I talked about driving large loads in Chapter 6, Adding Cameras to Our Security System, but now is probably a good time to expand on this a bit and talk about driving inductive loads such as bells and incandescent lamps. In the previous circuit example, I used the TIP120 Darlington transistor to drive an LED array that was not inductive. With inductive loads, you need to add a bit of diode protection to protect the circuit against spikes generated by the coils within relays and bells as they switch on and off.

Here's the modified circuit for our digital load driver with a 1N4007 rectifier diode for protection:

The digital load driver with diode protection