I posted about my 12 volt timer conversion a couple of weeks ago (see here). It’s now all done, and sitting in the greenhouse running the hydroponics pump very nicely. There were a few minor bumps along the way, so for any fellow travellers, I thought I should note them down. You may need to refer to the circuit diagram on the last post.
Firstly, I couldn’t get any plug compatible relays with 12v coils, so I had to build a daughter board with the relays on. When the old relays, and mains control circuitry was removed from the original power board, there wasn’t much left, as you can see in the photo below (daughter board is the one with the blue relays on).
I was also nervous about the current consumption of the relays, since the best I could find pulled 4-5 times more current than the 24v relays. I went for these YX97F ones from Maplin. However, I had to make a couple of circuit changes to cope. Firstly, I had to put 220 Ohm resistors in parallel with the indicator LEDs, to prevent them blowing on the 50mA relay coil current. Secondly, I had to change the base resistors on the relay drive transistors from 47k to 22k, since the gain of the transistors was less than I had expected, so they were not turned on hard, and were moderating the current below what was required to allow the relays to function. However, with these changes it worked a treat.
The rest of the circuitry worked on 12 volts just fine, as expected, since there was an independent 3.6volt supply for the timer board that was just as happy regulating 12v as 24v.
I did the connections with long flexible leads, with car accessory plugs and sockets (Maplin again – part number BV457B). These work well, but the weight of the leads is larger than the weight of the timer, so making it less stable than I’d have liked. If I did it again I’d look for some sockets to put in the case, not on a lead, though it might have been hard to get it in the case.
Finally, the case needed closing, since there were two outlet and an inlet socket that had left large holes. I used one of the outlet sockets to plug the inlet hole on the back as it was the same size. I used a piece of varnished wood on the front to cover both holes, with new indicator LEDs drilled through them. If I had done it again, I’d have found some plastic of the right size and bonded it in – it would have been a neater job. I did look at using a new case, but it was helpful to have the buttons and display already mounted.
All in all though, it has worked great. A pretty easy project, and still surprising that they can’t be just bought. Per last post though, let me note again that you shouldn’t play with mains powered devices unless you know what you’re doing.