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Help with circuit to light "sport mode" indicator

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rlcanon

New Member
I have a car with throttle by wire. There is a sport mode in the ECU that provides a quicker throttle map but it was not enabled at the factory. Sport mode is activated by sending a momentary ground to a pin on the ECU. There is also a sport mode indicator light position in the guage cluster but it did not have a bulb initially.

I have added the momentary switch and wiring to activate the sport mode and it works perfectly except for lighting the indicartor.

I recently built the circuit below that I got off of a web forum devoted to this particular car and installed a bulb in the sport mode indicator position. The problem is the circuit lights the indicator the opposite of the actual sport mode status. Sport mode is off by default at start-up, but the indicator is on. When I activate sport mode with the momentary ground the indicator goes off. I've rechecked the components in the circuit and everything looks correct, but something is obviously amiss. Any suggestions? Thanks! BTW, if there is a better circuit design I'm open to that!

The circuit:
]
thecircuit-1.gif



More background than you probably want:
https://www.bmwmotorsports.org/mods/E3678/sportmode.htm
 

Roff

Well-Known Member
This latch will toggle every time you apply the momentary pulse. In other words, the relay will change state every time you push the button. It should initialize with the relay de-energized when you apply power. If the relay is as drawn, it should power up with the light off. If your relay has normally closed instead of normally open contacts, then the opposite will occur.
 

rlcanon

New Member
Ron H said:
...It should initialize with the relay de-energized when you apply power. If the relay is as drawn, it should power up with the light off. If your relay has normally closed instead of normally open contacts, then the opposite will occur.

I double checked the relays over lunch and they are both normally open. Is there anything else you can think of that would cause this?

Thanks! I know diagnosing some moron's circuit sight unseen is all supposition...
 

Roff

Well-Known Member
I simulated this circuit, and it doesn't latch, even with a 0.5 second long momentary pulse. I get good results by changing the 220 ohm resistor to 470k, and the original 470k to 10k. I sim'ed it with pulse widths from 10 milliseconds to 5 seconds, and it always toggled. Let us know what you find out.

Ron
 

rlcanon

New Member
But it didn't come up latched like mine did? I must have something wrong... Planning on redoing it, I had a friend look at and he recommended loosing the relays and driving the lamp off the MOSFET directly. Any thoughts on this:

To start with you could eliminate both relays. The lamp relay contacts are sourcing voltage to the lamp when activated. That tells you that the other side of the lamp is ground or common. You can ditch the relay and connect the source or S connection of the FET to lamp. It's a 4 amp FET for christ sake so unless that little indicator lamp is 50 watts or so you should be fine. The other relay coil is driven directly by the switch so there is no need for it either. Just connect the two terminals of the switch in place of the relay contacts.
 

Roff

Well-Known Member
rlcanon said:
But it didn't come up latched like mine did? I must have something wrong... Planning on redoing it, I had a friend look at and he recommended loosing the relays and driving the lamp off the MOSFET directly. Any thoughts on this:

To start with you could eliminate both relays. The lamp relay contacts are sourcing voltage to the lamp when activated. That tells you that the other side of the lamp is ground or common. You can ditch the relay and connect the source or S connection of the FET to lamp. It's a 4 amp FET for christ sake so unless that little indicator lamp is 50 watts or so you should be fine. The other relay coil is driven directly by the switch so there is no need for it either. Just connect the two terminals of the switch in place of the relay contacts.

No, my sim of the original circuit came up with the light off, and it stayed off. However, if the diode is installed backwards, the circuit toggles, but comes up latched in the wrong state, just like you describe.
Regarding your friend's comments:
If you connect the lamp to the source of the IRF510, you have to tie the drain to +12v. There is no place to get the inverted output you need to provide the latching function. Not only that, you will lose about 3 or 4 volts across your lamp, because the FET is now a source follower, and the source will be lower than the gate by whatever the threshold voltage is at the lamp current. The only way I could see to make this work would be to tie the other end of the lamp to +12v and replace the relay with it. This is generally not possible in an automobile.
Doesn't the other switch also have to supply a pulse to the computer? How does that work, unless the relay has 2 poles? You can't just use a SPST switch, because one end has to be connected to the 1uF cap, and you can't run that voltage to the computer. There are ways to eliminate the relays, but I don't think your friend's suggestions will work.

Ron
 

rlcanon

New Member
OK, now this is odd...

In order to avoid taking my interior apart repeatedly and having to work in the cramped, and stultifyingly hot, confines of the car I connected my existing circuit to a different lamp and momentary switch and used jumpers to power it with my truck battery.

This is the odd part: Some of the time when I power it up with the hot lead to the positive terminal of the battery, the circuit comes up latched and the light is on. Other times it comes up with the light off, like I want it. Either way it comes up at power on, the momentary toggles it consistently. I'm not sure what the ramifications are for using the circuit in the car where power is supplied by the ignition switch... I imagine the ignition switch provides a more consistent power on/power off than jamming a alligator clip onto the positive terminal of the battery... Maybe I could connect the power lead of the circuit to the keyed hot with a spade connector and plug/unplug it until the circuit comes up unlatched then hope the key turning it on and off consistently brings it up unlatched...

Someone on the car forum where I got the circuit is working on an IC controlled circuit to do the same thing... Maybe I should just wait for that to be available. Still, if it's too expensive, a simple, but reliable, circuit I could put together might be preferable...

Someone who seems to enjoy teasing me with cryptic suggestions sent me this, but refuses to elaborate:

"A simple latching relay, capacitor, and diode will do the same job as that perf-board circuit . "

Any suggestions on how to arrange the components mentioned?
 
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