I need to switch on a relay with the output of the LM393. The relay is a 5vdc unit from RadioShack. Does the LM393 have what it takes to trigger this relay? If not what should I put between the relay and the LM393 to permit the 'output' of the relay to properly 'close' the relay?
Unless the relay is very small (low power), probably not. The LM393 saturates fully on only when the pull up current is a few milliamperes. Even a relay with a .1 watt coil draws 20 milliamps.
I would add a driver transistor with a clamp diode across the coil.
If you can reverse the sense of the comparator (reverse its inputs, so that the relay is on when the comparator is off) all you need is to connect an NPN transistor like a 2N3904 or 2N4401 emitter to ground, base to comparator output, collector to coil, other end of coil to +5, 1k resistor base to +5.
The diode connects cathode to +5, anode to collector.
If you cannot reverse the sense of the comparator, then it is easier to use a PNP transistor, like a 2N3906 or 2N4403, and connect emitter to +5, collector to coil, other end of coil to ground, 1k resistor base to output, 1k resistor base to +5.
In this version, the diode connects cathode to collector and anode to ground.
Note that if the relay includes a diode (many 5 volt DC units do) you have to connect its coil so that the internal diode is right way round, in place of the external diode described above.
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