This circuit is from a 1969 Electronics World. The transistors may be modern general-purpose equivalents. The circuit is for frequency ranges between 250 Hz and 250 kHz.
With this circuit, from Newton C. Braga's Mechatronics Sourcebook it is possible to control up to 5...
This alarm, from a 1996 magazine, can be fitted with two BC548s in the original configuration...
This circuit was obtained in a June 1987 magazine Hands-On. It is a circuit that indicates on two...