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Remote Mains Relay Mk.2

The PC board used for the USB Sensing Power Switch can also be used to build a remote mains relay. This can be used for switching mains power to lights or appliances using a remote switch (or relay) linked by low-voltage wiring. It can also be used for controlling security lights from the relay contacts on a PIR sensor.

By Jim Rowe

HERE ARE many devices with relay outputs that are unable to safely switch mains voltages. Often, the relay contacts will not be rated for 240VAC mains or, even worse, there are exposed tracks on the PC board which were never designed to carry mains voltages and currents.

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Fig.1: the circuit is triggered by closing the contacts of a remote switch. When that happens, transistor Q2 turns on and activates the relay which then switches power through to the GPO. LED1 indicates when the relay is on.

This Remote Mains Relay Mk.2 neatly solves that problem. It can be activated using a switch, relay contacts or even a switching semiconductor such as an open-collector transistor.

Because the remote switch is in the low-voltage section of the circuit, it is completely safe. In addition, the switch leads only carry a couple of milliamps, so light-duty cable can be used to connect the switch.

Fig.1 shows the modified circuit for the Remote Mains Relay. As shown, the ICs, transistors and USB sockets used in the USB Sensing Power Switch are deleted. Instead, jumper JP2 is installed and the +2V supply that appears across power indicator LED2 (red) is fed to a remote switch via connector CON3.

The remote switch is used to turn transistor Q2 on or off, simply by switching its base current on or off (via a 560Ù resistor). When the switch closes, Q2 turns on. This then turns on relay RLY1 which closes its contacts and switches power through to the mains socket (GPO).

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