I may be asking for too much here, but I am looking at coming up with two circuits to monitor voltage. The first will monitor a voltage and light an LED if it's from +3 to +12. The second does the same except the range is -1 to -10. It will hopefully be powered by +5v.

 

I may be asking for too much here, but I am looking at coming up with two circuits to monitor voltage. The first will monitor a voltage and light an LED if it's from +3 to +12. The second does the same except the range is -1 to -10. It will hopefully be powered by +5v.

You can use op-amp connected as comparators. search in goole for comparator circuits or on this site :

http://www.allaboutcircuits.com/vol_3/chpt_8/index.html

 

You will have problems with the voltages. In most cases, an op amp or comparitor can only have inputs that are not more positive or more negative than the power rails. You could reduce the voltage magnitudes with a divider, but you will probably need to run on +/- 5 volts volts.

 

I think this is one of those "It depends" kind of problems.

Is the +5v supply for your circuit completely isolated from the other supply voltages? (eg: a battery of some sort) If not, you'll need to use a DC-DC converter/inverter IC to measure the negative voltage, or another negative supply to measure the negative voltages. Or, a double-pole double-throw 5v coil relay or something like a MAX333 analog switch (Vcc to +5v, ground to the - voltage) driven by an astable multivibrator and a couple of non-polarized capacitors to invert the polarity of the negative voltage.

 

If the voltages you're monitoring can supply a little current, you can, in principle, use a scheme like this. The inputs are attenuated and level shifted to get them within the common mode range of the comparators.
You might need to add some pots and a voltage reference if your tolerance requirements are tight.
The LED part number is just something I had a spice model for. You can change it and the current limiting resistors to meet your requirements.

 

Thanks for the feedback and circuits. I will give them a try.