With this circuit it is possible to vary the resistance by adjusting the mark/space reaction of the 555. The circuit was found in the November 1989 Radio Electronics magazine.
This type of circuit and with this integrated circuit appears several times on this website. The...
Using transistors of the time, this circuit was published in Radio Electronics magazine in...
The integrated circuit used in this 1982 project is not very common today. It can be adapted to...