Battery-free two-tone doorbell

47. Battery-free dual-tone doorbell. With the increasing popularity of telephones, more and more families have residential telephones. However, the usage rate of most residential telephones is very low. The 48V provided by the telephone feeder to the home is used ( 60V) DC feed as the working energy of the electronic doorbell is economical and practical. Now we introduce a two-tone doorbell circuit that does not require batteries. The circuit principle is shown in the figure. It is easy to see that the circuit in the figure is a variation of the conventional telephone ringing circuit. a and b are the positive and negative ends of the telephone line respectively. AN is a normally open doorbell button. When the phone is waiting, press AN and the 48V (or 60V) voltage provided by the program-controlled switch will charge the capacitor C1 through VD1 and R1. When the voltage Vc at the C1 terminal reaches the IC1 When the control voltage is turned on, IC1 vibrates and sends out a two-tone electronic ring current to make buzzer B sound, informing the owner that a guest is coming. When the phone is in use, the voltage between a and b in the figure is too low to reach the starting control voltage of IC1. At this time, even if the AN doorbell button is pressed, it will not work. This is because the value of R1 is relatively small. Large, much larger than the impedance of a telephone. Therefore, pressing AN has no effect on the normal conversation of the telephone. It also has no adverse effects on the program-controlled switchboard, and only makes incoming calls busy when using the doorbell.