We found this code key or coded keyboard in a publication from the 90s. The circuit uses components that are still common today and is powered by a 12 V voltage according to the relay used.
The amplifier in the figure is based on a single LM108 and has low noise compensation. In this...
This simple megaphone is from old documentation, but uses still current components. The circuit is...
The circuit presented was taken from an old English publication in 1981 but it can be easily...