Off-hook Telephone Indicator (CB5994E)

This circuit was found in a 1980s US publication. It indicates when the phone is off-hook by triggering an indicator.

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40 mA Constant Current Source (CB6632E)
The setting of this power supply is only 0.1%. We found it in a 1969 US documentation. Transistors...
20 Hz to 20 kHz Sinusoidal Generator (CB19206E)
We found this interesting transistor generator in European documentation from the 1980s. The...
VCO With Operational (CB15466E)
The circuit shown in the figure uses two of the four operational amplifiers found on the LM324...