Electronic Timer Conversion

Converting Mains Voltage Electronic Timer to 12volts
January 2011

I picked up a Elto Electronic Timer from Bunnings last week for under $10. I was amazed that they could make it for this price. Electronic timer with a rechargeable battery backup, I opened it up for a look inside.

The bit to the right is the timer function, lcd screen and buttons. Three wires run it, ground, +1.2v and the control wire, which sits at 1.2v when the timer is "on" and 0v when "off".

The left bit has the backup battery, the big blue relay and some resisters, diodes and capacitors to give the DC voltages, which was 1.2volts for the logic and about 50v to drive the relay.

My goal was to convert this to run on 12volts, so I started by desoldering all the bits on the left with the exception of the rechargeable battery and the switching transistor.

To give the 1.2v needed for the panel I decided to use a very low drop voltage regulator (LE33CZ). For some reason the lower output voltage models are no longer available so I used the 3.3v model and reused a couple of the diodes to drop the voltage down. The regulator called for a 2.2uF capacitor in the output leg, so I added one of these in too.