WE-139 Monitor Circuit electronic kits.mov

It often connects in parallel when several telephones share one couple line. If users want to prevent interception when they are using the phone, then this is the best choice to use this circuit, which is connected in parallel on telephone, will not interference normal usage. This circuit has two NPN and a PNP transistor. The voltage on telephone line is 40 ~ 50V. If a user picks up the phone, it just likes connecting a 200ohm loading in series. Thus, voltage drops to 9V, Q1 (NPN transistor) turns off and Q2 (NPN transistor) turns on. Since Q3 (PNP transistor) also turns on, only LED2 lights up. It means that it is normal conversation. Current flow is around 2 ~ 3mA. If another user picks up the phone, voltage drops to 5V and Q3 turns off. LED1 and LED2 both light up, showing that someone intercepts the phone call. Adjusting variable resistor R1 can make this circuit works properly. electronic kits