Office Phone

Step 4 Pick up the Telephone

Before dialing out, the phone needs to be off the hook. We'll also want to be able to hang up if no one answers on the other end.

This will take some exploration on your part with a multimeter and some patience. You are going to insert a relay into the hook circuit that exists on your phone, and mimic that mechanical switch with a relay. I'll take this opportunity to point out that I think Physical Computings treatment of relays is not as strong as it should be. The illustration shows what I believe the correct schematic is (though in the particular phone I was using, the relay's switches are slightly different).

In the phone I was using had a switch that was thrown when the receiver was picked up. It was configured NC, NO, NO. With a little probing with the multimeter, I was able to figure out which wires were which, and I simply inserted the relay circuit inline with the wires. The important thing to note is that I have no idea what the wires do, or what the switch does when it is thrown. I am simply able to mimic the behavior of the mechanical hookswitch with my relay, while preserving the function of the hookswitch itself. I have no idea if the same configuration is used on every phone, but the basic idea should hold.

Hook the relay circuit up to the BS2 so that taking a pin high throws the relay, and we can pick up the phone with the push of a button. Now we're ready to dial.