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DPRG: More info on circuit

Subject: DPRG: More info on circuit
From: Alan Bredon alanb at cryogen.com
Date: Tue Nov 23 11:05:23 CST 1999

On Otto, I put the switches (in Otto's case, magnetic reed switches) in
series with 0.1 capacitors.  This differentiates the signal and
basically makes it fire once then go away.  This acts as a cheap
one-shot that signals the limit, then lets go of the signal line so
other switches on the same line can fire if need be.

If you want to operate the limits that way you state, without
supervision, then put a diode on the switch to block the power to the
motor for only one polarity.  The diode shunts the switch so the
opposite polarity can still pass.  The switch opens on activation
allowing only the opposite polarity to pass current to the motor.

I think this was what you wanted all along.  :-)

Alan Bredon

ElectroDFW at cs.com wrote:
> Sorry for the crossposting...but I wanted to make sure everybody who replied
> got this info... besides, we all probably already know each other
> (virtually), anyway.
> It seems my original piece didn't give enough specifics for you to help me
> enough.
> Let's see... Where do I start?
> It's a 3V permanent magnet DC motor
> I have a robot arm I want to put end of travel limit switches on (for the
> 180* elbow movement).
> Since the motor is near the axis, I want only two wires going to it from
> other parts of the robot
> I have the primary direction control on the other end of the wires (but I
> don't mind hacking in to the motor side of the wires)
> I also only want to use two RadioShack SPDT lever switches as the limiters
> (one on each side)
> If I overshoot moving the elbow, I want the limit switches to stop movement
> in that direction BUT also allow it to move in the *other* direction.
> For that, a straight cutoff won't do it; it won't allow me to free the switch
> by going in the other direction.
> So basically, I need a directional cutoff using only two SPDT switches, wired
> in such a fashion with the motor as to allow the above.
> Can it be done, and with few other components as possible?
> Hope that clears things up!
> David


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