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
1521 22 August 2007
This circuit was floating around on the web apparently untried and since I had recently acquired a school bell I thought I'd give it a try.
Audio from a telephone line can be obtained using a transformer and capacitor to isolate the line from external equipment.