Note – this may seem a deeply detailed post, but I’m writing it up as I can see others having the same irritated search for 12v timers with the types of functionality you find on mains timers, and thought I might give them something on the internet to find 🙂
One of the projects I started last year, but haven’t yet finished, is to establish proper hydroponic vegetable greenhouse growing. We thought it would be good to try a ‘flood and drain’ system, as it is reputed to give the best results. This is a system where the roots are flooded with nutrient containing water for 30 mins or so every few hours, and left drained at other times. The normal way of controlling this is to use a mains timer that allows a pump to be turned on for 15 or 30 mins periodically. When the pump stops the water drains back down through the pump. Unfortunately we don’t have easy access to mains in the greenhouse, and while I could install it, I have got quite caught up with the idea of doing it on 12 volts, and more efficiently than an operating model that runs a pump for 30 mins when the roots are flooded in about 1.
So, I started looking for a 12 volt equivalent of the mains timers that sell for hardly anything at B&Q et al. I reasoned that even if they weren’t available in normal stores, I’d find them in a chandler for boating. Amazingly, I didn’t, so I thought I might try converting a digital mains timer for 12v usage. My logic was that the digital circuit would run from some relatively low voltage (like 3v), so it would be possible to convert. I have started with a Philex 2 channel timer (part number 19528R) that I got from B&Q. Don’t know if they still stock them as I got this last year.
WARNING – IF YOU COPY THIS, DO IT AT YOUR OWN RISK. THE FOLLOWING INVOLVES ACCESS TO LIVE MAINS VOLTAGES. IF YOU AREN’T CONFIDENT THEN DON’T ATTEMPT.
I removed the security screws (designed to stop access, given mains voltages), then examined the innards. There is a timer control board in the top that I’ve left alone, and a power and switching board in the base. I started to map out the circuitry on that base board, but I found I was making mistakes as I continually flipped the board to look at the PCB tracks. So I created an image of the board, with the circuitry from the back superimposed so I could easily see how the circuit worked :-
Anyway, I finally got the circuit diagram out (click link for larger version):-
Annoyingly, it uses 24volts as an intermediate voltage for the relays, meaning that they also need to change to run from 12v. I spend a fair while trying to find plug compatible replacements with 12v coils, but to no avail, so I may just re-create the small amount of relevant circuitry on a new board (most of the circuitry on this board relates to handling the mains – on 12v it’s much simpler).