Telephone Monitor - Maker Factory ELECTRONICS EXPERIMENTS BOX Manual

2.68 Telephone monitor

Circuit 144: Telephone monitor for analogue telephone systems
Setup information
Install the transistor, the four diodes, the six resistors and the two electrolytic capacitors as shown in
Fig. 2.144. The thirteen jumper wires come next. Check the placement on the board again and only then
connect the battery.
Experiment
Even if everything is wired correctly, nothing will happen at first. But when you connect the circuit to the
analogue telephone network and a call is made, the red LED will go on. This signals that there is call.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.144.
170
Components required:
1 x 2N3904 transis-
tor, 2 x 1N4148 silicon
diodes, 2 x red LED,
1 x 1 kΩ resistor,
3 x 10 kΩ resistors,
2 x 22 kΩ resistors,
2 x 10 μF electro-
lytic capacitors,
1 x breadboard, 1 x 9V
battery with clip con-
nector
Fig. 2.144: Telephone moni-
tor for analogue telephone
systems