Automatic switch for a shower halogen downlighter (or other light)


Introduction

One of our showers is in a cubicle that does not receive much daylight and the ceiling light in the room does not light the shower cubicle well either. I could have added a second light in parallel with the existing ceiling light but that would have come on every time irrespective of whether the shower was being used and the ceiling light would be on when you wanted a shower... The shower has an electrical heater so I could put the light on the shower circuit but that would mean wiring a ceiling light onto a 40A domestic power circuit and not the lighting circuit which was something I did not want to do. I could install a second light switch. None of these I found particularly appealing so I eventually came up with this alternative (which is an approach used by many others).


Schematic

The base of Q1 is connected to a short wire that is wrapped around the outside of the power feed cable into the shower after the shower isolation switch. When power is present on this cable (even if the shower unit is still switched off) the AC signal is picked up by the wire sensor and amplified by Q1-Q2. Q3 then drives the relay to apply mains power to the halogen lamp in the shower cubicle (via a 12V transformer). C1 holds the relay in the on position and prevents the relay chattering at 50 Hz.

The power supply is taken from the mains using C3, R3, D3, D2 and C2 to provide about 12V DC for the amplifier and relay.

Works a treat - two wraps of wire around the power cable gives sufficient signal for the relay to engage and turn the light on when the isolator switch is closed.

The mains supply (230VAC) was taken from the lighting circuit using a fused, switched spur box.


PCB

The schematics and PCB layouts were created using Eagle and are available for download here.