[sebhc] Motor control

Lee Hart leeahart at earthlink.net
Wed Jun 28 10:52:56 CDT 2006

Carroll Waddell wrote:
> Thanks Lee. I have the schematic, but I was just wondering how
> the circuit works. It seems that the 555 timer functions as a
> monostable multiv. It appears that the pulse width is about
> 1.67 msec. At 300 RPM, that would be about 1/2 of a revolution.

Not having the circuit to look at, my guess would be that the 555 drives
a transistor, which drives the motor. The 555 switches on/off at some
duty cycle. The duty cycle produces an average voltage to the motor; for
example, 25% on-time = 12v x 0.25 = 3v, 50% on-time = 12v x 0.5 = 6v,
etc. Thus, it controls motor speed by controlling the duty cycle of the

The tachometer voltage would be rectified and filtered to produce a DC
voltage proportional to speed. This voltage then controls the duty cycle
of the 555 pulse-width modulator. The circuit would be arranged so if
the motor speeds up, the tach voltage rises, which reduces the on-time
of the 555.
Ring the bells that you can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in
	-- Leonard Cohen
Lee A. Hart   814 8th Ave N   Sartell MN 56377  
leeahart_ at _earthlink.net

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