Newsgroups: comp.robotics
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From: jbottoms@world.std.com (John W Bottoms)
Subject: Driving relays
Message-ID: <Cv1pK4.DEK@world.std.com>
Organization: The World Public Access UNIX, Brookline, MA
Date: Wed, 24 Aug 1994 15:26:27 GMT
Lines: 32

Here's the circuit I used to interface an RS-232 circuit to a
RatShack relay. It ain't pretty but it works. The 33ohm is to
limit current through the relay. The RS power supply actually
put out 13volts. Drain voltage should be about 10volts, grounding
the input (or -12v) should turn on the fet. The MHT20N15 TMOS FET 
I used is rated at 1.5 amps. The power diode across the FET protects 
it from transients when the relay is energized. (Blew up 2 before I 
realized that the internal diode on the TMOS FET was not sufficient 
to protect it.)

The circuit should also work for most small motors. Haven't tried
it. I'm planning on driving this from either the RTS or DTR signal
on the serial port.

                   +12V
                    C ||
                    C || relay
                    C >|
                    |
                    \  
                    /  33 ohm
                    \
                    /
       68ohm        |--------+
                  |/         - power
   --/\/\/\/--+---| tmos fet ^ diode
              /   |<---------+
         1.2K \     \
              /      |
              \      gnd
              -
             gnd
