simple automotive reversing switch

[DarrenElmslie]

Hello all from a new member.

I'm hoping someone can help me devise a solution to an automotive electronics challenge: I have a sequential gearbox that uses a gear position potentiometer to output gear position but no facility to fit an electromechnical reversing switch. It's fitted to a kitcar that I would like to install some reversing lights to.

The gear potentiometer has a +5v reference supply, an output signal and a ground. Would the following work?

12v supply to a relay
relay output connected to reversing lights
relay ground connected to gear signal wire (in essence, the gear potentiometer wiper)

(IF when the box was in reverse, the gear pot signal was 0v)

I think this would mean the gear pot acted as a switched ground for the relay, effectively operating it when the potentiometer was at 0v (reverse)

Would I need any additional circuitry to safeguard pot/relay when the gear potentiometer was not at 0v (ie, in a gear other than reverse)?

This all relies on reverse being zero volts of course, and what the difference in signal voltage is at other gear positions. If this simple approach isn't sufficient I guess I'd need to start looking at something like a voltage comparator to give me an output when the gear signal matched whatever reverse was determined to be..

Thanks in advance for any help you can give and apologies for any stupidity in the question, my grasp of electronics is pretty rudimentary!

 

[MikeMl]

It would be easier to offer constructive suggestions if you sketch your proposed circuit and post it.

 

[DarrenElmslie]

Will do Mike, thanks for the suggestion.

The more I think about it, the more I think some sort of Op-amp comparator circuit will be the best solution. I'll sketch something up and ask for opinions. Many thanks!

 

[Chippie]

Something like this...

 

[DarrenElmslie]

That's the puppy! Thanks Chippie.

If I wanted to go posh I suppose I could use a couple of OpAmps to provide a VRefHigh and VRefLow which'd give me a reverse detection window within which the lights could operate. This could be worth doing to keep it flexible and allow for any small changes

 

[MikeMl]

...
If I wanted to go posh I suppose I could use a couple of OpAmps to provide a VRefHigh and VRefLow which'd give me a reverse detection window within which the lights could operate. This could be worth doing to keep it flexible and allow for any small changes

Don't you want to pull-in the relay if the sensor pot wiper voltage is less than a few mV? If so, derive the "few mV" with a resistive divider fed from the same regulated 5V that feeds the pot. Power the opamp and relay off the 12V automotive bus (not the 5V ref). Use an opAmp like a LM358 whose common-mode input voltage range includes its own ground. Put a little positive feed back around the opamp to give it some hysteresis to prevent chattering. Use the output of the opamp to drive a NPN transistor through a resistor to pull-in the relay.

 

[DarrenElmslie]

That sounds perfect Mike. I'll get my figures and design the circuit then get back with any questions.

Thanks a million for your help