This circuit was published in an article in Radio Electronics magazine in October 1988. It uses components that are still common today, except for the sensor, which is a special type from that time.
This circuit uses a CA3140, an operational amplifier with FETs in the input stages. Equivalent can be...
Found in a Popular Electronics from the 90s, this circuit has a configuration that has been widely...
This circuit was found in an American 80s circuit manual. The components used are unusual. The...