11 years, 2 months ago.

Are pullup resistors needed?

I never have figured out what pull down resistors do, & read that the mbed has them. However I haven't read about pull ups, and wonder if they are needed?

Experimenting with PWM and some LEDs (digital out) I encountered some strange behavior, with enough voltage appearing on the wrong LEDs to turn them on.

It's probably in my program, if it persists I'll post the program.


2 Answers

11 years, 2 months ago.

See http://mbed.org/users/4180_1/notebook/pushbuttons/ for pullup and pulldown resistors.

Many LEDs will also need a series resistor to drop the voltage. Depends on the LED but likely 100-220 ohms or so. Most LEDs want to see a voltage of around 2.2V, the series resistor drops the voltage across the LED.

Accepted Answer
11 years, 1 month ago.

Most of the time pull-up and pull-down resistors are used on inputs to set the input to a known logic level and stop the input 'floating'. The mbed has internal pull up and pull downs that can be selected in software. You shouldn't need any when using digital outputs.