This simple general-purpose transistor mixer was taken from an article in Electronics Now magazine in March 1994. You can choose a low-noise transistor such as the BC549.
This circuit is from a 1982 documentation. It can be modernized using LEDs in series with 470 ohms...
We found this circuit in a 1985 technical publication. The circuit uses PUTs to drive lamps....
This circuit appears with variations in other points of this section. This version is from...