Heat Sensor Circuit Guidance

Thought for 2 mins 28 sec

Got it. You’ve shared a “Heat sensor with fan cooling” circuit using a 741 op-amp. Here’s a concise walkthrough and a safer modern tweak if you plan to build it.

What the original circuit does

Reference rail: R5 + Z1 create about 10 V from the 12 V supply to power the LM741. This stabilizes the op-amp supply.
Temperature sensor: A forward‑biased 1N4148 diode acts as the sensor. Its forward voltage drops roughly 2 mV/°C, so as temperature rises, the sensed voltage changes.
Threshold set + hysteresis: VR1 (100 k) sets the trip point; R4 (100 k) feeds back a little of the output to add hysteresis so the fan doesn’t chatter near the threshold.
Output stage: The op-amp drives a PNP BD140 high‑side transistor through R6 (1 k). When hot, the op‑amp output pulls the base low, turning on the transistor and powering the 12 V fan. D2 across the fan is a flyback diode (use a 1N4007) to clamp motor transients. C1 (10 µF/16 V) decouples the rail.

Important build notes

Single‑supply 741 caveats: On single supplies you must bias signals within the op‑amp’s valid input range; do not assume ground is valid. The 741 has limited input common‑mode near either rail and limited output swing. In single‑supply designs, explicitly bias around mid‑rail and check output headroom and input headroom.
Orientation: Observe the sensor diode polarity, the flyback diode polarity (cathode to +12 V), and wire the BD140 as a high‑side switch (emitter to +12 V, collector to fan+, fan− to GND).
Calibration: With the fan off at room temp, adjust VR1 until the fan just turns on at your desired temperature, then back off slightly. Hysteresis from R4 will give a few degrees of separation between on/off.

If you’re going to build it today, consider this improvement

Swap the 741 for a single‑supply‑friendly part: LM358 (op‑amp) or, even better for thresholds, LM393 (comparator). These accept inputs down to ground and have outputs that are easier to interface, reducing edge‑cases around common‑mode and output swing.
Keep the rest the same: sensor 1N4148, flyback 1N4007, PNP BD140, C1 10 µF, VR1 100 k, etc. Or upgrade sensing with a 10k NTC Thermistor for a larger, more linear temperature swing.

Would you like me to recreate this in Flux as a small PCB, using LM358 or LM393, and add proper connectors for a 12 V fan and power input? If yes, tell me:

Keep the 1N4148 sensor or use a 10 k NTC?
Target turn‑on temperature and desired hysteresis (°C)?
Fan current (so I can size the transistor and traces)?

ask for schematic capture

choose modern op-amp

thermistor instead of diode

set temperature thresholds

prepare for PCB design

D_4

D_3

D_2

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S_3

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