Found in documentation from the 1980s, this circuit can have several adaptations to use modern transistors and LEDs instead of the lamp. The power supply voltage can also be changed.
This configuration, which produces a rectangular signal compatible with CMOS and TTL logic, was...
We found this circuit in a 1977 Radio Shack brochure. Transistors can be the most modern BC548 for NPN...
The basic idea of this circuit is to convert the indication of a magnetic sensor to an audio tone...