Components:

• 12 V DC power supply (or 13 V battery)
• LED (5 mm)
• Electrolytic capacitor (100 µF)
• Resistors: 2 × 10 kΩ, 1 × 3.3 kΩ
• Potentiometer (4.7 kΩ)
• TYN612M SCR
• BC547 NPN transistor
• NTC thermistor (10 kΩ)

Tools & Accessories:

• Multimeter (for calibration)
• Wire cutters/strippers
• Hook‑up wire / jumper cables
• Solderless breadboard (or perfboard + soldering iron)

• Place the Thermistor: Insert your NTC into one row.
• Add Q1 (BC547): Place the transistor so its flat face points toward you; note Emitter (E), Base (B), Collector (C).
• Mount the SCR: Position the TYN612M nearby, aligning its pins (Anode, Cathode, Gate) per your schematic.
• Install the Potentiometer (RV1): Put the three leads across three consecutive breadboard rows.

• Base Bias for Q1:
  • Run R1 (10 kΩ) from +12 V rail to Q1’s base row.
  • Run R2 (10 kΩ) from +12 V rail to Q1’s base row.
  • Connect the wiper of RV1 (4.7 kΩ pot) between those same two 10 kΩ resistors and to Q1’s base.

• Thermistor Connection:
  • One lead to Q1’s base row.
  • The other lead to ground rail.

• SCR Gate Resistor: Place the 3.3 kΩ resistor from +12 V rail to the SCR’s gate pin.
• LED & Series Resistor:
  • Connect the SCR’s cathode to ground.
  • From the SCR’s anode, run the LED (with its built‑in resistor or an external ~220 Ω resistor) to +12 V.

• Tie the 100 µF electrolytic capacitor between +12 V and ground as close to the transistor/SCR as possible to filter noise.

1. Attach your 12 V supply (observe polarity).
2. Using a multimeter, adjust RV1 until Q1 just barely conducts (Collector sits near 0 V). This keeps the SCR gate grounded and the LED off at room temperature.

1. Cold Test: The LED should remain off at ambient temperature.
2. Heat Test: Gently warm the thermistor (e.g., with a hair dryer or hot water) and watch for the LED to snap on around ~90 °C.
3. Reset: Cut power to reset the SCR latch and repeat as needed.

• No LED when hot? Verify your gate resistor and SCR pinout; ensure RV1 is not set too high.
• LED always on? RV1 may be set too low—reduce the base bias so Q1 stays firmly ON at room temp.
• Voltage sag: If you used a 9 V battery and the circuit refuses to trigger, switch to 12 V.