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PATHFINDER

2014-11-08 15:18  
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PATHFINDER
This circuit is little more than a simple two-transistor oscillator (a relaxation type) whose frequency is controlled by a light-dependent resistor (LDR). The oscillator can be built using nearly any NPN and PNP transistor combination desired, and its operation is simple, as its parts count is low. The resistance of R1 and the value of C1 determine the oscillating frequency of the circuit, which varies from a slow tick in darkness (when R1 is about 100,000 Ω) to a high-pitched tone in bright light (when R1 is 1000Ω). The change in frequency is heard through the transducer (BZ1). Power for the circuit is furnished by a single 1.5-V AA battery; because the Pathfinder draws so little power, it will function normally until the battery voltage drops to about 0.9 V.