This circuit was found in an English publication from the 70s, an article that teaches how to calculate instruments. The circuit is still current and can be built with an analog indicator of 100 uA.
This circuit for the car was found in the April 1990 Radio Electronics magazine. The speaker is...
We found this circuit in a Texas Instruments documentation from 1973. Other operational amplifiers can...
This circuit was found in a 90's EDN. The circuit has a well-known configuration. The voltage...