# Circuit notes

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Some miscellaneous analog & digital circuit notes:

## Analog filter

The frequency at which the filter attenuates to half it's original power is:

f = 1 / ( 2 x pi x R x C )
Where:
f = frequency in Hz
R = resistance in Ohms
C = capacitance in Farads

According to Atmel - a 10 kohm resistor and a 100 nF capacitor produces a filter with a 1 kHz 'crossover' frequency?!? Evil Mad scientist labs suggest a 33 kohm resistor and a 22 uF capacitor. I've been using 1.8kohm and a 10nF capacitor.

To make a high-pass filter, swap the resistor and capacitor.

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## Variable Resistor

On my Arduino, I see numbers in the range ~90 to ~1020 returned from analogRead(0) depending on the position of the knob. It's very non-linear though.

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## Switch

The resistor is needed to avoid shorting out the Arduino pin if it's programmed as an output by mistake.

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## Hooking up a Relay

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 Wikiid Pages relating to Arduino (edit) Arduino Command line Arduino Startup code for Arduino Low level functions for Arduino Putting data in flash on the Arduino External resources for Arduino Board schematics for Arduino Misc notes: Circuit notes, Music notes, Stepper motors