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I want an LED to turn off when the input voltage falls below a certain point, e.g., 3.3V. Originally, I was going to use a basic 2N2222 or 2N4401 transistor, however, because they act as "amplifiers" rather than ideal switches, they still allow current through at 2.5V, for example.

Is there a better (as well as inexpensive) solid state switching technology that will cut power sharply at the threshold level?

Warren Young
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Soviero
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2 Answers2

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One way is to use a comparator to compare the input against a 3.3 Volt reference.

The output of the comparator can be used to drive a BJT or MOSFET to provide the requisite current gain - or the LED can be driven directly by the comparator if a suitably high output current comparator is used.

schematic

simulate this circuit – Schematic created using CircuitLab

Note that comparators often have a current sink output, so a pull-up resistor would be needed at the comparator output for this to work... but that is getting into specifics.

Anindo Ghosh
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You need a little positive feedback, which gives hysteresis and "snap action".

Comparators have some hysteresis built in, or Schmitt trigger logic gates.

For inexpensive, it's hard to beat TL431. Here is a circuit from EDN that adds a transistor to the TL431 to give a little hysteresis. This circuit happens to do exactly what you want:

enter image description here

markrages
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    Note that LED action above is opposite of what you want. Easily altered. TL431 is superb for this BUT Vref of 2.5V will limit lowest useful voltage. TLV431 has a 1.25V ref so allows lower voltages. Also has substantially lower minimim in-regulation current - which can be useful (typically about 80 uA versus about 400 uA). – Russell McMahon Sep 03 '13 at 11:17
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    ... - Circuit above would have trouble with green LED at low voltage. Data sheet says Vcathode should not go below Vref but in practice it will self limit about 1 diode drop lower. Even then with 1.25 Vref and Vc = 1.25-~0.5 = 0.75V, LED will see only 2.25V less drop in R6 when Vsupply = 3V. Red LED gives you about 1 V lower. – Russell McMahon Sep 03 '13 at 11:20
  • The LED action is what he wants: "On" above threshold == "Off" below threshold. TLV431 is the right choice if 3V operation is desired. I suppose you could always get more headroom by replacing the LED with another PNP. – markrages Sep 03 '13 at 14:57
  • *Some* comparators have hysteresis built in, not all. – Warren Young Sep 03 '13 at 15:07
  • @WarrenYoung, can you link one without? Isn't that just a decompensated op-amp? – markrages Sep 03 '13 at 15:18
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    Ye olde LM311. See item 6 on page 12 of [this datasheet](http://www.ti.com/lit/ds/symlink/lm311-n.pdf). It wouldn't be necessary to add the resistor if the LM311 had built-in hysteresis. [The Wikipedia article](http://en.wikipedia.org/wiki/Comparator#Hysteresis) also talks about this. – Warren Young Sep 03 '13 at 15:22
  • @markrages - Agh - yes, LED action is what he wants (if you can get it to run :-) ). I can't imagine what I was thinking. – Russell McMahon Sep 04 '13 at 11:23