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.

Precision Tone Switch 4046 (CB25256E)
Using a 4046 PLL this circuit recognizes a 1.6 to 1.8 kHz tone by driving an output LED. The...
Battery charger with current and voltage limiter (CB9537E)
This battery charger circuit was found in ancient American documentation. The components are...
Solar Battery Test (CB1355E )
This circuit is of great use today, when solar energy is becoming increasingly important. The...