Electronic generator for operating telephone ringers

When several telephones are connected to the same line, some method of ringing the instruments individually is desirable. This is usually a multi-frequency system, with each ringer tuned to a particular frequency (in the range 16-66 c/s). The device described is not new in principle, but has the merits of economy and efficiency. Each frequency generator consists basically of a 2-value unit (multivibrator and amplifier): its stability considerations are noted, and the assembly of a complete unit is illustrated.