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ULN2803A Application Circuit Diagram

2017-12-22 04:52  
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Notes on interfacing with the parallel port

The output pins of the parallel port are low power devices, typically capable of delivering only a few milliamperes. This may be enough for driving a LED or an optocoupler, but for relays and motors a hardware interface is necessary.

The simplest solution I know of is the following circuit, based on theULN2803Achip:


The interface delivers up to 500 mA of output driving current. Included in the diagram is a 30 V zener diode for inverse current protection. Although an external 9 V power supply is indicated, the circuit works fine with 12 V. Thanks to Miguel Grassi  for the design.

The parallel port's inputs (pins 10, 11, 12, 13, and 15) require TTL logic levels, so any logic-high outputs that connect to the parallel port inputs should be at least 2.4V.