I am replacing a mechanical "horn" overheat alarm on my boat. I am using a radioshack 76dB Piezo Buzzer. The new buzzer operates on 6-18VDC, carrying 10mA current at 12V. It would work right out of the box, but the ECM on the boat motor is throwing me a curve ball.

The non-alarm situation is 12.8V 20mA
The alarm situation is 12.8V 10A

The old burned-out buzzer needed many amps to start sounding, so this worked great for it. However, the 20mA "always there" current is more than enough to sound the new piezo buzzer.

My question: Is there a way to divert/short the 20mA so that the piezo does not sound.. and when the 10A becomes available, the previous diversion become bypassed?
I'm trying to avoid having to order specialty ICs or transistors. My next step if I can't get any good suggestions from this forum is to just buy a doorbell or similar that won't sound until multiple amps are present.

But, thanks in advance for any help you can provide!

--Vince

 

To clarify, the alarm and non-alarm situation is the voltage/amperage provided by the ECM to the two wires I need to hook up an audible alarm.

 

I may be out in left field here but perhaps a normally closed 12V relay that requires more than 20mA to activate. When the current returns to 10A the relay will open, turning off the horn. In other words, you want to open the circuit path to the piezo buzzer.

 

A resistor connected in parallel with the buzzer should do the job. The resistor needs to be low enough in value so that with only 20mA flowing, the voltage across the buzzer will be low enough so that it remains silent. You might find that this would need to be just a few tens of ohms though, so the resistor would have to have quite a big power rating.

That said, are you sure that a 76dB buzzer is loud enough in the first place? You might be better off with something more powerful, which might use more current in any case.

@ iONic : Preferably, the relay guaranteed minimum drop-out current would need to be under 20mA: typically the drop-out current is less than the pull-in or "operate" current, sometimes by quite a wide margin. If this condition is not met the relay might operate, but never release unless the system is powered off. That might be OK if the power is always going to be cut off if the engine alarm sounds.

 

A resistor connected in parallel with the buzzer should do the job. The resistor needs to be low enough in value so that with only 20mA flowing, the voltage across the buzzer will be low enough so that it remains silent. You might find that this would need to be just a few tens of ohms though, so the resistor would have to have quite a big power rating.

I agree that trying resistors of decreasing ohmage is a good approach to solving this, if you've got the resistors to test with. Or maybe a speaker or light bulb (maybe a useful visual warning?)you could try hooking in parallel. I don't think the "off" current is any problem but the resistor does need to handle whatever wattage the horn was rated to when "on". It may have been an 8Ω impedance but I'd guess a higher value resistor will do the trick.
I have a hard time believing this thing ever really put out a 130W signal - that's an enormous amount of sound.

 

Right, using a relay was my first thought, but the ones I have laying around would still be energized at 20mA.

The resistor solution might be good if my local radioshack had some with the proper rating.

The light bulb besides diverting the "off" current of 20mA, would serve an excellent second purpose of a visual indicator. This is my solution of choice! Thanks very much.

The original buzzer was some kind of mechanical buzzer in a metal box underneath the driver's console. The new piezo buzzer is mounted so the hole in the buzzer emits the sound through a hole in the dash directly in front of the driver... it was loud enough in my tests.

Thanks again. Problem solved.

 

...emits the sound through a hole in the dash directly in front of the driver... it was loud enough in my tests.

I once left a cap on my SeaDoo jetboat's water inlet, so after a while on the water, this ear-splitting alarm went off and scared the crap out of all onboard. It was obviously an alarm, but it so rattled me it took a few seconds to remember how to even shut off the engines. And a bit longer to remember I left that cap on.