Telephone Circuits

Telephone Ring Generator Using Switching Supply

The telephone ring generator shown below generates the needed high voltage from a simple switching mode power supply (SMPS) which employs a CMOS Schmitt Trigger square wave oscillator, 10 mH inductor, high voltage switching transistor (TIP47 or other high voltage, 1 amp transistor) and a driver ...

Telephone Ring Generator Using Small Power Transformer

This ring generator will ring a telephone once every 10 seconds. The interval between rings can be lengthened or shortened by varying the value of ...

Telephone In-Use Indicator

Telephone In-Use Relay Controller

Circuit to close a relay when any phone extension is off-hook. Voltage at the gate of the MOSFET should be ...

Telephone Audio Interface

Audio from a telephone line can be obtained using a transformer and capacitor to isolate the line from external equipment. A non-polarized capacitor is placed in series with the transformer line connection to prevent DC current from flowing in the transformer winding which may prevent the line from returning to the on-hook state. The capacitor should have a voltage rating above the peak ring voltage of 90 volts plus the on-hook voltage of...

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