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.
If you have higher valued resistors, try those. I've had up to 20K series resistance with a red LED (high luminosity, admittedly) and it was still clearly visible in office/lab lighting. You can go for more intensity later, but this may be sufficiently visible for bench debug. How high you can go on resistance is probably different for each device, and I've tried this with exactly one sample, so YMMV.