As shown in the figure, when the electronic clock reaches the alarm time, the alarm point contact S in the original movement is closed (the contact is closed for about 2 minutes), VT5 has the base current and Ib>Ics/β, VT5 is saturated. The circuit starts to work. The 5-pin output of IC1 (1/2 556) is low level (normal), the relay K1 is closed, the K1-1 and K1-2 contacts are closed, and the IC of the movement is the music circuit of the alarm. jobs. The sound of the music signal is transmitted through the VT6, and the amplification of the VT7 pushes the speaker to make a loud noise. Reflective infrared photodetector is composed of VD3 and VT1, and 1/2 556, R9, R10 and C7 form an oscillator with a frequency of 38KhZ. The 9-pin of the 556 outputs a square wave of 38kHZ, which causes the VD3 to emit infrared rays modulated by 38kHz. When the hand is close to the photodetector, the infrared light reflected from the surface of the hand is amplified by VT1. After the frequency selection by VT2, the Uc of VT3 rises, VT4 changes from off to saturation, and a low level is output, triggered by 1/1. 2 556, R11, C8 monostable circuit, so that IC1's 5 pin output high level (transient state), relay K1 is released, contacts K1-1, K1-2 are disconnected, BL stops ringing. The transient state of this monotonic state is about 70 seconds, that is, one minute after the wave is stopped, it will ring again, and then wave it again to stop until K1 is disconnected, and the circuit stops working. After K1 is disconnected, Ib of VT5 is zero, and the current flowing through c and e, that is, the current of the whole circuit is only Iceo. Since VT5 is a silicon tube, its Iceo is extremely small, so the circuit does not consume power at ordinary times.
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