Making a simple and practical telephone quick tester

1. Circuit working principle

　　As shown in the figure below. Tl is the 79 V AC (115 V/36 V) generated by the secondary winding of the transformer and resistor Rl to form a telephone ringing circuit. The 36V voltage generated by the secondary of transformer T is rectified and filtered by VD1~VD4, Cl, R2, and R4 to form an analog telephone line network. LED1~LED4 are detection status indicators, Kl is a single-machine and double-machine conversion switch, and K2 is a ringing and key-talk function conversion switch.

　　2. Component selection

　　Tl is a 5W high-quality finished transformer, the primary voltage is 220V, and the secondary voltages are 36V and 115V respectively. Pay attention to the power of resistors R1, R2, and R4, which should meet the requirements of the circuit. It is best to use different colors for LED1~LED4 to easily distinguish their functions. Others are subject to the diagram.

　　3. Usage

　　(1) Ringing maintenance: Turn K2 to the ringing position. When the telephone is normal, LED3 and LED4 should light up.

　　(2) Inspection of single-machine buttons and call functions. K1 is set to the single position, and K2 is set to the key-to-call position. At this time, LED4 is on. When the phone is normal, LED4 goes out and LED2 is on after picking up the phone. After pressing the "number" key or dialing, if it is pulse mode, LED2 will flash accordingly and the phone will make a corresponding "beep" sound; if it is audio mode, LED2 will be always on and the phone will make a corresponding audio sound. When inspecting calls, the relevant voltage and current can be detected as on the telephone line network.

　　(3) Two-phone call maintenance. K1 is set to the dual position, K2 is set to the key-talk position, and two telephones are connected at the same output end. If the telephones are normal, LED4 will light up. After the handsets are picked up at the same time, LED4 will go out. When the LED is on, the two telephones can talk to each other.
The following figure is the assembly diagram of the printed circuit board.