This circuit was found in Electronics Today International of the 80's. The circuit uses still common components, providing the indication through LEDs.
The frequency of this circuit depends on the crystal. The transistor supports a modern equivalent. We...
Indicated in Maxim's documentation from the 90's, this circuit has a gain of 40 dB and an output...
The time obtained in this circuit, from a 1978 publication, depends on R3 and C. R3 can have...