Bypass circuit bmw M3

[j0nny]

I have a boring problem that i need to fix. Easiest way i think is to bypass it.( ive already tried several other ways to fix the problem properly)

Excuse the very amateur language

I would like to build a 'circuit' that works as follows.

You have microswitch(the sort that doesnt remember if its on or off, so its the same everytime you press it)

I would like this switch to turn a bulb on or off, so push it once the bulb comes on, push again, the bulb goes off. However I only want this switch to work when another wire has a live going through it ( appalling language i know!!!). If the live through that wire is ever lost, then the bulb has to turn off (assuming it was on, if the bulb was off then losing the live in this wire has no effect). When/if the live returns to that wire, then the bulb must be off. So the bulb always starts by being off.

This is to replicate the rear fog light functionality, where the live feed it to the rear lights, the bulb is the rear fog light bulb and the switch is the fog light switch. So you understand that you can only have the rear fog on if the rear lights are on and if you ever turn off the rear lights then the fog light must turn off, when you turn the rear lights back on again , the fog light must not come back on unless you press the fog light switch again.

Dull problem i know.

Can anyone offer a simple circuit that can do this? it would be very much appreciated! For this problem that has driven me nuts over the last few months.

 

[duke37]

Take a feed from the rear light circuit so that the fog light can only be on when the rear lights are on.

Use a relay which is normally off and have a switch to energise the coil. Use a contact to energise the coil so that the relay stays in when the switch is de-energised. The fog light will stay on and you can use another switch to break the coil current. Press one button to start and another to stop. This system is used on motor control boxes, green button to go, red button to stop.

You do not need the stop button if you are happy to stop the fog lights by turning the rear lights off for an instance.

Check that the rear light circuit can handle the extra load.

 

[j0nny]

This would work, but in order to keep the functionality as is, it would be really good if I can use the one switch to turn the fog on and off. So one press to switch on (assuming rear lights are on) and another press to turn it off. (if I do this I can put that micro switch behind the existing fog light button)

If I don't get this I can just take a feed straight of the rear light's and bang a bog standard lit toggle switch in there. Ugly but effective.

Any ideas on how to make that one microswitch, both turn the fog on and off?

 

[j0nny]

Im thinking something with some latching relays, non latching relays a capacitor and a normal microswitch! Im sure this secondary school stuff...?

 

[duke37]

I went to school too many years ago and my brain cell is discharged.

I am sure that it can be done with two relays but I do not have time to work it out now.

CMOS can be used to make a counter. A 4013 with Q/ to D or 4027 with J and K high will do it. A transistor to drive an output relay would be needed.

 

[j0nny]

Ok new solution. The 'functionality' I need is already displayed by the foglight indicator lamp that is lit on the dashboard cluster(i really didnt want to mess around with this, which was why i didnt suggest before). So there is a tiny led on the dash cluster that goes on and off with the foglight button, i can take a feed off this(it will/may alter the brightness of the dash cluster led but im not bothered) to operate a relay(mosfet?), that can switch on the rear foglight. So i need a relay(mosfet?) that will switch on this really low current(only enough to light a single led on the dash cluster), but the other part of the relay(mostfet) can handle the 21watt rear fog bulb.

I can measure the current drawn by the tiny led this week. Any ideas on which relay or mosfet i might need(do they do the same as a relay??)

thanks.

 

[duke37]

You do not need to measure the led current, voltage measurements will do so you will not need to discaonnect anything. A led is usually fed with a resistance, the voltage 'top side' this resistance may be a better source.

A fat fet will deal with the current you want or a diddy fet to drive a relay.

Let us know the voltages that you have so that a circuit can be suggested. It may be as simple as a fet driving a relay but some voltage shift may be necessary.

If you wish to use a fet to drive the lights directly, then you will need a P channel fet as the lights will be grounded.

 

[j0nny]

here is what i have done, not very pretty eh....

http://www.facebook.com/photo.php?f...698136411.1073741825.732496410&type=3&theater

You can see the circuit here...

Red circle is the rear fog light led
Blue circle is my wire
Purple circle is the front flog light led
Green circle is the common earth(or live)
Yellow is just a little circle of silver that they leave bare, very handy cos i used the same on the other block above to attach my new wire to.

When you said this "the voltage 'top side' this resistance may be a better source"
I think you were saying i should have put my wire before the 1470 resistor??

I will attach another wire to the common earth(or common live im not sure what it is) The bit in the green circle.

How do i now check the voltage? or current? or whatever it is i need to check? simply put the multimeter across these two new wires? and choose Amps on the multimeter or volts?