voltage sensitve relay

Hey,

Im looking for information on making a circuit  where the relay would either open or close when it hits 13volts normal operating voltage is 14-14.5.

thanks

One way would be to incorporate an analog to digital converter in to the circuit.  The only problem is that you are monitoring your rail voltage, which may make it difficult to get a good reference point. 

If you use a regulated 5vdc power supply and then a 1/3 voltage regulator you should be able to accurately turn a relay on or off at your desired voltage.  Your 5vdc would be your reference voltage, and you would compare it agaist 1/3 of your battery voltage.  When your scaled battery voltage is below 4.333 volts then you can control your output.

There may also be an easier way using a simple NPN tranistor and an extreme amount of resistance.  I've used this method to push the turn on voltage up to 2-3 volts, (using about 1.5 megs of resistance) but I've never tried to go any higher.  I would think there would come a point that the transistor would no longer reliably come on.

I built a circuit like this last night (for auto door locking with no 'true' igntion source).  I used an Atmel Tiny12 microcontroller that has an integrated analog comparator.  With this chip it is possible to set an interal reference of 1.32 volts.  I then built a voltage divider like I mentioned above, using a 100,000 ohm resistor with a 10,000 ohm resistor.  This combination divides the voltage by a factor of 10.  Now, anytime the voltage is over 13.2vdc the auto lock outputs a lock signal, when it drops below 13.2 it outputs an unlock signal. 

It would be possible to tweak the voltage divider resistors to turn the thing on and off at any desired setpoint.

thank you that info is awesome.  would you be able to draw me a diagram. and a list of part numbers. ex. what transistor, and what wattage are the resistors etc?  thanks again.....

Darren

Well, just to let you know, the idea didn't work for me for my application.  I was trying to build an auto door lock/unlock module based off of battery voltage, but the voltage varys way too much (when my power windows are moving my battery voltage dropped below 12vdc a few times). 

The circuit was simple, using the parts listed above.  The resistors were 1/6 watt, and don't really have a load on them.  The complicated part is programming the Atmel Tiny12, as its just a microcontroller until you write code and download it to it.

What exactly are you trying to do?