Electronic – Help understanding this Circuit

Ignore pin 4, that's the normally closed contact. It's that simple. Note that the diagram is looking at the bottom of the part, not from the top through it.

1 and 3 are the switch that closes when the coil is energized, it doesn't matter which is which, and they go to the keyboard.

2 and 5 are the coil.. again it doesn't matter which is which.

Test it out before hooking it up to the keyboard- you should hear the relay clacking as it operates with the "tumbler switch".

When you do hook it up, keep the connections neat, shorts kill parts.

All this hanging contacts around the relay coil can and does work .The red and green start and stop buttons seen in machine shops are older than me .There is another way that uses the fact that the pull in voltage is always much greater than the dropout voltage .Get your relay and a DC power source thats the same as the coil volts .The relay coil will off course pull in reliably.Now place a series resister thats the same as the coil resistance in your relay coil power supply circuit.The coil now only gets 6V and wont pull in .If you momentarily short the resister the coil will pull in .If you momentarily remove the power the relay will drop out .So you can make a normal relay have a bistable latch function .The time to do this rather than the orthodox approach is when you need the contacts for other things and you are not worried about power.I have tried this with small DC relays but have not done this on AC relays.