IntermediateSensors & Indicators
NTC Temperature-Threshold Switch
Compare an NTC divider with an adjustable reference and light an indicator when temperature crosses the chosen threshold.
Parts from the database
- LM393P × 1Threshold comparator
Other supplies
- breadboardCommon build supply
- regulated 5 V comparator supplyCommon build supply
- regulated 3.3 V sensor/reference supply with shared groundCommon build supply
- 10 kΩ NTCCommon build supply
- 10 kΩ resistorCommon build supply
- 10 kΩ potentiometerCommon build supply
- 4.7 kΩ pull-upCommon build supply
- LED and 330 Ω resistorCommon build supply
- 100 nF bypassCommon build supply
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Connections
- Power LM393 pin 8 from +5 V and pin 4 from ground; bypass. Comparator A uses pin 3 (+), pin 2 (−), pin 1 (open-collector output).
- Connect 10 kΩ from +3.3 V to sensor node and NTC from node to ground; connect node to pin 3. Warming lowers NTC resistance and lowers this node voltage.
- Connect a 10 kΩ potentiometer across +3.3 V/ground with wiper to pin 2. Pull pin 1 up through 4.7 kΩ; LED and 330 Ω in series from +5 V to pin 1. Keep both comparator inputs on the 3.3 V divider/reference rail.
- Tie unused comparator inputs to defined levels.
Build steps
- Measure the divider node at room temperature.
- Turn the reference pot until the LED is just off.
- Warm the NTC gently; with this wiring its node falls and the output sinks to light the LED.
- Repeat while cooling; use a thermistor table for temperature calibration.
Software and firmware setup
This is a hardware-only circuit; no firmware is required.
Test your build
LED changes as the NTC divider crosses the reference; the pot setting is an approximate threshold, not calibrated temperature.
Troubleshooting
- Opposite switching: swap comparator inputs.
- Unstable threshold: shorten leads and bypass the supply.
- No high level at output: restore the open-collector pull-up.
Safety and limitations
Not a safety thermostat or heater/battery cutoff; use certified protection for hazardous temperatures.
Manufacturer and platform documentation
Community Notes for This Project
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