Most people hate unsolicited calls, and it’s worse in the dead of night when we’re all trying to sleep. Smartphones are easy to configure to block nuisance calls, but what if you need a solution for your Plain Old Telephone System (POTS)? [Molecular Descriptor] has built a system to invisibly stop landline phones ringing after hours.
The basic principle relies on an analog circuit that detects the AC ringing signal from the phone network, and then switches in an impedance to make the phone company think the phone has been picked up. The circuit is able to operate solely on the voltage from the phone line itself, thanks to the use of the LM2936 – a regulator with an ultra-low quiescent current. It’s important if you’re going to place a load on the phone line that it be as miniscule as possible, otherwise you’ll have phone company technicians snooping around your house in short order wondering what’s going on.
The aforementioned circuitry is just to block the phone line. To enable the system to only work at night, more sophistication was needed. An Arduino Mega was used to program an advanced RTC with two alarm outputs, and then disconnected. The RTC is then connected to a flip-flop which connects the blocking circuit only during the requisite “quiet” hours programmed by the Arduino. The RTC / flip-flop combination is an elegant way of allowing the circuit to remain solely powered by the phone line in use, as they use far less power when properly configured than a full-blown microcontroller.
It’s a cool project, with perhaps the only pitfall being that telecommunications companies aren’t always cool with hackers attaching their latest homebrewed creations to the network. Your mileage may vary. For more old-school telephony goodness, check out this home PBX rig.