The smoke alarm featured here uses the optical technique to detect the presence of smoke. With this technique, the use of an ionization chamber with radioactive elements is avoided.
To make the entire circuit simple to construct, a single IC LM1801 is used. Take note that the circuit is directly connected to the power line, so be careful.
The actual smoke sensor is the combination of LED D2 and the two LDRs. The LED shines on the two LDRs. When a smoke appears between the LED and LDRs,
the balance of the bridge circuit inside the IC changes. The IC then triggers the triac into conduction which in turn activates any alarm device connected to it.
The sensitivity can be adjusted through P1.
The LDRs and the LED must be constructed inside a lightproof box. They must be installed in such a way that the smoke can pass between them.
Electronic Circuits volume 1.0 - Circuit Nr. 31
The complete data of the electronic circuit described above can be found in the following book and is available from Amazon.com.
Click on the image to view the book.