Project taken from the site: Blue Ring Tester Kit

Tester to measure the quality of the transformer windings used in switching power supplies. As you can see, few and inexpensive components are needed. The operating principle is as follows: the CD4016 integrated circuit generates a pulse lasting approximately 2.3 mS after which it waits for 104 mS and then repeats the cycle. Furthermore, using the button it works as On-Off. At this point the generated pulse is sent to the capacitor shown in the diagram as *100nF* which, if connected in parallel to the winding to be tested, generates a series of pulses which in turn are amplified by the two transistors and the IC1C gate sent to the input of the CD4015 integrated circuit. Being said to be integrated, a Shift-Register turns on the various LEDs connected to its outputs in relation to the number of oscillations on the input. Since the oscillations generated by the capacitor and the winding that make up a tuned circuit are not self-powered, they cease after a certain number. So this happens if the winding of the transformer is intact and we will have all the green LEDs lighting up: if on the other hand the winding is interrupted or shorted these oscillations do not occur and obviously only the red LEDs will remain lit. The circuit is also able to signal if only some coils are short-circuited and in this case we will still have the green LEDs off. As mentioned previously, the circuit works very well with the transformers used in switching power supplies: I made some attempts to see if it was possible to use it also in transformers with a 50 Hz mains voltage, trying to vary the capacitor marked in the diagram * 100nF * and also changing the pulse duration, but I didn't get reliable results. Two things before closing the topic: as you can see in the photo we have the version on the left which is the precise replica of the original scheme, while the version on the right uses the frequency meter always described on this site under PIC/FREQ 70 MHz for display. : in this case values read below 100 are to be considered as faulty windings. The other thing is that I preferred to use the external 12 Volts reduced to the appropriate value via a 7808 to power the circuit instead of the battery.