need help, possibly a comparitor?

[scampo77]

I need an ultra low tech way of getting an LED to come on between the voltages of 11-14vdc and a seperate LED to come on for everything out of that range.

I also need this same circuit for the range 2.9v-3.1v

I have a box of opamps here of every different size,shape and purpose but im not sure which one to use or if I should buy a new one, or possibly I wouldnt even use a opamp.

Any help would sure go a long way

[ben7]

Re: need help, possibly a comparitor?

What you need to make is a window comparator. A window comparator has two set points, the low point, and the high point. When the input voltage is in between the high point and the low point, the output will be OFF. This is the 'window,' hence the name 'window comparator'. The circuit uses just two op-amps, and two diodes. (also 4 resistors for the high/low point settings, if you aren't using the voltages from some other circuit)

Here is an example:


The schematic shows both of the op-amps in an LM1458 chip being used. You can use mostly any other op-amp chip though. BTW, Op-amps can be used as comparators too!

Just wire the LED - with a current limiting resistor - from the positive supply rail to the output of the window comparator (after the diodes). Be aware that, for this use, you will need to flip the diodes around! Then the LED should come on when the output gets pulled low, which is when the input is in the 'window' voltage range.

The purpose of the diodes are to keep the op-amp outputs from shorting each other.

Now to let that soak into your brain

P.S. Going to give us [some of] the chip numbers from the lot of op-amps you have?

-Ben