This circuit is from an article that teaches how to assemble it in the magazine Electronics Now from July 1993. It uses a liquid crystal display module from the time with 3 and a half digits.
This circuit is suggested by Motorola, using a 4N26 type coupler and an MC1733 type operational....
The two amplifiers are practically the same, except for the gain. They have the following...
This source only uses passive components (without transistors or integrated circuits) and even so it...