Modulated light receiver (CB21790E)

We found this circuit in a Mullard manual from 1960. The transistor is a phototransistor of the time. The gain is 3 dB.

Crystal converter (CB7327E)
This circuit was found in a Popular Electronics from the 90's. The circuit can be used to convert...
Two way intercom (CB20505E)
This circuit works with only 1.5 V and can be powered by a solar panel. The circuit is from 1991....
FM transmitter for 800 m Range (CB20409E)
In fact, the original American publication exaggerated a little about the range that a good...