Output

This page is a summary of various output devices and circuits we have used in our home.

Direct Relay

The outputs on the I/O board can drive up 10mA and this is enough for some small reed 5V relays. The downside of this approach is that small relays like this have contacts that can only switch about 1A at 100V ac maximum. This is a nice simply option that works in some situations though. The relay shown here is from Maplin and requires only 10mA at 5V to activate it.

Relay

By using a transistor it is possible to switch more powerful and high current relays, such as those used in automotive applications. This is often the best approach for large 12V lighting loads and such relays can be rated up to 40A at 12V dc. As with all relays, the driving circuit needs to handle back EMF from the relay.

Power Transistor

The I/O board can be connected to a power transistor which is used directly to switch a load. This provides simplicity, reliability and potential to switch high current loads (when a heatsink is used). Power transistors can't be used with to switch ac voltages though.

We use the 2N3055 power transistor for switching alarms, sounders and lighting.

Solid State Relay

We have purchased some solid state relays that can switch mains voltages at up to 40A. These are a simply but, expensive way to control things that require mains voltage. These can be connected directly to the output pins on our I/O board.