This is a light alarm circuit. This circuit uses Light dependent resistor (LDR). When there is no light fall on the LDR, the transistor driving the speaker is not turned on, because its resistance is high. When the LDR receives light, its resistance decreases and the collector of the second transistor falls.
The first transistor turn off slightly, via the second 100n. The base of the second transistor receives additional spike from the first 100n. It continues Until the second transistor is turned on. The second transistor cannot be kept turned on because The first 100n is now nearly charged. To produce the second half of the cycle, both transistors swap conditions and The second transistor is turned off.
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