We found this circuit in an old documentation from the 70s, but it can be assembled with more modern components. The transistors can be the BC558 and the SCR the TIC106. We suggest using a relay instead of an alarm and supplying the circuit with 12 V. The LDR can be common and the adjustment trimpot R1 can be 220 k. The detector accuses when the passage of smoke between a lamp and the LDR reduces the light intensity. D1 can be a 1N4148 diode or equivalent.
