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2017-01-08 22:03  
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The system is formed of two separate circuits-a key and a keyhole. The key engages the keyhole by means of a mating pair of connectors. The key is a tone-generator circuit consisting of a 4049 hex inverter CMOS IC (U1), switches (S1 and S2), a resistor (R1), and a capacitor (C1). The value of the tone generated by that circuit, in hertz, is determined by 1/(1.4XR1C1) The keyhole is a 567 tone-decoder circuit that can be configured to detect any frequency from 0.01 Hz to 500 kHz. The frequency it detects (fo)is set by resistor R2 and capacitor CZ, according to fo=1.1/(R2C2). When the key is inserted into the lock and switch S1 is pressed, a tone is supplied to the input of the keyhole circuit. If the tone frequency is close enough to fo, the 567IC turns on the relay (K1), which should be connected to the electronic locking device. Components R3 and D1 are used to latch the circuit, so that the output stays on even after the input tone is removed. When S2 is pressed, the system is reset. Switch S3 resets the circuit from inside. Note: The accuracy with which the keyhole circuit detects fo can be set by changing the values of three components. First, R2 should be between 2000 and 20,000 Ω . Second, the value in microfarads of capacitor C4 should be n/fo, where n is a value between 1300 and 0000 (which gives a detection accuracy of up to 14 percent of fo). Finally, capacitor C3 should have about twice the capacitance of capacitor C4. Battery B1 supplies both circuits.