Understanding a Thermistor-Based Temperature Alarm Latch

 Understanding a Thermistor-Based Temperature Alarm Latch

 

 

Power block: PWR, +5V, GND
Power comes in through the PWR JST connector onto the +5V and GND rails. Every other block below is referenced to these two rails.

Sensing block: R19 + R21 — the resistor divider
R19 (the NTC thermistor) sits between +5V and the divider junction. R21 (the trimmer pot, wired as a two-terminal rheostat) sits between that same junction and GND. So the divider is: +5V → R19 → divider junction → R21 → GND. As temperature rises, R19's resistance drops, so the divider junction's voltage rises. R21 sets how far it needs to rise before the next stage reacts — that's your trip-point adjustment. D2's anode taps directly into this divider junction.

Trigger block: D2
D2's cathode sits on the latch's output node — the same node shared by Q2's collector, R10, R16, and C3. Normally that node sits low (latch off), so as the divider junction climbs with temperature, once it gets about a diode-drop above the output node, D2 starts conducting and pushes a small current into it. This is the "detect" event — a small trickle of current appearing there the moment the voltage condition is met. D2 is really acting as the voltage-sensitive trigger interface; the transistors that follow provide the actual amplification and memory.

What that trigger current does — through R10 into T2's base
That injected current at the output node has one meaningful path onward: through R10 (4.7K) into T2's base, which it shares with reset switch S2. So the small trigger current nudges T2 slightly into conduction.

Latch block: T2 + Q2 — the regenerative pair
This is the core mechanism:

• T2 (NPN) starts turning on → pulls its collector down.
• T2's collector feeds Q2's base through R15 (2.2K), so as it drops, Q2's base gets pulled down → Q2 (PNP) turns on.
• Q2's emitter is on +5V, so turning on pulls its collector — which is that same output node D2 feeds into — up toward +5V.
• That rise pushes more current through R10 into T2's base than the thermistor alone ever could.

That's the loop: T2 on → Q2 on → output node rises → drives T2 harder. This is regenerative switching — exactly how fast it snaps over depends on the transistor characteristics, capacitances, and resistor values, but the direction is unambiguous: once it starts, it runs away to fully on. Once latched, D2 and the thermistor stop mattering — the output node sits higher than the divider junction, so D2 goes reverse-biased and is out of the picture. The circuit is now holding the trip state purely through this feedback, not through any capacitor.

Supporting resistors and caps on the latch:

• R17 (2.2K) is the pull-up from +5V to T2's collector. With T2 off it holds T2's collector, and therefore Q2's base, high — keeping Q2 off. When T2 turns on, it pulls that node low, pulling Q2's base low and turning Q2 on.
• C4 (33µF, +5V to T2's collector) and C3 (1µF, output node to GND) provide filtering and transient stabilization around the regenerative nodes, making the circuit less sensitive to rapid voltage fluctuations.
• R16 (1K, output node to GND) gives that node a defined path to ground when Q2 is off, and also sets how much current Q2 has to supply once it turns on.

Output block: SG2 + R18, and LED2 + R13/R14
Once latched, the output node sits near +5V. From there:

• R18 (10R) current-limits into the buzzer SG2, whose return goes to GND — this sounds the alarm.
• Separately, T2's emitter node drives the indicator: R13 (330R) feeds LED2's anode, LED2's cathode goes to GND, lighting it up. R14 (10K) sits from that same emitter node to GND in parallel, giving it a defined load/pull-down path too.

Reset block: S2
S2 connects T2's base directly to GND when pressed, forcing T2 off regardless of the latch state. With T2 off, R17 pulls its collector back up, releasing the pull-down on Q2's base, so Q2 turns off too, and the output node falls back low — buzzer and LED both go out. The important caveat: resetting doesn't remove the temperature condition. If the divider junction is still sufficiently above the output node, D2 conducts again the moment you release S2, and the regenerative process starts right back up.
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old youtube video which contain the demo+circuit+pcb files :
https://www.youtube.com/watch?v=_q9TxOG8NQc
fire detector circuit (උෂ්ණත්ව සංවේදී පරිපථය).