The ADC801 is not an easy component to obtain today, but the configuration is worth reference. The circuit is from National Semiconductor's Linear Applications Handbook.
Any general-purpose germanium or silicon PNP transistor can be used on this receiver. The handset...
When this circuit is activated the LED flashes for a moment indicating that the function has been...
In the circuit of the figure, when pressing S1 the sound varies according to the value of R2 and...