Many radio Amateurs, who installed two telephones, is aware of the situation when talking on one phone, and the second, not making sure that the line is free, you start to dial a number that creates a nuisance in the form of clicks (during pulse dialing) or tonal acoustic signals that interfere conversation.
To correct this, in telephones or telephone outlets offer to install one of the following alarms or blocker, allowing at the same time to get rid of eavesdropping on the conversation and "Pozvoni" phone in the time dial the second telephone device (Fig. 3).
The first indicator is included in the gap of the telephone line and ignition on led indicates that the handset is on the unit is removed (Fig. 1).
The second indicator, on the contrary, when the burning led indicates that the line free. It connects in parallel to a telephone line (Fig. 2).
Capacitors C1, C2 (Fig. 3) is chosen such that the frequency of the oscillating the contour formed by the relay coil and the capacitor, does not coincide with the frequency ringing signal from the station (30...60 Hz) and the frequency of the tone (500 Hz), and when tone dialing - even with the frequency of the tone. Better if the frequency of this oscillating circuit is above 10 kHz.
As the relays K1 and K2 will fit any low-power relay with a coil resistance to 200 Ohms and current response of no more than 15 mA.
When receiving a call signal from the PBX, when the lock will be to call both phones. Will turn on the same, which will be the first to pick up the phone.
Author: A. Filipovic, Dzerzhinsk