Finished the 12v time switch

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).

12v timer

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.

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13 Responses to Finished the 12v time switch

  1. Douglas Coates says:

    HI Greg. I too have looked far & wide for a 12v timer with no luck. I tried to convert a bog standard & cheap mains timer but with no success & conclude I have nither the knowledge or the ability. I am the bloke that your warning in red was written for! So is there any chance that you could build a unit for me – and if so how much?
    Regards, Douglas.

  2. Greg says:

    Well, I could, but I haven’t really got enough spare time, so I’m afraid that I can’t. Sorry. But, it really isn’t that hard if you find a hobbyist electronics geek.

  3. Douglas Coates says:

    Ok, thanks. Do understand.

  4. Rick Boylan says:

    I too have been looking for a 12v timer for my greenhouse watering system.

    For the not so technical among us I have found a 7 day timer meant for car use. I have not ordered one because it seems to have only one on and one off per day.

    see it at:

  5. Greg says:

    The timeswitch seems to be working well. The one I used has 12 on-off times a day which has been great for hydroponics. It also has two channels, so I’ve used one for the main hydroponics and another on a second pump for a once a day watering system for other stuff.

  6. Neil says:

    Here are details of how to convert the Tesco TE7 seven-day programmable mains powered digital timer to operate at low voltage DC instead of mains voltage:

    This is by far the easiest timer we have found to convert so far, and Tesco sell them at only £8.00 in most stores.

  7. Pingback: Playing with watering « Greg Pyes blog

  8. Dav Frayne says:

    I’ve been looking for a 12v timer too, to run my off the grid ebb & flow aquaponics system.

    As a kid I remember making simple delay circuits with my 100-in-1 electronics projects kit. It should only require a capacitor, transistor, a few resistors, and a very small cheap relay. I think you might need two of these simple circuits, one to turn it on after 45 minute delay, and one to turn it off after a 15 minute delay, but they can probably be integrated together somehow.

    Anyway, my quick solution is to use a $10 regular 110v timer, and plug it into a $5 car lighter inverter, and voila, 12 volt timer.

  9. Tom McMurtrie says:

    Hi Greg Pyes
    Great article, just the ticket for what I’ve been looking to making a 12Vdc device to feed out grain for birds. I live down-under here in New Zealand and the Mains Digital Timer I have to convert is a HPM product and the circuit layout is similar to the one you noted in your article.

    Can you please help with some more information? Would it be possible if you could please provide a further enhancement to your Mains Digital Timer schematic diagram which includes with the daughter board items along the wiring connections back to the Mains Digital Timer schematic.

    Kind regards Tom

  10. Greg says:

    I probably have it somewhere, but I might not be able to find it until next weekend. It’s still working well by the way, with the battery now fully solar powered.

  11. Tom McMurtrie says:

    Thanks Greg for this response and I look forward to your reply with further circuit information in due course. If it is not too much of a problem I would like to see also the details and schematic diagram/sketch how you incorparated the solar power with the battery.

    Regards Tom

  12. Andy Davies says:

    I wonder whether a cheap water timer could be adapted for this e.g. ELECTRONIC HOSE TIMER GARDEN WATER @ Amazon

    I’ve got a Gardena one where the connection between the timer and the value is a simple phono socket/plug – wire a relay across that and “Bob’s your uncle?”

    Planning to get a greenhouse this year so I may get one and have a play – will report back.


  13. Greg says:

    Sounds a good plan Andy

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