Describing operation of the two sensing circuits

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 04/20/2018 at 08:140 Comments

Please excuse the hand drawn schematic. I can draw by hand faster than learning how to operate a new drawing program. I am finding hackaday a challenge without having my data entry disappear and having to re-enter it over and over.

The ON HOOK  sensor has two 14 volt .5 watt zener diodes rather than one 28 volt .5 watt zener diode as I had the two 1N5244 diodes left over from another project, and the combined voltage drop with the forward voltage drop from the opto LED put the operating voltage around 29 to 32 volts. When the phone is hung up (ON HOOK) the phone line is around 48 volts, when OFF HOOK (making a call) the  voltage drops to around 5 to 8 volts DC. So when the phone is ON HOOK , current flows thru the opto LED and the output photo transistor  turns on pulling its pullup to 0 volts. When OFF hook the the voltage of 5 to 8 volts is too low to allow current to flow thru the reverse biased zener diodes. The zener diodes act like a reverse biased diode until the reverse breakdown voltage is reached, then the voltage regulating property of the zener diode starts working and the zener voltage is held unless the zener current is starved when the series resistor value is too big or the load current draw is too much. I am not too concerned as the purpose is to keep the opto isolator current off until the phone line voltage is certain to indicate an ON hook condition.

The combined voltage for the two series zener diodes is 28 volts. When wiring zener diodes in series to produce a combined voltage, they must have the same wattage. 

I must mention that some telephone devices have a maximum ON hook line voltage of 24 Volts DC. The zener voltage would need to be set lower than that possibly to 20 volts.

The AT tiny 85 input pin 7 is programmed as a digital input and sees the 0 volts as an low. 

The bridge rectifier in front of the ON HOOK sensor is there as the sensor is polarity sensitive and phone line polarity may be opposite of what is needed. I forgot to label the connections to the bridge rectifier formed with the 4 diodes. The 2 diodes with their Cathodes connected is +, and the 2 diodes with their Anodes connected is -, all other connections with one Cathode and one Anode are the AC connections.

The current sense transformer is converting the small audio currents into a higher AC voltage which is limited to a maximum positive voltage if it got there of +4.3 volts when the zener diode is reverse biased  (on the positive 1/2 AC cycle), and a maximum - .7 volts when the zener diode is forward biased (on the negative 1/2 AC cycle). The series 1N4002 diode only passes the positive voltage to the .1 ufd capacitor and its bleed / pull down resistor of 220 K ohms. The 1N4002 diode also blocks the capacitor from draining thru the 10 K ohm resistor and the current sense transformer winding.