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555 light control automatic flashing road-sign light circuit

2017-02-02 07:52  
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555 light control automatic flashing road-sign light circuit
As the figure 17-44 shows, the circuit is composed of the transformer rectifier circuit, the light control switch and the 555 astable multivibrator.etc. The VT tube is the 3DU-type phototransistor, when there is the illumination, the port voltage of this transistor is less than 1.6V, the power switching tube TWH8778 cuts off because the control voltage of pin-5 is low, 555 has no power supply voltage. At night, the photodiode resistance increases, the circuit opens, the TWH8778 control port pin-5's voltage is higher than 1.6V, 555 get power to work, the oscillation frequency f=1/1.44(R2+R3)C2.