a DC
 generator for an instant.
>
> This method takes more parts than measururing ON current, usually needs an op-
amp circuit, but way cheaper than a tach.  This is probably to complex a method
for an H bridge driver, but its quite easy for a PWM single direction of rotatio
n application.
>   Regards,
>      Ron Fial
>
> At 12:56 PM 6/22/98 +0200, you wrote:
> >Hi all,
> >
> >As a newcomer to the list, I am interested in speed regulation of a
> >small DC brush motor (e.g. as used in model trains) by using a PIC. Most
>
>
> Regards,
>   Ron Fial
>