Operating 10 relays using LPT-16-O or ISA-24-IO

Connection

With an minimum 11 ribbon cable connecting 10 outputs and 1 ground.

The Board

There are 10 relays DPCO(dual pole change over), which are activated using a 12V supply, and around 60mA current.

Note that the diode D1 is there to protect the rest of the circuitry (the buffer) from the current which flows once the relay is switched off. The current is produced because the relay coil builds a back emf to counter act the magentic field built across it. When the buffer is switched off, the magnetic field will still exist and will need to discharge via the coil . The diode enables the current to flow in a loop without damaging the circuitry.

Buffering

The chips available in the output boards LPT-16-O and ISA8255-24-IO board have to sink that much current. The LPT-16-O has already the ULN2803 which does this job. Therefore a type of buffer must be placed in between the IO card and the relays.

However there are multiple ways to build a buffer.

1. Using a ULN2803 as shown in the LPT-16-0 and below

2. Using discrete transistors

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