Found in an old Popular Electronics, this circuit warns when a remote lighting system is not working. The circuit uses an LDR as a sensor and the components used are quite common today.
Figure Bellow shows how to use a 7490 (TTL) IC to divide the frequency of a TTL signal by 6; The...
This circuit was obtained in an old US 1974 documentation when Nixie tube displays were common. The...
This source draws a fixed constant current of 1 mA using a Norton operational amplifier. Other...