Over Temperature Indicator using SCR

Imagine you’re in the lab late at night, power tools buzzing and solder fumes in the air. You need a simple, breadboard‑friendly circuit that can scream “Too hot!” the instant your device crosses a danger threshold. That’s exactly where our Over‑Temperature Indicator Using SCR (Silicon Controlled Rectifier) was born. Although this project was a part of exhibition but the use cases for safety or similar purposes are endless. It’s a low-cost, budget friendly circuit that can help protect equipment worth thousands.

TL;DR: A <$5 analog over-temperature alarm that trips at ~90 °C - no microcontroller required.

From Spark to Circuit

As Electronics & Communication majors at BIT Mesra, we’ve seen every simulation tool paint a perfect picture - Falstad’s glowing wires, ideal battery voltages, and transistor currents that never waver. We sketched our design around a humble NTC thermistor, a BC547 transistor and a TYN612M SCR, confident that a 9 V battery and a 4.7 kΩ preset would keep everything in check.

Reality Check

But the first breadboard prototype crumbled. Our trusty 9 V snapped under the gate‑current demand, the SCR refused to latch, and the LED sat dark. Did wire checks, recalculated all values, but nothing worked. Frustration mounted until we realized: simulations assume ideal parts.

In real life, the SCR’s trigger current and the BJT’s gain demanded a beefier 12 V supply. Once we upgraded, the LED flared to life the moment our hotplate crossed 90 °C - and we knew we had a winner.

Objective

The objective of the project is to design and implement a circuit with a focus on achieving the following three key objectives:

1. Energy-Efficient Circuit: Optimize power use during both active and standby states.

2. Temperature Monitoring: Accurately detect and respond to temperature changes using a thermistor.

3. Equipment Protection: Trigger LED alerts to prevent heat damage and extend device lifespan.

Why You’ll Love It

• Zero µC required: No firmware. Just analog parts and pure instant‑on safety.
• Ultra‑low cost: Spend ₹150 ($2) to potentially save ₹10,000+ ($100s) of equipment.
• Breadboard‑friendly: All parts are through‑hole and cheap, so you can rebuild in minutes
• Fully customizable: Tweak the preset for any threshold: batteries, 3D‑printer nozzles, transformers.

About Components

• TYN612M SCR: A silicon-controlled rectifier; a unidirectional device that latches ON when triggered by a gate pulse.
• NTC Thermistor: A temperature-dependent resistor whose resistance decreases as temperature increases.
• BC547 Transistor: An NPN bipolar junction transistor used for amplification or switching applications.
• Preset Resistor (Potentiometer): A variable resistor used to adjust circuit parameters like voltage or current.
• Resistor: A passive component that limits or regulates the flow of electrical current in a circuit.
• Electrolytic Capacitor: A polarized capacitor used for energy storage, filtering, or timing applications.
• LED (Light Emitting Diode): A diode that emits light when forward-biased current flows through it.
• DC Power Supply (12V): Provides a constant direct current voltage to power electronic circuits.

How It Works

1. Power the Circuit:

• Supply 12-13V DC to the circuit.
• The circuit is built as shown in schematic.

2. Initial Setup:

• Adjust the preset (RV1) so that Transistor Q1 (BC547) remains ON when the circuit is first powered.
• Q1 turns ON when its base current (IB) is ≥ 0.5 mA.
• IB is provided through R1, R2 (10 kΩ each), and RV1 (set to around 4.7 kΩ).

3. SCR in Standby:

• When Q1 is ON, it pulls the SCR gate to ground, preventing it from triggering.
• The gate needs at least 1.5 mA (IT) to turn the SCR ON—but with Q1 conducting, this current is bypassed.

4. Temperature Rises:

• As temperature increases, the NTC thermistor's resistance decreases.
• When it drops to around 1.1 kΩ (depending on RV1 setting), base bias to Q1 reduces.

5. Transistor Q1 Turns OFF:

• With insufficient base current, Q1 turns OFF.
• This releases the SCR gate from ground.

6. SCR Gets Triggered:

• Now, current flows through the 3.3 kΩ resistor into the SCR gate, delivering the needed 1.5 mA.
• The SCR triggers and latches ON.

7. LED Lights Up:

• Once the SCR is ON, it conducts from anode to cathode, completing the circuit for the LED.
• The LED turns ON, clearly indicating an over-temperature condition.

Once temperature decreases that is back to room temperature, device shuts down LED turn OFF.

Results

Upon reaching temperatures exceeding 85°C (theoretical) and 90°C (practical), the resistance of the thermistor decreases below 1.1KΩ, causing the BJT (Q1) to Turn OFF and triggering the SCR at a gate current of 1.5 mA. Also, the 1 μF capacitor in the circuit to control the time constant of the circuit with value of 3.3 milliseconds, influencing the system's response time to temperature changes.

The flame temperature that we obtained practically is 90°C, so the percentage change between theoretical and practical values is 5.88% .

We’ve poured countless hours into tweaking resistor values, hunting down stray currents, and perfecting our breadboard layout—so you don’t have to. Now it’s your turn: grab your parts, follow our guide, and never let overheating catch you by surprise again! Let us know if you have any doubts.

Special Thanks to my team partner Samyak Jain and faculty members of BIT Mesra for their support.

Checkout our GitHub Repo for all the calculations: https://github.com/harshitmishra13/over-temperature-scr