Question: I've got a silly situation. I have an Arduino board (2009). There's just one inbuilt LED but can't do much with it beyond blinking. I have five LEDs (3 yellow, 2 green), breadboards, jumper wires but not resistors. (And there is a total lockdown here) I wish to play with these LEDs but can't risk it putting it directly. I tried using them in series but that did not light them up.
I know each LED needs a 330ohm resistor but that I don't have. I even have a 2x 7 segment display too but the same problem.

Any way to use them?
Thanks.

Update:

On suggestions, we have got two resistors from a faulty solder gun (blue-grey-orange-gold = 68k) and a (brown-black-brown-gold = 100) and 1 IN4000 diode.

Answer: 
```
#include<avr/io.h>
#include<util/delay.h>

int main(void)
{
    SFIOR &=~ (0<<PUD);
    PORTC |= 0x01;

    while(1)
    {
    DDRC ^= (1<<0);
    _delay_ms(10);
    }
}  

```

AVR microcontroller has a built-in 10K resistor with every output pins. Arduino should have libraries related to it. They are called pull up resistors. I checked it using the code I provided.
[![enter image description here](https://i.stack.imgur.com/QvsnF.png)](https://i.stack.imgur.com/QvsnF.png)  

This screenshot is taken from the AVR datasheet. You can see that there is a Resistor inside that can be operated by switching the internal MOSFET.

So, ask in the Arduino forum about the library associated to it.
In case a 5V VCC if you dont want any resistors you can connect 2 leds of voltage drop of 2.7V in series. One of the LED's wont be completely forward biased and will limit current.