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

Direct Relay

5V reed 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.


Automotive high-current 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.

Typically we use a BC337 transistor for applications such as this.


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.

BC337 transistor
This is an output circuit we use a lot. The use of an ILQ74 opto-isolator to drive a BC337 transistor means that pretty much any processor (such as a Raspberry Pi) can be used to provide the very low current to switch on the ILQ74 output.

The ILQ74 has limted current capability but when used with a BC337 transistor like this, it can switch higher voltage loads (up to about 40V dc) and supply currents up to about 800mA.

Medium Power Transistors

Medium power transistor
A medium power NPN transistor such as a BC140 or 2N3053 will enable control of devices that draw up to about 1A.

TIP120 transistor
This is another output circuit we use a lot. This time we are using an ILQ74 opto-isolator to drive a TIP120 power transistor, which can supply currents up to about 5A as long as a suitable heat-sink is used.

High Power Transistors

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

Solid State Relay

Solid state relay
We have purchased some solid state relays that can switch mains voltages at up to 40A. These are a simply but, quite expensive way to control things that require mains voltage.

These can be connected directly to the output pins on our I/O board.

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