Hi,
I am a mechanic and I’m designing my own latching relay circuit to turn power to my fuel pump on/off with a pulse provided by my remote/immobiliser.
I’m not an electrical engineer and I need some help designing this circuit. I have an Omron 12VDC latching relay but did not realise that you have to change the polarity to flop the relay back the other way.
Is there any electronic component that can automatically change the polarity on the 2nd pulse?
(Attached is a schematic of the 1 pole relay)

 

Welcome to AAC.

Do you want the switching to be automatic, e.g., after a set delay, or can you use a manual switch? The latter is easily done with a DPDT switch. Power to center pair, latch and reset to the outer pairs of contacts.

 

I should have also said that my remote pulses a 200mA ground only.

Also this is not a vehicle modification as the vehicle is from the year 1964! It is rather a security upgrade for my aftermarket keyless entry system.

 

With a pulse like that, you might considers a relay similarly wired. Do you also want a "no power" (e.g., center off) position?

 

Welcome to AAC.

Do you want the switching to be automatic, e.g., after a set delay, or can you use a manual switch? The latter is easily done with a DPDT switch. Power to center pair, latch and reset to the outer pairs of contacts.

I would like it to be automatic so I don’t have to open the boot and press a button. Just using my remote.

 

With a pulse like that, you might considers a relay similarly wired. Do you also want a "no power" (e.g., center off) position?

Yes, there must be a NO position (which I think the latching relay has)

 

Is it too late to get a different latching relay? Here is one I have used successfully. It is a true toggle type - the same polarity coil pulses latch it on - off - on - off ...

ak

 

If you are just pulsing it, how will you know whether the relay is on or off?

 

If you now have two momentary pulses for on & off, you could use two small bridge rectifiers to 'steer' the opposite polarity to the relay.
Max.

 

Is it too late to get a different latching relay? Here is one I have used successfully. It is a true toggle type - the same polarity coil pulses latch it on - off - on - off ...

ak

This is what I need but it’s quite large. I’m only switching 5amps max

 

If you are just pulsing it, how will you know whether the relay is on or off?

I will wire in an LED, plus I will remember what it’s set to when I drive the car.

 

Talking about a latch that controls a latch
Heres my way of doing it:

Assuming your pulse is 12V
This circuit remembers the state and switch it automatically.
How it works:
It centered around a D-Flip Flop, the input pulse triggers the flip flop, the flip flop then trigger a 'one shot' pulse to turn on/off the relay oppose to the previous set state

Edit: Note: LEDs resistors need to be calculated. I forgot to change them

 

I would build in a safety factor in case the button gets pushed and the fuel pump is turned on. If left your battery goes dead. Something along the lines that it has to see an ignition pulse or charging system pulse or it shuts the pump off after x seconds.

 

I would build in a safety factor in case the button gets pushed and the fuel pump is turned on. If left your battery goes dead. Something along the lines that it has to see an ignition pulse or charging system pulse or it shuts the pump off after x seconds.

Fuel pump won’t turn on unless there is oil pressure. I have an oil pressure switch.

 

Surely there is some type o

Talking about a latch that controls a latch
Heres my way of doing it:
View attachment 183643
Assuming your pulse is 12V
This circuit remembers the state and switch it automatically.
How it works:
It centered around a D-Flip Flop, the input pulse triggers the flip flop, the flip flop then trigger a 'one shot' pulse to turn on/off the relay oppose to the previous set state

Edit: Note: LEDs resistors need to be calculated. I forgot to change them

can you describe to me how this circuit functions in other words?

 

If you now have two momentary pulses for on & off, you could use two small bridge rectifiers to 'steer' the opposite polarity to the relay.
Max.

What do you mean by two momentary pulses? I have one momentary pulse for both on/off

 

can you describe to me how this circuit functions in other words?

Its a simple 2 transistors latch on/off. When a button is pressed, current flows into C1, charging it up, when current flows, voltage drop, thus Q1 turns off. At this point Q1 no longer draws current from the base of Q2 and since no current draw, voltage rise therefore, Q2 turns on. Q2 draws current from the base of Q1 thus keeping it off. When the button is pressed again, C1 discharged to the base of Q1, thus turns it on, as it turns on it pulls the base of Q2 low, Q2 turns off. Q1 keeps C1 discharged. C1 is pretty much a memory cell.
The circuit in post #12 replaces the switch with with an N-Mos so that we can apply a pulse to change the state of this circuit. Since Q1 and Q2 can only be in an on or off state, we need some way to generates a pulse when they are on so that we can use that pulse to trigger the relay, thats what the other 2 mosfets are for.

 

View attachment 183852
Its a simple 2 transistors latch on/off. When a button is pressed, current flows into C1, charging it up, when current flows, voltage drop, thus Q1 turns off. At this point Q1 no longer draws current from the base of Q2 and since no current draw, voltage rise therefore, Q2 turns on. Q2 draws current from the base of Q1 thus keeping it off. When the button is pressed again, C1 discharged to the base of Q1, thus turns it on, as it turns on it pulls the base of Q2 low, Q2 turns off. Q1 keeps C1 discharged. C1 is pretty much a memory cell.
The circuit in post #12 replaces the switch with with an N-Mos so that we can apply a pulse to change the state of this circuit. Since Q1 and Q2 can only be in an on or off state, we need some way to generates a pulse when they are on so that we can use that pulse to trigger the relay, thats what the other 2 mosfets are for.

This is brilliant thank you! Now all I need is to make it! I really don’t like my chances, I think I need to find somebody to help me or to do it for me…

 

its not to hard to build on perfboard with through hole components.
I like 2.54mm hole spacing. It should be common enough you can find all the components to fit.
https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=perfboard+2.54mm&_sacat=0

 

If you can live with two SPDT momentary pushbutton switches (one ON, one OFF), this would be a simple implementation. Each switch would supply a momentary pulse with one switch setting the relay and the other switch resetting it. It is basically a pushbutton H-Bridge.