VERY simple dc motor drive circuit?

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Either an OpAmp with enough power capability to run the motor directly, or an OpAmp with a boost transistor added at the output.
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Yes.
No, VC is the "command"
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...Jim Thompson

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Don't forget the diode on the motor to eat the spike as the MOSFET turns off.
If you run the pin 5 of the 555 to a zener, you can compensate for the variations in the input voltage.

John E. wrote:
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Put an emittor follower behind the potentiometer.

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You want one part called a rheostat. Assuming the motor near full load at 500 mA then the resistance will be E/I = 10 ohms. Filament control rheostats were often zero to 25 ohms at around 5 watts. Odds are you will end up using a 10 ohm resistor. If thats not enough, try 20 ohms or so.

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wart
feedback,
http://www.uoguelph.ca/~antoon/circ/pwm555.html
JT
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LM317, a pot, & a couple of resistors should do the trick easily & cheaply. Suitable circuits can be found on most LM317 data sheets.

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That's not good. Every problem must be solved by a circuit using either an LM555 or a PIC, perferably both :>
Earlier, I suggested the LT1083 as a better way to go than the LM317 because of the heat issue. I have also thought of a very ugly circuit like this:
Input +V ------+------------------+----------------+----- To motor ! ! ! ! \ ! ! / --- ! -- \ --------- ^ ! ! ! ! ! ! --- +---/\/\---++----+----- To motor ! --- ! ! \ ! !/e ! / <----/\/\--+---! PNP ! \ !\ !/ ! +---------! NPN ! ! !\e \ \ ! / / GND \ \ ! ! GND GND
This requires much less current in the POT than the single transistor one. The NPN's heat is a lot lower too.

On Sun, 4 Mar 2007 18:09:11 +0000 (UTC), snipped-for-privacy@green.rahul.net (Ken Smith) wrote:
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Hey, LM317s are /nearly/ as traditional an answer to any kind of electronics problem as the classic 555, & much more traditional than those new-fangled PICs. ;) (And my science teacher in highschool used LM3909 as his solution for most things.)

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Take a look at the links here http://www.epanorama.net/links/motorcontrol.html#dc