Switch light on/off using a transistor based on input voltage range?

[enuf4u]

Hello,

I am a ME with very basic understanding of electronics. I am trying to design a simple circuit for use with one wire alternators to allow you to use the idiot light (indicates the system is not charging correctly).

My concept is that the normal voltage of the system should be between 12.5 and 14.5 volts (or some similar range). Can a transistor be used in conjunction with some resistors and diodes to switch on and off a light that would indicate the system voltage is within or outside the given voltage range?

If this is possible, I need some help with sizing the components and general circuit layout.

Thanks in advance for any help you may can give.

 

[benta]

Try a search for "window comparator".

Cheers,

Benta.

 

[powerjunx]

also try search "practical electronics" (e-book)


"..the more, the merrier" Genghis Khan

 

[OperaHouse]

If memory serves me, look up LM431 amplified zener. This 3 terminal device with 2 resistors will drive a small lamp of about .1A or a relay. When voltage goes above a threshold, the light or relay comes on. an added transistor can invert this. Just follow application notes.

 

[biff44]

you could easily make a circuit with one transistor, one zener diode, and 2 resistors that would sense if the voltage was too low.

But to measure both too low and too high, you would probably need an op amp window comparator circuit.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[potteryshard]

I have to ask... Why use a one-wire alternator, and then attempt to re-engineer the charge light drive? A regular two-wire alternator already has the necessary circuitry to drive the light, and because it uses the current through the light to tell it when the car's ignition is on, it can charge at much lower RPM than a one-wire alternator.

 

[enuf4u]

Well, like I said, I am a ME, but none the less an engineer. Engineers like to conquer the unknown and right now I just want to prove that this can be done. The tractor I bought has already been retrofitted with a one wire alternator and a separate wire was run to the battery for charging and the existing wiring was left in tact. I could change back to the three wire alternator with relative ease, but what fun would that be and what could I learn by doing that? In what little spare time I have, I have been playing with some electro/mechanical automation devices such as relays and mechanical switches. I have built a couple boxes that use mechanical thermal switches to switch a relay that turns a 110 plug on and off depending on temperature. I put the box in my water pump house or camper. When the temp goes below freezing, the thermal switch (placed outside the building or camper to read outside ambient temperature) energizes the relay, turning power on to the plug, thus turning a 110 heater on that keeps my water pumps and lines from freezing. If it is not below freezing outside, the heater has no need to run, thus saving my electric bill. At the end of the day, one of my hobbies has been playing with automation devices. I have just begun to get into IC's with timers, counters, and now op amps and zener shunts. I have looked on google for a solution to the one wire alternator idiot light and did not find anything solid and got to thinking about using a big diode on the line coming from the one wire alternator so I did not get feedback from the battery and then using a transistor as a switch using current from the alternator as the input signal. I then got to thinking that would only tell me a charging, non charging state. I got to thinking it could be charging but at a low voltage and not charge the battery or it could be overcharging and fry the batter. When I seen the window comparator, I thought yeah, that will work! My last hurdle to overcome is figuring out how to get the high reference voltage of say 15 volts out of a 12 volt battery and charging system that runs as say 14 volts? It is easy enough to use resistors to get it down to 12 volts for the low reference voltage... I have ordered the zener shunts and dual op amps from eBay and am waiting for them to arrive. Maybe I can work on this during the holidays!

Thanks for everyone's ideas and help!

 

[benta]

There's no need for a higher reference voltage, just scale down the input voltage to 6.25...7.25 v, for instance (easily done with a voltage diveder). You can use any voltage you like, what's important is the trigger points.

Cheers,

Benta

 

[LionelHutz]

Try a MAX6458 or a similar non surface-mount version if they have such a thing.