I don't know how to accomplish this task - please assist..

[PSUViper]

Hi Everyone. I'm sure most of you can answer this question in minutes or less...

I'm trying to use a temporary-pulse ground-control switch (A fog-lamp switch in a car) to activate an ECM which will then send power to a simple relay which in turn sends power to the fog-lamps of the car.

I have NO IDEA where to find this simple of an ECU - something that will read a single, ground current pulse (non-constant) and use it to activate OR DEACTIVATE a separate circuit which will remain constant.

That, or I need an electro-mechanical latching relay that can use the same source-switch to open OR close itself.

I'm stuck - please help.

Jeffrey Knapp
WyoTech Automotive Technical Training School

http://www.JeffreyStephen.com

 

[logbook]

The most obvious solution is to use a magnetically latching relay with two coil; one to open and one to close the relay. Use a double pole relay and use one coil to select which coil is powered from the fog lamp. This will require experimentation. At the moment I see two problems. When you activate the switch, the coil will be energised. As soon as the contact starts to open the coil energisation will disappear. The relay may open fully or it may fall back to its previous state and the buzz, continuously trying to open but failing. This would probably be fixed by using a suitable sized capacitor across the coil, probably in the region of tens of microfarads, depending on the voltage rating of the coil. This brings us to problem number 2. If the relay opens then the contact closes on the other side, the tendency will be to swap states again. This may again make a buzzer, continuously opening and closing whilst the fog switch is closed.

It may work with a capacitor in series with the fog switch and a recharge resistor in the circuit. It is certainly not a guaranteed solution, but it is simple in the sense of not needing any sort of electronics.

The alternative requires electronics, which whilst not complicated, will need all sorts of protection, potting, etc. The circuit would be a B-series CMOS dual D flip flop with D wired to Q. A clock pulse makes it change state. This would need power supply protection using a zener and resistor. A relay driver transistor, and a few R’s and C’ to create a pulse from the switch. Perhaps a dozen cheap components. This is not a big deal for an electronics designer, but may be difficult to get a rugged design for a non-specialist.

 

[felixc]

Why do you need a momentary switch?

 

[PSUViper]

The car comes with a momentary switch. The challenge to myself is to use the stock switch.

I could save myself tons of time (and a few bucks) if I installed a simple on-off switch that supplied ground current directly up into my factory-installed fog-lamp relay.

But it's a challenge - stock switch, on, and off...

 

[RobWard]

You just need an impulse relay (or a ratchet relay, at a pinch)as this changes the contacts' states (closed/open/closed/open...etc)on an electrical impulse (ie from your momentry switch)

Ratchet relay does a similar thing but has more outputs which close in turn, so you'd have to connect alternative ones together)

Hope that helps.

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[VEBill]

Make sure that the solution that you choose does exactly what you want it to do when the whole car is turned off and then turned back on again (like the next morning).

It is likely that you would want it to switch off with the shut down, and default to ~off~ (NOT the last selected state) when you turn the car back on in the morning.

Some obvious solutions, such as magnetically latching relays, might wake up in the ~on~ mode.

There are probably special 'relays' for the automotive market that perform this exact function. If you can find the right info, then it would be available at a junkyard for maybe $5...

 

[VEBill]

Googling: "push on" "push off" circuit

http://www.oldradio.com/current/pushon-pushoff.htm

http://beradio.com/news/radio_pushonpushoff_switch_2/

http://home.maine.rr.com/randylinscott/aug21.htm

http://www.drobinson.com.au/projects/FlipFlop/

Further to previous, many rear window defogger switches in cars do exactly what you want, often with the addition of an automatic timeout. If you were to disable the timeout (a simple mod), you'd be left with the right kind of automotive 'relay'.

 

[VEBill]

A recent EDN magazine 'Design Idea' describes a latching power switch using a momentary-action switch.

http://www.edn.com/article/CA472837.html?spacedesc=designIdeas

Note that the switch is not grounded at either end. Also the power capability is at the low end. Both issues could be dealt with in the obvious manner using very simple relays.

 

[BrianG]

This integrated circuit may help:
visit

http://www.atmel.com/products

and look under "automotive & industrial" section for long-time timer , part number U6032B.
This has the "toggle" function (push-on, push-off) needed to and drive a relay for the fog lamps. It uses a few external components (resistor and capacitor) to de-bounce the switch contact.

The only downside I can see is that, as designed, the timer I.C. expects the switch to have a "hot feed" via a resistor from battery +ve to the toggle input, i.e. not a switch closure to ground, but this could be overcome with the addition of a transistor to level-shift the switch ground signal.