Buzzers - Speed Warning, Key Reminder - Theory of Operation?

[davepl]

Unable to find a reasonably priced (ie: under 60-100 bucks) speed warning relay, I bought one for a Camaro. Looking at all the wiring diagrams there are three poles on the relay and only two are used (plus ground).

Here's my dilemma.

If I were to make one from scratch (and I did with a modern relay, but it was almost silent, so no dice) I would do the following:

- Get a relay that has both NC (normally closed) and NO (normally open) pins
- Ground it
- Send 12V to the normally closed pin
- Connect input pin to 12V to buzz it

When you apply power the power goes into and through the NC, activates the magnet, which pulls it away from the NC for a moment, then it falls back under spring pressure, repeating. As a buzz. Easy.

If you wanted to be fancy, you could also wire the NO pin such that it grounds the power pin, thereby cancelling the magnet faster. Didn't try this part, might work better. But that "grounding power" approach leads to the more important problem below.

Problem is, the GM speed buzzer only has ground plus two connections: power and a second ground, uses as the "turn buzzer on" pin. It gets grounded by speed warning head, so when activated, the little lock/spring mechanism grounds it.

How's it possible to buzz with just power and ground, and no "normally closed" pin whatsoever? Heck, only 2 of the available 3 pins are even used. So, to make a buzzer out of it, here's what I did:

- Connected power
- Grounded the relay
- Connected ground trigger
- Send relay output, which is ground, back to power.

How's that work? When the ground trigger is activated, it fires the magnet which closes the relay. I use that relay switch to ground the power input, which is a short circuit, so the relay magnet is no longer energize, so it collapses. Repeat. Buzzer. Works fine. Except...

That's putting quite a load on the power wire, since it's momentary completely grounded into a short circuit. In fact, that cooked the little 24 gauge alligator clip wires I was using to test this all out... melted them.

Granted, the factory wiring is more like 16 gauge, maybe it can handle that without incident.

However, if I have some fundamental misunderstanding of -how- the speed warning circuit is supposed to work, building a buzzer that shorts itself out because I don't have enough pins for my liking would be silly.

FWIW, I'm using a '69 Camaro speed warning buzzer since they are far easier to get. Pinout could be different. I took apart a non-working (broken coil) original truck SW buzzer to look inside, and it's the same deal, a single magnet, single pin, two wire harness.

Thoughts?

[davepl]

50 views and not one bite? I'm a software guy, there must be an electrical guy around somewhere!

[Andy4639]

Well I'm not electrical guy but I do know enough about my truck wiring since I did all of it myself years ago before this board had all the great info on it it has now.

I don't know about the relay you are speaking of but I bought a seat buzzer and the wiring and wired it to the speed warning and it just had a hot from fuse panel to buzzer then the ground went to the speed warning needle. When the speed needle made contact it buzzed! That simple.

[Longhorn 70]

The cool thing about using a relay with the coil acting as a buzzer is it freaks out radar detectors in the area.

If you'll excuse the crude drawing, I believe this is the way they used to do it.

When the speed warn contact connects to the speedo needle, it closes the circuit. This pulls the relay coil down opening the normally closed contact. This disengages the coil which makes the N.C. contact re-energize the coil.

You must make sure you use a HIGH impedance coil on the relay. 1Kohm or better to limit the current through the circuit. Also not shown is a small cap across the contact to keep RFI down.

[davepl]

I can't tell what the impedance is from labelling (factory GM unit). Can I measure a coil trivially with a meter or does magnetism come into play and mess that up? If it were too low, could I add a 1/2W resistor? Seems not big enough to dissipate the heat offhand.

Problem is everyone I've talked assumes a NC contact on the relay. That's not how they seem to be set up. I'll pull the cover off and get a photo of the coil/contacts before we speculate more, sorry. If it does have one I was done three days ago but didn't know it...

[Longhorn 70]

Yeah you can just measure it and be close. Also try to get a relay with a metal cover. They make a better noise than the plastic ones...

Also I recall that Olds and Pontiac used something similar in the early 70's.

Measure the D.C. resistance of the coil. We can work out a resistor value/wattage to give you 9V at the coil and not go poof.

Yes do please post a pic of the relay.

[Big Kev-O]

The camaro buzzer has 3 terminals but only two were used to activate the buzzer. The third terminal was used to keep factory plug orientated so that the wires didnt get reversed. The camaro buzzer has 12+ running to it and is activated when the circuit is grounded by the speedometer when it hits the limit. GM cars that were originally equipped with speed warning buzzers had a speedometer with a special terminal to ground the circuit. You need a way to complete a 12v circuit at a given speed. Does your speedometer have the connector/terminal that provides a ground? If it does you can use the Camaro buzzer or a typical 12v relay in conjunction with any 12v audible device for a speed buzzer.

[Longhorn 70]

This is the one I am used to seeing.

[davepl]

Originally Posted by Big Kev-O

The camaro buzzer has 3 terminals but only two were used to activate the buzzer. The third terminal was used to keep factory plug orientated so that the wires didnt get reversed. The camaro buzzer has 12+ running to it and is activated when the circuit is grounded by the speedometer when it hits the limit. GM cars that were originally equipped with speed warning buzzers had a speedometer with a special terminal to ground the circuit. You need a way to complete a 12v circuit at a given speed. Does your speedometer have the connector/terminal that provides a ground? If it does you can use the Camaro buzzer or a typical 12v relay in conjunction with any 12v audible device for a speed buzzer.

It's got all that of course and that all works. The problem is only the buzzer part of the circuit. There is no normally-closed contact that I can find. So you have to power the buzzer with 12V and then have the trigger from the speedometer ground out that 12V so the coil collapses and the relay 'clicks', and it repeats.

That grounding melted the little test wires, but before I just assume the factory harness can handle that load and I burn down my truck, I need to understand it better.

Maybe a resistor in line? If some electrical genius wants to draw a diagram with a better way, I'd love to see it. I'm trying to find a schematic in another thread, no luck yet.

[davepl]

Turns out mine was not suited for out truck. My approach of shorting itself out actually does work, though. But with an external wire you need something like 2 gauge and a heavy rated resistor.

Which, it turns out, is exactly what's inside the one I finally got that works. There's a heavy rod with a resistor inline that does exactly what my approach above does, but because it's internal and integral they don't have to worry about wire gauge and wires heating up and so on.

Moral of the story: don't spend a week working on a part that you don't know to be functional in the first place!

[jerry moss]

or, you can go to napa and buy a universal buzzer that works just fine,lol. i have to do some digging and see if i still have the box/part number.