Crash Course Electronics/Resistors

Resistors as parts

German: Widerstand, Widerstände

Symbol: R (shortened for Resistance)
Unit for resistance is Ohms, represented by a capital omega: Ω (Example: 1 ohm = 1 Ω). For convenience the capital omega is replaced: 22 Ω is often written as 22 R. Decimal marks like 2.2 Ω are replaced by the unit: 2R2.

Accordingly values with a prefix like 47 kΩ (47 000 Ω) are shortened to 47k. 1 MΩ is written 1M. Decimal marks are replaced like before:
4.7 kΩ is written 4k7
2.2 MΩ is written 2M2

Common Appliances

• Pull up / Pull down for digital IO
• current limiting (LED, Transistor, directly dimming lamps and small motors)
• Voltage dividers for reference voltages
• Sensors made of changing parts changing their resistance.

Resistive Sensors

Resistive sensors with large resistance changes (doubling the resistance during use or more) are easy to connect to an an analog input. A simple voltage divider is sufficient to read the resistance value in this case.

Resistors with small changes of resistance need additional circuits to amplify the small changes measured on the voltage divider. This can be achieved with an operational amplifier and a bunch of resistors and potentiometers for calibration.

Potentiometers

Standard Linear and Rotary Potentiometers Magnetopot

Touch

Rotary and linear touch sensors. Similar to connect like Potentiometers: When touched they react like a potentiometer, when not touched the "middle pin" of the potentiometer doesn't touch the resistive layer at all. Single and dual touch is detectable. Single touch is like measuring potentiometers, dual touch needs 2 ADC-Channels and some calculations for the resistances in series.

Temperature

PT100 and PT1000: Made of thin platinum wire. Low resolution compared to other sensors when directly hooked up to an analog input since change of resistance is small. Can be used to make measurements very precise when pre-amplification is used. Industry standard - simple to replace when broken, pricey.

NTC and PTC Thermistors: NTC Lowers Resistance when heated, PTC increases resistance when heated. Simple to use, cheap, not very accurate (5%), nonlinear, but can be linearized, usable for many appliances where high accuracy isn't important.

http://reprap.org/wiki/Thermistor

Light

LDR (Light Dependent Resistor) Available in different dark/bright resistance values. Bright resistance: The resistance of the sensor when lit with a defined amount of light (Usually 25 Lux are used as "bright"). Varies from 1k to several 100k for 25 Lx.

Pressure / Force

DIY Sensors from resistive Materials: http://www.kobakant.at/DIY/?tag=velostat

Conductive Foam Velostat Cunductive yarn Conductive(!) video Tape - beware: most tapes aren't conductive.

Elongation / Strain

Strain gauges: Made of thin resistive wires, resistance increases when enlongated. Very small change of resistance - needs to be amplified for mikrocontroller's ADC. Used in personal scales to measure weight, also usable as bend sensor when glued on solid surfaces.

Bend Sensors

change of resistance when bent, destroyed when fold. Small hysteresis, appearing when unbent. Power Glove: contains 5 of those sensors - one for each finger, can by bought in different lenghths.

DIY: Velostat and shrink tube - less accurate, some hysteresis.

Resistor Color Codes

http://www.elektronik-kompendium.de/sites/bau/1109051.htm (German)
http://www.bader-frankfurt.de/widerstandscode.htm (German)
http://www.csgnetwork.com/resistcolcalc.html (English)

• Read Resistor codes interactively on www.dannyg.com or on samengstrom.com
• Read Resistor codes by a table overview elektron-bbs.de