Office Phone

Step 6 Dialing

Dialing uses the same approach as manipulating the hookswitch: we'll take the existing infrastructure and try to slip some relays in between.

In this case, we want to use the existing number pad. Luckily, on my phone and most that I have seen of a certain age, there are solder terminals that expose the particular behavior of the pad. Placing the leads of your multimeter on the terminals and methodically testing the keys, you will discover the layout- each terminal corresponds to a row or a column. Duplicate this layout with 10 SPST NO relay/FETs, and make the output of the circuit that closes go to each terminal you sussed out. This is (schematically, if not physically) laid out as a grid of relays. Each relay's FET is hooked up to an output pin on the BS2, and when the pin is taken high, the number pad terminals seem to close, and send the correct tone.

I won't belabor this one in great detail- like so much in the world of making, it only really makes sense when you try it yourself.

For a more thorough discussion of standard grid switches, see pp.389-391 of Physical Computing: Row/Column Scanning.

Now we have the hardware to pick up the phone and dial... let's get the visitors' attention.