- Build the high-resolution microscope
- Build the low-cost microscope
- Build the upright microscope
- Customisations and alternatives
- Troubleshooting
- Knowledge base

# Why does the LED need a resistor?

LEDs are designed to be driven at constant current. Simply connecting an LED to a 5V power supply is likely to result in too much current being drawn, potentially damaging either the power supply or the LED. A resistor is a very basic way of driving the LED safely, but for low power LEDs such as the ones we use here, it is generally sufficient.

## Calculating the right resistance

To calculate the right resistor value, we simply use Ohm's law, V=I R. In this case, the voltage V is 5V minus the operating voltage of your LED, typically 3.2V for a white LED, giving us 1.8V. The operating current of the LED, I, then sets the required resistance: R=V/I. For 30mA current, we therefore need R=1.8/0.03=60 Ohms. We specify 150 Ohms to drive the LED below its maximum current rating - we don't need the extra brightness, and it saves on power and heat.

## Flicker and better drive electronics

Using a resistor only provides a constant current if the voltage is constant; it means any ripple in supply voltage will result in a slight flickering of the LED. It is also difficult to perform wire-to-wire soldering reliably. For these reasons, the recommended illumination from V7 of the OpenFlexure microscope is a small PCB to mount the LED and a constant current chip, which will more reliably provide a constant current. This assembly is also easier to mount, and has a proper connector on it thus eliminating wire-to-wire soldering.