Circuits / 555 and op-amps
LM393 comparator (open collector)
The output only pulls down: the LED, fed from the supply, lights while V− > V+
Tap a part to see its voltages and currents. It is a model, not a measurement.
What do you need?
- Potentiometer 10 kΩSearch on Amazon
- Resistor 10 kΩ × 2Search on Amazon
- Integrated circuit LM393Search on Amazon
- Resistor 330 ΩSearch on Amazon
- Red LEDSearch on Amazon
- Jumper wires × 10Search on Amazon
- BreadboardSearch on Amazon
How does an open-collector comparator like the LM393 work?
The LM393 is a comparator with an open-collector output: it can pull its output down to ground, but it cannot push it up. To get a high level you add a pull-up, and here that is R3 (330 Ω) with the LED, fed from the 5 V supply. R1 and R2 set a 2.5 V reference on V+, and the potentiometer feeds V−. With the knob at 30 %, V− is at 3.5 V, above the reference, so the output pulls down and current flows from the supply through R3 and the LED into the chip: about 8.4 mA, and the LED lights. Turn the knob until V− drops below 2.5 V and the output lets go: no path to ground, so the LED goes off. An open collector lets the pull-up go to a different voltage than the chip's own supply.
Values from the Protoboard ISO simulation: a model, not a bench measurement.
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