It can only used with a PC's parrallel output and a special programm running the parallel port control program.
It doesn't overoading the pc's port because it use JFET BS250 which acts like a switch.
Those swithes make a short-circuit between Drain - Source when a zero voltage appears in Gate through a 470 Ohm resistance.
When the JFET short-circuited a led lights through a 1K Ohm resistance and the relay change state.
The diode 1N4148 parallel to the relay is for protection.
The resistors R5 and R6 is connected to pin 10 and 21 and is used to simulate that a printer is connected to the parallel port.
Also this electronic circuit is powered by a regulated power supply LM7806 (6Volt output).
If you like to expand this circuit to 8 switch controler you can add 4 more BS250 or built the NPN transistor circuit below.
If you use a 7806 regulator with 8 relays you must add a big enough heatsink or use an external power supply.
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A schematic of the switch is shown below.
Notice that in this case parallel port supply more current to the circuit.
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You must built 8 of those to control 8 different relays.
Also notice that only Parallel Port Control Program v2.0 can handle 8 different relays.
If you use a 7806 regulator with 8 relays you must add a big enough heatsink.
Use Parallel Port Control Program (ppcp) v2.1 to control parallel port.
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ppcp v.2.1 source.zip- (55.11 KiB) Downloaded 567 times
