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.

200 mW FM transmitter (CB14878E)
This transmitter has a range of a few hundred meters depending on site conditions. The coil...
Multivibrator Lock Out Input LM139 (CB14239E)
The circuit shown below was obtained from the National Semiconductor Linear Applications Handbook...
Variable astable 4093 (CB21127E)
This circuit was found in a Popular Electronics from the 90s. The circuit has 4 frequency ranges...