Driving a transformer

Other Hardware General Electronics
1

Just wrote down a basic square wave to drive the transformer which is 230V AC input and 11V AC output.
This was a transformer extracted from a Guitar amplifier.
Connected the arduino from 11V side and expected to see at least half of the 230 V (If I'm not mistaken arduino supplies around 3V, right)

However when I touched the LED to 230 V side of the transformer, LED was barely blinking with a pale light.

What might be wrong with this ? Why the voltage drops even under the 5V when it passed through the transformer ?

digitalWrite(led, HIGH);
delay(20);
digitalWrite(led, LOW);
delay(20);

2

DO NOT connect an arduino directly to a transformer you will damage the output pin.
The point at which damage starts to be done is 40mA. That means the transformer must have a resistance of about 125R for it to be safe.
Drive the transformer through a transistor with a diode across the coil.

If I'm not mistaken arduino supplies around 3V, right

No it normally gives a 5V output. Well it gives about the same output as the chip is powered by so a 3V3 arduino can give 3V.