Uploaded May 2019 | Updated September 2026, 11 minutes ago

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This is a simple Smoke detector alarm circuit using LED and LDR element.

This circuit uses light scattering.

Materials
1. LDR(Light Dependent Resistor); CdS
2. LED & 300 Ohm Resistor
3. TL 431 Shunt Regulator
4. 5V Buzzer(LED Indicator is optional)
5. 1 Mega ohm Trimpot & 300k resistor
6. 5V constant DC Power supply

- The TL431 can be found in the SMPS circuit. TL431 is conductd when the reference voltage exceeds 2.5V.
- Install a steel mesh to prevent malfunction caused by insects.
- In wet areas, the hard board may be deformed.