Is there a DC voltage sensor that is on/off at a predetermined voltage?

I'd like a sensor/switch that when it senses 17 volts or greater, it turns on a light. Below 17 volts and the light is off.

Does such a thing exist - or will it need to built?

If it needs built, I need a mad-scientist out there to build it for me!

Thanks for any input you can provide.

I want that mad scientist to build me a zapper that will fry peoples' cell phones when they go off in meetings (with or without smoke). Here's a BTT for ya

What you are looking for is a comparator. The comparator will compare an input voltage against a reference voltage and will output either a high or low voltage depending on where the input signal is compared to the reference.

So in your case the reference voltage is 17V, as long as the input voltage is less then 17V the comparator will output 0V and the light will be off. Once the input is slightly greater then 17V the comparator will output a voltage (something >0V, usually whatever it's supply voltage is) and this output voltage could be used to power the light. You might need additional circuity after the comparator to power the light just depends on the design.

Typically this type of gadget is something that an electronics guy has to design up on a printed circuit board.

i want one that will zap the guy in front of me that is texting when the light turns green.BTT

How precise does it need to be?
Is this just turning a light on/off?

I think you might be able to do this with a 14 volt Zener Diode, but it won't be as accurate as the comparator circuit

Example of simple circuit using a Zener diode:
http://www.learningaboutelectronics.com/Articles/Voltage-level-indicator-with-a-zener-diode.php

Thanks Loco - great info! I figured I'd have to find a Radio Shack Forum and hire one of them there guys to build it! Thanks again for your time and knowledge!

I don't reckon none too precise. My 84 Ram is possessed. About every 4 to 6 months, the voltage regulator goes full tilt at 17+ volts. 3 alternators and 8 VR's later, I've accepted the fact that it is what it is and so now I carry a spare VR and just swap it out when it goes. I don't always glance down at the aftermarket gauge set I have so it can be a few days that go by before I notice it. As such, I'd like a little dash mounted LED dummy light that will fire off and let me know we're cooking again! An ACME fix!!

I have seen Zener diodes in electronic voltage regulators do this but not at 17 volts. Ron

It could be done...

You would need a voltage regulator to provide a fixed reference, then the comparator, and a voltage divider to knock your 17v down to something lower.

It could likely be done on a breadboard (cheaper and faster than having a circuit board fabbed).

As for your original problem, you've made sure everything has a very good ground?

Ouch! That's no fun. In that situation I would be tempted to either

- replace every single wire and connector in the charge and regulation circuit, clean every ground to bare metal, add hard wire grounds and buy a new high end (not parts store) regulator

or

- Install a self regulated alternator such as a Powermaster with a stout 6 gauge charge cable to the battery or fender relay.

My ground wires have ground wires..........

So that's a yes...

I can sketch out a circuit later today that would use radioshack components to do this if you're interested.

Use the simple Zener diode circuit. With he comparator you could build in some hysteresis (amount of voltage change around your set point before the output changes), but for what you are doing you don't need it and the circuitry would be much more complex.

It sounds like the regulated ground wire from the alternator field to the regulator may be grounding out on something which puts full field current to the alternator and max alternator output?

If it does it again, do some troubleshooting before changing out parts.
I did have a shorted out voltage regulator on my '84 Ram.

There is also the possibility something is pulling down the engine compartment (ignition) +12 volt power, and the regulator is sensing a lower voltage than the output voltage of the alternator.

Thanks 451. When it goes full field to 17+, I've shook more wires than Elvis Presley shook his leg trying to find one grounding out. Even when its a new VR I've moved wires to try and get it to fail. That's why I say I'm convinced its possessed. I love the truck too much to give up on it so until (IF) it can be found / discovered, I'm just gonna keep on throwing VR in it!

Nice truck. Pic of my '84 RamCharger before I rolled it.

A twin! Sorry to hear about the roll.....

While I spun it 180 degrees and slid backwards into a curb that caught the tire and tipped the truck onto the drivers side.
I guess drifting a 95" wheelbase truck with a 1/2 tank of fuel sitting behind the rear axle, around a corner was a bad idea?
I may have been reparable, but it already had a bent frame. When I was fixing it up, I noticed the passenger side front frame was twisted when I tried to replace the spring bushings.

I have the wiring harness from the RamCharger, but I may have taken the charging wiring to use in a friends '68 W100.