Note the following points:

1. The display on the CRO screen is a rectified AC which is equivalent to an unsmoothed DC

2. This is done by using a LED connected in series with the AC input voltage which is fed into the Y-plates of the CRO

3. Should the LED be reversed in its connection along the circuit, the upper part of the sine wave of the AC input will be displayed.

4. The sweep of the CRO has to be turned on to obtain the display.

Left: Sine waves of an AC input into the Y-plates with the sweep (time-base) on.