Found in Popular Electronics from the 90s, this circuit uses a reed switch on the rails as a sensor. Instead of lamps, in a modern version, LEDs can be used.
We found this circuit with an operational transconductance amplifier in the Motorola Linear...
We found this circuit in the 1994 Linear Applications Handbook by National Semiconductor. The...
In the figure we have a differentiator with the LM3900 transconductance amplifier. The circuit is...