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?