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Mobile Phone Sensor-900mhz RF Indicator Circuit

2014-11-01 18:09  
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Mobile Phone Sensor-900mhz RF Indicator Circuit
When the mobile phone is in the standby mode and receives the calling signal from the base station, it would transmitter the answer signal. So that the mobile phone's antenna would also have a short time while the RF signal is sending. At this time, is there is a micro sensor beside the mobile phone, it would flash the red light. Some other kinds of sensors could also have the music. By the analysis of the sensor circuit, it is found that it is practically a 900MHz RF indicator. Here the analysis of its working principle is introduced.The 900MHz RF indicator's principle circuit is shown in the picture. The RF signal is received by the spring steel wire L1 which is used as the antenna (w) (L2 is a current-limiting coil with only one cycle). The signal then is sended to VT1 for amplification through C1. The VT1 is in the critical conduction mode at the functions of bias resistor R1 and R2. When no signal the VT1's collector polar is in high voltage level to make the PNP tube VT2 closed. So the multivibrator would not work which is made up of VT3 and VT4.