I am new to doing electronical things, so I bought some components and I am just trying thing out. Now, I am using a LM 2901-comparator, whichs documents you can find here.

Using a power supply of 3V, I measured the following pins,

Pin 3 (V+): 3.0V
Pin 4 (Input 1-): 1.5V
Pin 5 (Input 1+): 2.2V
Pin 12 (GND): 0.0V

according to the documents you would expect pin 2 to be 3.0 V since Vout = 1 if Vin+ => Vin-, but it is not giving an output at all.

Anyone that can explain why this is not working?


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ What do you mean it's "not giving an output at all"? What is happening? What are you measuring? Is it powered on? Are you sources connected correctly? \$\endgroup\$ – sherrellbc Jun 29 '14 at 20:23
  • \$\begingroup\$ It is giving 0V. I am pretty sure the rest of the pins are wired up correctly. All other pins not mentioned are not connected to anything, could that be a problem? \$\endgroup\$ – Merijn Den Houting Jun 29 '14 at 20:25
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    \$\begingroup\$ Take a 3K - 5K resistor and connect the output to Vcc \$\endgroup\$ – user34920 Jun 29 '14 at 20:26

An LM339 needs a pull-up resistor on the output or you'll just get diddly-squat ZERO out of it.

enter image description here

  • \$\begingroup\$ I've seen the images in the datasheet, but is it ever explicitly written that this resistor is required? \$\endgroup\$ – sherrellbc Jun 29 '14 at 20:33
  • \$\begingroup\$ @sherrellbc yes, page 10 last paragraph \$\endgroup\$ – user34920 Jun 29 '14 at 20:34
  • \$\begingroup\$ @sherrellbc Nearly every comparator I remember of this vintage (if not all?) needed a pull-up - I guess it made it more generally suitable to mixed rail supplies. These days there are plenty that are push-pull - I guess the sea of change hapened at some point somewhere!! \$\endgroup\$ – Andy aka Jun 29 '14 at 20:35
  • \$\begingroup\$ There was probably a use case for tying multiple comparator outputs together in the days before cheap digital processing - e.g. if any single one triggered, sound an alarm, shut off power, etc. The pullup allows this to work. \$\endgroup\$ – Brian Drummond Jun 29 '14 at 23:21
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    \$\begingroup\$ Yes; tying multiple op amp outputs together would be bad for that reason. But this is the likely reason why comparators were designed differently, as open collectors needing for a pullup resistor to +V. It is safe to tie several comparator outputs together as they cannot pull to different voltages. \$\endgroup\$ – Brian Drummond Jun 30 '14 at 8:49

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