This circuit was found in a Popular Electronics of the 1990s. The sensor is a rotating mechanical system with a propeller-driven magnet to count pulses with a reedswitch.
The shield circuit shown is from the 1994 Linear Applications Handbook by National Semiconductor of 1994....
This configuration was found in a 1990s documentation. Modern operational amplifiers can be used...
The LED frequency flashes depends on the value of the capacitor tested according to the table. We...