Hardware Reference
In-Depth Information
Figure 12-4:
he door lock
circuit diagram
for an electro-
mechanical
keeper.
A Reminder About Relays
Relays are switches that are operated electrically. They have contacts that are moved by
passing an electric current through a coil of wire. Relays are available with different conigu-
rations of contacts. The relays you will use here are changeover, or sometimes called
dou-
ble throw.
This means that with no current lowing through the coil, the centre contact is
connected to one pin on the relay. When current lows the centre contact changes-over and
becomes connected to the other pin.
In line with a test-as-you-go strategy, now is a good time to check whether you have control
of the door lock. Connect up the power supply for the door lock and use the PiFace emulator
(as described in Chapter 9, “Test Your Reactions”) to check the lock holds and releases when
you change output 0.
