The circuit is a remotely operated DTMF telephone. The circuit can be used to change up to 9 devices using the keys 0 to 9 phone. Digit 0 is used to switch between the phone system remotely and change the way of normal conversation. IC KT3170 (DTMF to BCD decoder) is used to decode DTMF signals transmitted through the telephone line to the corresponding BCD format. IC 74 154 (4 to 16 demultiplexer) and IC CD4023 (dual flip flop) is used to change the device according to the DTMF signal reception.
Circuit operation is as follows. After hearing the dial tone from the phone at the end of the receiver, press 0 on the phone remotely. IC1 decodes this as the pin 11 of IC2 1010.The investment will be low and after the door of NO in the IC3 will be high. This will change the flip flop IC5a and the transistor Q1 is activated. This causes the K1 relay ON. The two contacts C1 and C2 of relay K1 is closed. C1 220 Ohm form a loop through the phone line to disconnect the ringing of the telephone line (this condition is similar to taking the phone off the hook). C2 is connected to a source of 10 KHz audio to the telephone line to inform you that the system is now in the way remote switch. Now if you press a transmitter in the phone, IC1 will decode as 0001 and pin 2 of IC2 is low.
After the investment by NO within IC3 corresponding door, which will be high. This will change IC5b flip flop and turns on transistor Q2. The relay is activated and the device connected through its contacts are switched. Pressing the 1 key will change the status of the device. In the same ways Keys 2-9 on the telephone transmitter can be used to change the status of the device connected to channel O2 O9. After the connection has finished, press the O key on the telephone transmitter to change the flip flop IC5a to deactivate the relay K1. 200 Ohm loop line is disconnected, the source of 10 kHz audio will be removed and the phone is ready to receive new calls.
