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I want to build a low-voltage detection circuit, and while I found many online, I think I prefer the 741-based ones as they seem simple enough - i.e. circuits such as these:

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My question is that in both of these circuits, there is an "ambiguous" connection to a resistor (i.e. the 100k in the first one, and the 10k in the second one) - how am I supposed to connect it, as it looks like 3 wires are going into a resistor (1 wire in the middle?)

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The two resistors you mention are potentiometers, not simple resistors.

A potentiometer has three terminals: the two ends of the resistance element, and a moving contact that slides along the resistance element. The resistance between the moving contact and either end contact will vary as the contact is moved. The arrow pointing into the side of the resistor box represents the moving contact of the pot.

A common application of a potentiometer (or "pot") is as a volume control.

In your circuits, the pots are used to set the threshold voltage - the voltage that the circuit compares the input voltage against.

By the way, the 741 is a very old op-amp, and requires + and - power supplies, and can't get its output closer than 3 volts or so to the power supplies. For this application an analog comparator would be much more appropriate.

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  • \$\begingroup\$ Would you happen to have a link to a sample circuit? I searched, but I find a lot of options for using an op-amp as a voltage compactors instead, and only complicated circuits and packages (4 comparators, 14 pins, etc.) at that - yes, the 741 is old, but I don't think any of those are particularly troublesome for me, and there seem to be plenty of drop-in replacements for it (I was thinking of using a LM358 actually, but google brings up more 741 circuits) - and again, it's still more complicated than what I need. \$\endgroup\$ – user2813274 Oct 24 '15 at 19:07
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    \$\begingroup\$ I don't have a circuit in mind, but if you google for "LM311 circuit" you should find lots of suggestions. The LM311 is a single comparator in an 98 pin DIP package that can work on a +5V power supply. the LM339 has four comparators in a 14 pin package, and can operate with a 2V to 36V power supply. \$\endgroup\$ – Peter Bennett Oct 24 '15 at 22:52
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    \$\begingroup\$ Arrrgh! The LM311 is in an 8 pin DIP package. \$\endgroup\$ – Peter Bennett Oct 24 '15 at 23:42
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That's no resistor, it's a potentiometerpotentiometer, or pot for short. It consists of a resistive track with a sliding contact or "wiper" which makes contact at differing points depending on shaft rotation. So it really is a 3-terminal device, and it looks like 2 resistors connected at the wiper. As the wiper is moved, the ratio of the two resistors changes, but the total resistance stays the same.

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