I came into Electronics through Arduino, so pardon me if my knowledge about non-Arduino circuits is somewhat silly. I am preparing a small electronics project for my school with LDRs to detect light levels. With, an Arduino I could easily detect light levels and when they go down a threshold, I turn on a relay.

But, I wanted to make this without an Arduino, so I though of using an LM339 Comparator. I connected pin 3 of IC to Vcc and pin 12 to Ground. Pin 4 has an input lower than the input on pin 5 which receives input from Vcc. Lastly, I had the output 1 from pin 2 connected to an LED.

This circuit is only to test the comparator. If it works I am going to change the reference voltage and in place of the LED place a transistor. Here is a quick circuit Diagram I created:enter image description here

But, the LED does not glow. Even if I switch the inupts, the LED does not glow. I tested this in circuits.io, the online simulator. It works perfect there. But, the problem persists on the breadboard.

How do I get the LED to glow in my circuit on the bench?

UPDATE: I just got the LED glowing with the help of Raj, but how can I connect the base of a transistor in place of the LED.

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    \$\begingroup\$ 1) ALWAYS use a series resistor when using a LED, in case of doubt use 1 k ohm. 2) The LM339 cannot supply current, it can only sink it. So place the LED + resistor between Vcc and the output, swap the inputs (as now the LED will be on when the output is LOW) and the LED will light up. \$\endgroup\$ Jan 31, 2017 at 15:30
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    \$\begingroup\$ You should also be aware of the common-mode input range of the LM339, which is zero to Vcc - 2V. If both inputs are above this range, the output will be low regardless of which is higher \$\endgroup\$
    – user28910
    Jan 31, 2017 at 15:51
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    \$\begingroup\$ It sounds the online simulator isn't very good, to be honest. \$\endgroup\$ Jan 31, 2017 at 20:34

3 Answers 3


LM339 comparator output is open collector mean you can make the led glow by connecting the anode to vcc and cathode to the output pin.

please note:

add a series resistor to the LED or else the LED will make a short to the open collector output transistor of the LM339

simple comparator

  • \$\begingroup\$ Hah, beat me to it! You need a resistor between Vcc and the output. (5 k ohm traditional?) \$\endgroup\$ Jan 31, 2017 at 15:29
  • \$\begingroup\$ Can you also tell me how I can connect the output to base of a NPN transistor ? That's what I plan on doing later. \$\endgroup\$ Feb 1, 2017 at 4:59
  • \$\begingroup\$ I had updated the circuit, the comparator has a internal NPN transistor, with collector connected to output, it can sink current around 10 - 15 ma check the data sheet of your device \$\endgroup\$
    – Raj
    Feb 1, 2017 at 5:17
  • \$\begingroup\$ Thanks Raj, the circuit works perfectly ! Chose this as the correct answer. Thanks ! \$\endgroup\$ Feb 1, 2017 at 7:34
  • \$\begingroup\$ Hey, your updated circuit looks to be malfunctioning. The LED keeps on glowing. Can you tell me whats wrong ? \$\endgroup\$ Feb 4, 2017 at 15:34

The LM339 has open-collector outputs, which means that they can sink current but not source it.

In order to see any changes in the output state, you'll have to connect your LED (WITH a current-limiting resistor!) between the positive supply and the output.

  • \$\begingroup\$ Is there any other comaparator which does not use an open-collector at the output. Like LM397, or LM393 ? \$\endgroup\$ Feb 1, 2017 at 5:03
  • \$\begingroup\$ @Delta_Shadow, Comparators traditionally use open collector style outputs, if you want bipolar drive select a suitable OP-Amp instead. Remember the commonmode input and output limits. \$\endgroup\$
    – KalleMP
    Feb 1, 2017 at 5:56

LM339 has open collector outputs. The load must be against the + supply voltage. LM339 can't push out the +, it only sinks current. You're lucky, because the led needs a series resistor to keep the current safely low.

The following link has several circuits, also some with LM339.



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