This circuit is Transformer dot convention detector that can detect the polarity of any transformer.

Dot convention refers to the phase shift between input voltage (primary) and the output voltage (Secondary) of any transformer. 

In some transformer applications its very important to know the transformer polarity (dot convention). 

Video: https://youtu.be/zMc17zLqZAU

Parts list:

IC 555

IC 4030

IC LM358

Capacitor 100nF

Capacitor 10nF

Capacitor 100uF(2pcs) 

Capacitor 220uF

LED (2pcs)

Banana Plug and socket (4 pair)

Push button

Diode 1N4148 (4pcs)

Resistor 100K (2pcs)

Resistor 510R (2pcs)

Resistor 100R

Resistor 33K (2pcs)

IC Socket 8 pin (2pcs) 

IC Socket 14 pin