Telephone Ring Generator Using Small Power Transformer
By Bill Bowden
The interval between rings can be lengthened or shortened by varying the value of the 1 Meg resistor. The 70 volt/ 30 Hz ring voltage is produced from the 120 volt side of a small 12.6 VAC power transformer (Radio Shack 273-1365). Both capacitors connected across the transformer windings are non-polarized / 100 volts. Circuit draws about 300mA from the 12 volt DC power supply during the ringing interval.
Telephone Ring Generator Using Small Power Transformer circuit
1436 22 August 2007
The circuit presented here can be used for connecting two telephones in parallel and also as a 2-line intercom.
The telephone ring generator shown below generates the needed high voltage from a simple switching mode power supply.
Audio from a telephone line can be obtained using a transformer and capacitor to isolate the line from external equipment.