How do I work out what resistor to use?

I am trying to work out what resistor to use in a circuit.

The circuit is a series circuit. The power to the circuit is 5.5V, with a switch, which connects to a GPIO pin on the RasberryPi. I believe 3.3V is needed for the pull up resistor on the RPi.

I need to use a resistor to lower the power from 5.5V to 3.3V. I know Ohms law, but I'm struggling to use it in practice. How can I figure out the required resistor needed for this situation, and for future knowledge?

Is their a formula I can note down?

The Pi has 3.3 volts, from an internal regulator, available on the GPIO connector - you can use that as a 3.3 V source for your pull-up resistors.

Using a resistor to produce a voltage drop is not recommended, as the voltage across the resistor will depend on the current drawn through the resistor.

The relationship between voltage drop, current, and resistance is known as Ohm's Law: V = IR, where V = voltage, I = current in Amps, and R = resistance in ohms.

• Ah so it does. Thank you. In a situation where a drop is required (eg. if the RPi didn't have 3.3 V how would it be achieved? – James Jeffery Mar 22 '15 at 22:03
• In this case, since the Pi input pin is very high impedance, a voltage divider as Dwayne suggests would work. – Peter Bennett Mar 22 '15 at 22:26

I think that what you want to use is a voltage divider. Try using a 2.2k resistor as the series resistor with a 3.3k shunt resistor.

Here's what I mean:

5V input signal into one end of 2.2k resistor. Other end of 2.2k resistor connected to one end of 3.3k resistor and connected to Pi input. Other end of 3.3k resistor goes to ground.

• +1 This is the electrical equivalent of a 1.32K pullup resistor 1/(1/2.2 + 1/3.3). For many purposes you could multiply those values by 10. – Spehro Pefhany Mar 23 '15 at 1:49