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Im a beginnerish will electronic and im looking for a switch that is only ON if the reference voltage is matched -+ 0.25 volts. So if i have a reference of 2.7v to match and i feed the switch a 2.75v the switch would be ON. But if i feed it 2.5v or 2.85v the switch would is OFF.

This might be a simple google search, but i am lacking the correct terminology.

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    \$\begingroup\$ The term you're looking for is a window comparator (also a few slight variants such as Window detector) \$\endgroup\$ – helloworld922 Apr 29 '16 at 5:49
  • \$\begingroup\$ Awesome thanks @helloworld922 , I'm taking look. I would of never figured that one out. \$\endgroup\$ – colyton Apr 29 '16 at 5:51
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    \$\begingroup\$ You could also use two regular comparators and an AND gate, if you already have those. \$\endgroup\$ – Dmitry Grigoryev Apr 29 '16 at 6:28
  • \$\begingroup\$ Is the reference fixed or should it be an input of the circuit ? \$\endgroup\$ – dim Apr 29 '16 at 12:17
  • \$\begingroup\$ Hey @dim , the reference should be fixed. The reference voltage will never change \$\endgroup\$ – colyton May 1 '16 at 23:04
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Indeed, the window comparator is what you need. It requires two opamps. You don't need a "real" AND gate, however, if you use open collector comparators. In this case, you can wire the output of both comparators together.

The reference can be a simple resistor divider with three resistors appropriately sized, which gives both the 2.7V+0.25V and the 2.7V-0.25V. The references can be derived from a supply rail if you don't need it to be ultra high accuracy. Otherwise, you'll need a precision voltage reference (e.g. TL431).

Here is the circuit:

window comparator

The input is V1, and the circuit to supply when the conditions are met is represented by Rload. It can be a led, a relay (don't forget the protection diode), anything...

Note: The supply rail I used is 5V (I assumed that were your supply), and I used LM393 (because I had this model at hand), but this is not actually a good combination. The common-mode range of LM393 is Vcc-2V, which makes it too close from your highest reference point. Use this chip only if your supply voltage is higher. Otherwise, use an IC with a larger common-mode input range (there are cheap dual rail-to-rail input comparators with open collector output available from many manufacturers).

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