A versaile output interface that can control external devices direct from your computer. Uses freely available software and works with both windows and Linux.
Any PC that has a free parallel printer port available should work with this circuit. The schematic shown above requires little explanation. Any data sent to the printer port is already "latched" and just requires buffering to be able to drive an external ciruit. In this case ULN2083 octal transistor is used to array to buffer the output current from the parallel port. The IC pinout for the ULN2083 is shown below.
The pararell printer port on your PC is a 25 way D type connector. Its pinout is shown below. Pins 2 to 10 are the data output lines D0 to D7. Pins 18 through 25 are ground pins, the other pins are not used and left unconnected.
The output current is amplified by the ULN2083 and each output pin can supply a load of up to 50mA. The outputs are active low, meaning that a high logic input gives a low logic output. To get around this the output load is routed to a positive voltage, so that any high input will now produce a high output. The positive supply can be any value from +5 to 50Vdc.