Telephone off-hook indicator

This encoder utilizes a single contact per key keyboard and manages all other switching functions electronically. A diode connected between terminals 8 and 15 limits the output to no more than 1 V negative concerning the negative supply, Vss....

In this circuit, a simple calculator, in conjunction with a COB (chip-on-board) from an analogue quartz clock, is used to make a telephone call meter. The calculator enables conversion of STD/ISD calls to local call equivalents and always displays...

12V Battery Charge Nominal Discharge (Low) Indicator Circuit. This circuit monitors car battery voltage and provides an indication of nominal levels. The 12V Battery Charge Nominal Discharge Indicator Circuit is designed to monitor the voltage levels of a car battery...

The indicator described here is specifically designed for adjusting the dynamic limiter outlined elsewhere in this edition and checking if the maximum level of the reference voltage (P1) requires modification. A 4-to-16 decoder integrated circuit (IC) of type 4514...

The circuit presented utilizes modulated rectangular waves of varying time periods to generate ringing tones akin to those produced by a telephone. It requires four astable multivibrators for operation, implemented using two 556 integrated circuits (ICs). The 556 IC...

This circuit will allow you to connect any tape recorder that has a mic and remote input to a phone line and automatically record both sides of a conversation whenever the phone is in use. You will need to...