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.
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
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