[GreenKeys] ring signal

[Sam Hallas]

amourdutigre wrote:
>  I was curious, how did the telco COs generate their ring signal? I
>  believe that it is a 20 Cycle AC current.
Joe,
In the early days here in the UK, they used a dynamotor (a motor and 
dynamo on the same shaft). Ringing was 16 2/3 Hz at first and 25 Hz 
later. The dynamotor could generate 25Hz to ring bells as well as the 
progress tones (400 Hz etc), fed through a series of cams to interrupt 
them in the right cadence.

Very small exchanges used a vibrating relay to create both ringing and 
tone, though I have seen an example that used a tube to make tone and a 
vibrator for the ringing.

These days the ringing & tones are all electronic: very often on the 
line interface card or local shelf.
Cheers,
Sam