Two-tone doorbell circuit

Published:2011/5/9 8:48:00 Author:Christina | Keyword: Two-tone, doorbell | From:SeekIC

Use the 48V (60V) DC feedback electric current which is provided by the phone feedback line as the working energy of the electronic doorbell is very economical and practical. Now I introduce one kind of two-tone doorbell circuit with out battery. The circuit is as shown, circuit in this figure is the variant of conventional telephone ringing circuit. a, b are the positive and negative ports of the home telephone line. AN is the normally open doorbell button, when the telephone is in the standby mode, if you press AN, the 48V (or 60V) voltage (which is provided by the PBX) and the DC feedback current will charge capacitor C1 through VD1 and R1, when C1's voltage reaches IC1's start-control voltage, IC1 starts to send out the current of two-tone electronic bell to make the buzzer B sounds, to inform the host when guests are visiting. When the phone is in using, voltage between a and b can not reaches the IC1's start-control voltage, at this point, even if you press AN, the doorbell button will not work because the R1's value is ​​larger than the telephone's impedance. So if you press AN, there is no effect on the normal call, and also the PBX, but when you use the doorbell, the telephone will be busy.



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