Relays are awesome small devices which could be used to control other devices. The plus point of relays is,they can be used to drive a different voltage level device with complete isolation. In here, I'm going to show you how to build a simple relay drive board for your parallel port interfacing or microntroller interfacing. Basically we are using a simple NPN transistor as a switch to drive a relay. The diode is used to protect the transistor from back e.m.f. generated by the relay coil at transient points (relay on/off). You can use 6V or 12V or any other relay as you wish. The transistor used here is 2SD400 (Ic=1A,Pc=0.75W) which costs around Rs.10 ($0.08). You can use any other general purpose NPN transistor as you wish.
The schematic for a channel:
You can build any number of channels as you wish. Here is a 4 channel board build as above.
You can connect this board to parallel port's D0 to D7 (pin 2 to 9) which belongs to the DATA register of the port. A simple program could be coded to turn on/off the relays connected to the parallel port. Even you can connect this board to a microcontroller and do the same. (Image source:Wikipedia)
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