hypothetical electronics question

[k80rum]

If.... I had a gearbox with a gear position potentiometer but no facility to fit a reversing switch, in a car I wanted to use a reversing light, would the following be a solution?

 

12v supply into a relay

relay output connected to reversing light

relay ground connected to gear signal wire (the gear potentiometer wiper)

 

IF (and it's a bit of an if, I realise)

the gear pot had a +5v reference supply and a ground

and 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. if it's not, I suppose I'd need to use a voltage comparator to give me an output when the gear signal matched whatever I predetermined reverse to be..

 

Darren E

 

K80 RUM Website and Emerald maps library

 

Home of the long-term, supercharged Duratec project

[tbird]

Not sure how reliable that would be, depends at what Vdiff the relay would trip at, I suspect it might operate in all states as you would always have a Vdiff of at least 7 volts maybe you would want a diode circuit or similar in there to fix the switching voltage at say 11 volts

 

would it not be easier just to fit a toggle switch

 

 

 

Edited by - tbird on 22 Jun 2009 14:08:38

[jonboylaw]

It will not work...

 

The Pot will still pass current in the other positions leading to the relay closing and lighting the reverse lamp. It would be very indeterminate and would depend upon the current required by the relay to energise the coil and make the switch contact. This means that you will find your reverse lights coming on when you don't expect it and all sorts of other issues such as slow relay switching and potentially burning out or arcing the relay switch contacts due to slow switching.

 

A simpler way is to have a dash mounted toggle switch you can use to light up the reverse lights when you want. I am not sure why you need it to be gearbox mounted.

 

 

 

Jon

 

 

Edit,

Ok re-read the bit about the comparator circuit etc, but it is over complex and not a reliable solution, a toggle is the simplest way.

 

Edited by - jonboylaw on 22 Jun 2009 16:30:40

[k80rum]

Thanks both, I did have my doubts about it

 

Jon, I wonder if a toggle switch be acceptable from an MOT point of view? I'll check up.

 

I'd still be keen on solving it with a circuit (CC have managed it on the new R500s) but if it comes down to it I'd probably favour removing the reversing light altogether. I just know I'd leave/knock a manual switch on at some point..

 

Darren E

 

K80 RUM Website and Emerald maps library

 

Home of the long-term, supercharged Duratec project

 

Edited by - k80rum on 22 Jun 2009 17:00:25

[rj]

You can do it with a comperator (an OP-AMP with a pre-set level) that triggers the relay.

 

Quite simple and easy to make up.

 

 

/regin

[Bricol]

Lever arm micro-switch mounted anywhere in/on the tunnel to contact the gear lever when in reverse, operating the relay . .

 

You can get them with long (6" plus) whisker type arms so it doesn't even have to be close, or particularly accurately positioned.

 

I get away with a toggle on the dash and a warning light on the trials car - the tester seem to ignore the lamp is a rather powerful headlamp . . . must a work lamp rather than a reversing lamp . .

 

Bri

[tbird]

another possibility, a sprung toggle, relay and timed hold on circuit, plenty 555 circuits available on t'interweb

[Mr Locust]

Any configurable I/O on the ECU? Read the voltage into the ECU and drive a spare output based on signal range between a and b.

 

Like a fan relay being driven from an analogue temperature input.

 

Ian

 

Green and Silver Roadsport 😬

[k80rum]

Thanks chaps

 

Bri, thanks for the micro-switch suggestion. It'd be just the ticket for an H-pattern but this box would be sequential so unfortunately it'd be a non-starter ☹️

 

Regin, I'd been doing a bit more digging; the comparator circuit seems like the perfect choice. If I get a measurement from the pot with the box in reverse, then use a variable resistor on the other input of the comparator chip to fine tune a reference to compare against. Finally I can use a relay on the output to switch the lights. It'd be even better if I made a version with Vlow and Vhigh references to create a window against which I check the gear signal. Great

 

Ian - that'd certainly save some time. I think my Emerald is out of spare input/outputs I could use but I'll check. Thanks for the idea.

 

Darren E

 

K80 RUM Website and Emerald maps library

 

Home of the long-term, supercharged Duratec project