This circuit comes from U.S. Patent 3, 778,800, granted to BRK Electronics in Aurora, IL. The circuit provides a smoke detector with an alarm for both smoke and low batteries. The R6/R7 voltage divider monitors the battery and will turn Q2 and Ql off when the battery voltage falls too low. The smoke-detector chamber will also cut Ql off when it senses smoke.
Ql via Q3, triggers SCR Dl and sounds the alarm. Capacitor Cl provides feedback that causes the alarm to sound intermittently. The smoke detector and low-battery circuits sound the alarm at two different rates.