I've often seen a circuit posted by John DeRosa (Hotwaterwizard - see below) that would appear to do what I need, except I cannot see that it works.
I need a circuit that will enable me to switch headlights on and off with a single button that will provide 12v when pushed. So after vehicle is turned on at the Ignition switch, the first 12v 'pulse' will turn the H/L on and the next push of the same button will turn the lights off etc. and when car is turned off and power to the circuit is removed, it will return to its default state of lights off.
I have studied several circuits, but either the button required is connect to ground, or it has to accept current in either direction (neither of which is possible) or it contains electronic components that I want to avoid in this instance.
Can anyone suggest a circuit using the minimum number of relays that will do this?
I've often seen a circuit posted by John DeRosa (Hotwaterwizard - see below) that would appear to do what I need, except I cannot see that it works. However I look at it, it doesn't do what it purports to do. Maybe I'm just not understanding it, in which case can someone please explain how it does work. Otherwise, any other solutions?
Only relays? Because you could do this with one relay, two diodes, and momentary button.
I just spent 10 minutes drawing out the various stages of operation of that circuit, and I have come to the conclusion that it is rather flaky.
When pressing the push button to get it to toggle on, it looks as though you could end up with the relays buzzing and chattering while the button is pressed.
When you take your finger off the button, what state you are left with is down to chance, it may be latched or it may not.
It should be possible to make a simple circuit to do what you want using relays, but at the moment my stomach says that it is tea time!
JimB
Hold on...it might take some extra trickery to be able to turn the circuit off. I was thinking of a latching pushbutton, rather than a momentary one. But latching wouldn't reset the state on power-off.That's the conclusion I came too. Just not viable as far as I could make out and I've not been able to think of a way to make it actually work.
At work right now so don't think I can put more time into this. See the attached circuit. This circuit only turns off while the normally closed momentary pushbutton is held down so you will have to add in a second relay into the path of the horizontal diode. Probably a normally closed relay so that when turned off but ignition on, battery power keeps the relay open. But when off, it resets to the closed position to allow the circuit to self-turn on.
The horizontal diode is the one that boots the circuit on power-up but at the moment does not allow the circuit to be turned off without holding down the button.
You might be able to get away with just a single relay that has both NC and NO contacts but something like a big cap would have to be added to allow the relay to stay energized long enough for it to switch itself over to disconnect the horizontal diode so it could enter its feed-forward state where that diode is no longer required. That cap might not be feasible though because it might be too large or might not be acceptable for you.
Or maybe a diode across the NO relay (when a separate one is used) might slow the collapse of current just long enough to let the circuit properly establish itself before the horizontal diode current is cut.
Hold on...it might take some extra trickery to be able to turn the circuit off. I was thinking of a latching pushbutton, rather than a momentary one. But latching wouldn't reset the state on power-off.
But how long is this pulse? It's just a pulse? Not sustained? If that's the case, the fact that the pulse doesn't last forever does the work of the extra bit I'm trying to figure out right now. Instead of the horizontal diode connecting to +12V supply, connect it to the pulse source. So it turns on momentarily to switch the relay on and then removes itself from the circuit. Just two diodes, a normally closed pushbutton and one NO relay. Done.My requirement is quite specific. This circuit HAS to work from a 12v positive trigger because that's what is in the vehicle and it's not practically changeable to anything else.
There is miscommunication here. The bolded part...is this from the first button button press after ignition? Or from something else? I took it to mean a pulse from something else (like turning the ignition).I need a circuit that will enable me to switch headlights on and off with a single button that will provide 12v when pushed. So after vehicle is turned on at the Ignition switch, the first 12v 'pulse' will turn the H/L.
Ah okay. So you must use a momentary non-latching normally open pushbutton and all you want is to connect it to a toggling, latching circuit that resets to off state on power down.Ok, not a complicated requirement. Turning the ignition on just powers the vehicle and anything that needs it. I then have a button that for each press needs to toggle the headlights on and then off etc. That button will supply 12v while pressed. It is an existing button and its function cannot be altered. 12v while pressed, that's it. While the headlights are off, the circuit should use no power. So the relays are in their default state.
It seems like a simple problem. A relay circuit that will toggle from a 12v trigger. But I've never been able to work out a solution with 2 relays, even DPDT types. Maybe with 3, but I'd prefer to keep the item count and the size down.
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