This circuit could be used for blind to sense light. It’s a similar circuit to the police siren.
The two transistors have a current gain of 200 X 200 or 40,000 so even a very tiny Q1 base current – microamps – causes a very large Q2 collector current. So a very small amount of light will cause this circuit to oscillate at a high pitch. Some resistance needs to be in series with the photocell, maybe 1k or more? It helps that the supply voltage is only 1.5 Volts. Still, I think it should have a 22 ohm resistor in series with the speaker. I had this circuit that was running from a 9V battery, and it tore the voice coil wires loose and the speaker went open – cheap radio scrap speaker!
