I’ve got some additional matching switches for my van, however only one is latching, the rest are push-to-make momentary switches. (as are all the standard ones like the hazards) At £18 each I would like to avoid buying latching ones. Is there a simple way to get the switch to make a circuit stay on? I thought I’d be able to get a specific relay or similar but Googling is bringing up home built circuits with caps, resistors, mosfets etc.

I could probably work it out (eventually) and create the circuit using breadboard, but space is limited behind the dash and I also need to get some relays and additional wiring in there and it’ll turn into a spiders nest!

Use a relay, non latching switch to energise coil and then use a 12V feed through one of the contacts to feed the coil as well. This will then stay latched until the 12 V is removed.

Though presumably he’d want to be able to switch it off again by pressing the switch rather than turning off the ignition? Please clarify…

Thanks Ming, just stumbled across that method, ‘electrically latched’ It could work, I would just need a discrete push to break on the negative side which I could install on a switch blank.

It would be great to make the same switch turn off again, but I can’t see an easy method to do that!

I’ll have a think – in theory a bistable circuit would work. In practice switch bounce will cause you problems with that. Which is what’s going to cause you problems with any simple circuit (and why you tend to do things by using the right sort of switch!)

You can buy latching relays.

one pulse to turn relay on (close contacts)
one pulse to turn relay off (open contacts)

They use an mechanical over centre latching mech, much like that in a ball point pen, and only use power when changing state. Google for ‘latching relay’

How many contacts does the momentary switch have and what type are they?

So I’d discounted latching relays as the ones I looked at (magnetic and mechanical) need two momentary switches, one to switch one way, and another to switch back. But just had another look after Maxtorques comment and found there is another type, impulse relay, which looks like it is designed to work off one switch, that sounds perfect.

All the switches have four contacts, including the latching one which has an ‘on’ indicator light. Pretty sure they are all backlit as well. So I’d guess the latching one uses all four to enable the backlight and the ‘on’ lamp, and the others probably have an unused pin.

What type? errr…four pins in a vertical arrangement, pretty sure there are loads of spare plugs on the wiring loom that I can chop out and reuse…the switch blanks are used for things like raising and lowering air suspension, heated rear screen on minibuses, heated seats etc.

Van is going in for repairs next week (three days before the warranty expires!) It has an earthing issue I think which is affecting the instrument warning lamps. So I won’t be getting stuck in until its fixed.

Aracer…the facia included two worklight switches which I thought were both latching, I also naively assumed the front fog light switch was latching and could have checked the one I already have fitted. When it turned up, it turned out that they had fitted two types of worklamp switch and only one is latching! So in retrospect, I would have spent £18-20 on two or three switches rather than £30 on six!

Switch -> xor gate -> relay or transistor. Feedback from gaye output back into second gate input. May need some denounce in there two in the form of some capacitors.

“may”? As I mentioned above debounce is going to be the fundamental issue here – also likely to be an issue with the impulse relay mentioned (you need to check how quickly it will respond to a second pulse). The trouble with that is it’s the sort of issue which causes you intermittent problems – it might all work fine when you’re testing with the van on the drive, but not when you’re driving or parked up on a slope.

I’ve had a think, and there’s not really any way around it – you’ll need to debounce in some way, whether using capacitors or a specific debounce chip (you can buy such things). It all depends how much electronics you want to do, or whether it is just easier to buy the right switches.

Ok will.