Looking for (electrical) relay

[bigegg]

I have a project where I want to turn on a circuit at the first application of a 12v signal, but which will only turn off if it receives a signal on a separate line.

So a latching bistable relay won't work.

Basically, I want to turn on a raspberry pi with the car ignition, but keep it turned on even if the ignition is turned off: or turned off+on repeatedly.

The car doesn't have a "delayed accessory feed", and I don't just want to use a time delay relay.

Any ideas?

 

[Ubique]

Yep, basically you need three relays, the first one is triggered by the ignition, second one where you use the output becomes self triggering, the third one breaks the - ve line from your extenal signal - I'll draw it out and post later

 

[Ross365]

Isn't this an application where some form of solid-state switch would do well (e.g. a transistor or thyristor) and maybe you could keep it switched on by having a small battery of some kind to keep the switch biased on even when the ignition is off. I'm not sufficiently knowledgeable to be more specific, but as you're not switching much current, I'm sure this not hard to do.

 

[bigegg]

Isn't this an application where some form of solid-state switch would do well (e.g. a transistor or thyristor) and maybe you could keep it switched on by having a small battery of some kind to keep the switch biased on even when the ignition is off. I'm not sufficiently knowledgeable to be more specific, but as you're not switching much current, I'm sure this not hard to do.

Plan B is to use a USB powerbank to power the pi, and just have it charge up from the cig lighter feed.

Then I could just connect an interrupt via a GPIO pin to the ignition, and have the pi stay in "sleep" mode between journeys.

That would be a software solution, and I'm trying to force myself to learn the "hardware" route.

 

[ChrisBailey]

Big capacitor?
Have the cap being discharged by an resitor.
Then use a comparator to sense the level to switch on or off.
You could use a potentiometer to fiddle with the switch off period.

 

[DSR]

There are lots of ways to do it. What is your "signal on a separate line" ? Is it 12V, 3V logic level, 5V logic level ? Where does it come from ?

If the signal is to come from a switch and we're switching 12V then it can all be done with a relay and a diode, if its logic level etc. you will need something a bit more complicated...

 

[bigegg]

There are lots of ways to do it. What is your "signal on a separate line" ? Is it 12V, 3V logic level, 5V logic level ? Where does it come from ?

If the signal is to come from a switch and we're switching 12V then it can all be done with a relay and a diode, if its logic level etc. you will need something a bit more complicated...

View attachment 309010

"Signal on a separate line" is output from the Pi, so nominally 3.3V, but I'll be running it through optoisolator(s) so could be 5, or 12V - probably 12V

 

[DSR]

Something like this....

 

[DSR]

Thinking about it, you'll probably need the Mk2 version with added LED indication and 1N4001 diode or similar to prevent any residual voltage in the Pi PSU causing it to latch back up when the 'cancel' signal has been sent.

 

[big bbq]

I'm trying to do the same thing and can't see why this wouldn't work, I mean it probably won't work but I can't see why.

 

[Kayos]

Im sure you can buy a premade hat that does this

 

[bigegg]

Im sure you can buy a premade hat that does this

Now *that* would save me some work

Any idea what I should be looking for?

 

[Ubique]

I'm trying to do the same thing and can't see why this wouldn't work, I mean it probably won't work but I can't see why.

View attachment 309017

That's pretty much what I was going to post - however.... you need a relay triggered by the ignition switch before the self- holding relay, otherwise it will hold all the ignition circuit on. It looks fine in isolation but in reality, there are other circuits connected to the output of the ignition switch.

ETA a quick sketch

Sorry for the non-ISO layout!

 

[DSR]

I'm trying to do the same thing and can't see why this wouldn't work, I mean it probably won't work but I can't see why.

View attachment 309017

Almost there - but once it is latched on the ignition will always be powered on with it. You'd need a diode adding between the ignition switch and the relay.

 

[DSR]

Now *that* would save me some work

Any idea what I should be looking for?

Look for a high side switch.

The main problem is the boot time of the Pi. You could quite easily turn the ignition on and off in the time it takes the pi to boot and take control of the hat/high side switch and that doesn't meet your requirement of latching on because the Pi won't have enough time to set up the latch.

 

[bigegg]

Look for a high side switch.

The main problem is the boot time of the Pi. You could quite easily turn the ignition on and off in the time it takes the pi to boot and take control of the hat/high side switch and that doesn't meet your requirement of latching on because the Pi won't have enough time to set up the latch.

Yep, that's been my issue:
When I turn the ignition key, it activates the accessories, but then cuts the power whilst turning the starter motor.

 

[big bbq]

Almost there - but once it is latched on the ignition will always be powered on with it. You'd need a diode adding between the ignition switch and the relay.

That would be it right?

 

[Ubique]

Yep, that's been my issue:
When I turn the ignition key, it activates the accessories, but then cuts the power whilst turning the starter motor.

Don't use the ACC output, use a term. 15 output, that will stay live during cranking.

 

[500e]

Almost sure there is one of those cheap handy boards CPC, RS or Farnell that have a logic switch & jumpers to do what you want £3/4

 

[DSR]

View attachment 309020

That would be it right?

Yep