Door buzzer or alarm (CB20492E)

This solar powered circuit was found in a 1991 documentation. The transistor is a general purpose NPN.

Simple photometer (CB23873E)
This bench photometer, suitable for professional photographers, was published in Radio Electronics...
Unity Gain Second Order Filter (CB25595E)
This low-pass filter with 741 op and symmetrical source was found in the August 1993 issue of...
Modulated Siren 555 (CB6421E)
In the original publication of the 90s a 556 consisting of a double 555 is suggested. The circuit...