Preamplifier stage (CB8934E)

This circuit is from a 90s Popular Electronics. It appears elsewhere on this site. Q1 can be any general purpose transistor.

Fire Detector (CB13985E)
This circuit was found in an Italian magazine Elettronica Prática of 1977. The transistors can be...
Wien Bridge Oscillator with AGC (CB14041E)
The presented circuit generates a sinusoidal signal of frequency given by R and C, presenting an...
Car Battery Charger (CB16114E)
Car battery charging can be done very simply with the indicated circuit. Although in the "fast"...