This current meter with many scales was found in a 80s American documentation. The operational amplifier is from that time, uncommon nowadays.
This circuit was found in an old GE manual. It uses a thermocouple as a sensor. The SCR must be in...
This circuit was found in an English magazine from August 1982, but it can be easily assembled with...
This is the circuit of the commercial Japanese megaphone of the 60s model DM-218. The circuit uses...