[Here is a] really super-cheap 5V relay driver. It uses an emitter follower so it actually delivers only 4.4V or so to the relay but this has always worked fine for the 5V relays I have used. It does not require a protection diode because the emitter follower won't allow any undershoot.
Use a 2N3904 transistor for up to 100mA. A 2N2222 may be used for up to 200mA. A ZTX689B is good for up to 500mA.
This circuit will NOT work with the open collector outputs on a PC Parallel Port, but will work nicely with a PIC or other fully driven output capable of sourcing a few milliamps.
- Lowest parts count
- Lowest cost
- Only good for driven outputs (not OC) and relays that will pull in at 4V
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