Found in Radio Electronics from the 90s, this circuit uses components from that time. However, it can still be elaborated with some ease.
This circuit was found in an English magazine from 1979. The magazine no longer exists, but the...
This circuit was found in an 80's EDN. The circuit uses a current common CMOS, the 4093 powering...
The sensor of this circuit found in Popular Electronics can be formed by a piece of fabric between two...