A simple temperature indicator circuit walkthrough — components, circuit connections, working principle, temperature sensing, LED indication, wiring steps, and an interactive temperature indicator system you can explore in the browser.
Sensor electronics · LM35 + LM3914
The LM35 turns temperature into a voltage that rises by exactly 10mV per degree. The LM3914 turns that single voltage into a five-LED bar. Between them, there's no comparator to design and no threshold math to get wrong — you're just telling the ladder where to start and stop.
Fetching your live temperature…
Status: 3 of 5 LEDs lit — mild
Parts bin
One sensor, one driver IC, five LEDs. The LM3914 does the comparing internally, so there's no comparator IC or logic to wire by hand.
Operating principle
The LM35 does the sensing; the LM3914 does the deciding. Neither one needs to know anything about the other beyond a single wire.
The LM35 is a three-pin sensor that behaves like a tiny, linear voltage source: its output climbs by 10mV for every degree Celsius, with no calibration or lookup table needed on your part. At 22°C it's putting out roughly 220mV; at 40°C, roughly 400mV.
The LM3914 takes that single voltage at its SIG IN pin and compares it, internally, against ten evenly spaced points on a resistor ladder running between its RLO and RHI pins. RV1 and RV2 set where that ladder starts and stops — everything in between is divided automatically, five ways for a five-LED bar.
In bar mode, the chip doesn't just light the single LED nearest the reading — it lights that one and every LED below it, so the display fills like a thermometer rather than flickering between two neighbouring dots.
RV1 and RV2 together are the entire calibration. Whatever voltage sits at RHI becomes "top LED lit"; whatever sits at RLO becomes "first LED lit." There's no other threshold to set anywhere in the circuit.
R1 sets a single reference current that the LM3914 then mirrors, identically, to whichever output pin is active — which is why none of the five LEDs need a resistor of their own.
Swap RHI and RLO's wiring and the bar simply reads backwards: full when cold, empty when hot.
Assembly
Six steps — get the sensor and the two trim pots wired first, then check both reference voltages with a meter before you trust any LED to mean what you think it means.
Power the LM35 from +V and ground, and run its output straight to LM3914's SIG IN pin. There's no amplification or shaping needed — the sensor already outputs a clean 10mV per °C, so 25°C is simply 250mV arriving at that pin. Add C1 across the output to ground if the reading looks noisy on a breadboard.
RV1 sets the voltage at the RHI pin — the reading that should light the final, top LED. If you want the bar to max out at 50°C, RV1 needs to be trimmed until RHI sits at 500mV, matching what the LM35 would output at that temperature.
RV2 does the same job at the other end: it sets the RLO voltage, the reading where the very first LED just switches on. Between LO and HI, the LM3914's internal resistor ladder splits the range into five even steps automatically — you never calculate those steps yourself.
Tie all five LED anodes to +V, and run each cathode to its own output pin on the LM3914. R1, connected once at the RHI pin, sets a constant current that the IC mirrors across every output — so none of the five LEDs need their own current-limiting resistor.
Tie the MODE pin to V+. In this mode every LED at or below the current level stays lit, filling like a thermometer, rather than only the single nearest LED lighting on its own (that's "dot" mode, MODE tied to ground instead).
At room temperature, only the bottom one or two LEDs should be lit. Warm the LM35 with a fingertip and the bar should climb steadily toward red. If it jumps straight to the top, RV1's HI reference is set too low for the range you actually want.
If it misbehaves
Nearly every fault here traces back to RV1 and RV2 — which one is which, and what voltage each is actually sitting at.