Low battery indicator 555 (CB9551E)

This circuit can operate with different voltages, depending on the zener used. We found it in a 90s American documentation.

Telegraph Practice Oscillator (CB6474E)
Found in a 1965 Radio Shack documentation this oscillator can be used with modern general purpose...
Voltage Frequency Converter with the 4047 (CB12736E)
In the figure we have a voltage / frequency converter circuit that can be used as a transducer interface...
Headphone Amplifier LM3900 (CB13929E)
This small stereo amplifier for headphones was found in a documentation on integrated circuits from...