Story

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...