This circuit is from Mullard 1969 documentation. The circuit operates at a relatively high supply voltage and has a 10 Vpp output. The transistors are of the time, but admit equivalents.
This circuit was found in a 1980s US publication. It indicates when the phone is off-hook by...
This Radio Electronics circuit, in an 80's edition, decodes the 67 kHz signal used at the time to...
We found this circuit in an old American documentation. The table gives the values of the...