This circuit was obtained from an American 1980s documentation. The frequency depends on the crystal and the circuit may be a PNP that reaches the generated frequency.
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...