Sensor operation described

A project log for Cordless phone RESET when off - hook & no audio

My 2018 Hackaday Open Hardware Project involves resetting a cordless phone when an elderly neighbor does not hang up

Boelens, LelandBoelens, Leland 04/27/2018 at 03:180 Comments

The On Hook Sensor, is a voltage sensor that provides a logic low when the phone line voltage is 29 volts to 48 volts DC when the phone is On Hook ( hung up). The zener voltage(14 x2=28) plus the opto LED forward voltage drop determines the lowest voltage that will be sensed. Once the voltage drops below 29 volts the opto LED is off, and the output of the darlington  photo transistor will be high, indicating an OFF hook condition, On hook is low.

The Audio Detector uses a 115 volt AC to 12 volt AC, 1 Amp step down transformer wired backwards. The 12 volt secondary winding has fewer turns, and has possibly a 22 gauge or 18 gauge wire, and a low DC resistance.  When the 12 volt secondary is wired in series with one of the phone line wires to the cordless phone base, it becomes a current sense transformer. It actually has a DC current flowing in the secondary wire turns, but the fluctuations of this DC current is basically a moving magnetic field cutting across the hundreds of turns of the 115  volt primary, inducing an AC  voltage which is rectified by the 1N4002 diode into + DC, the 10 ufd capacitor stores this + DC voltage, the 220 K ohm resistor provides a discharge path for the capacitor. The formula T=RC was discussed before about the discharge time. The 330 ohm 1/2 watt resistor is to limit the current into the ATtiny 85 input only when the input voltage exceeds +5 volts. If higher than +5 volts is present, the input clamp  diode in the ATtiny 85 will become forward biased and limit the positive input voltage  to +5.7 volts. Originally I expected  a higher max AC voltage from the current sense transformer, and that is why the series resistor and zener diode were in the original design. This last version works very well, using fewer components