What sensors are appropriate to use in a sensor-focused circuits class? I listed a bunch that I’ve been thinking about in Why teach circuits to bioengineers?, but in this post I want to focus a bit more on what criteria we should use for deciding which sensors to include or exclude.
- Price. We want students to be able to buy a kit of parts for the course and take everything home with them when they are done. All sensors should be under $5 (ideally under $1) each in quantities of 10.
- Variety of things sensed. I’d like for there to be at least 5 and preferably 8 or 9 different properties detected.
- Variety of different electrical properties as sensor outputs (at least current, resistance, voltage, and capacitance, maybe also inductance or mutual inductance).
- Some sensors should be rapidly varying (for oscilloscope output).
- Some sensors should be slowly varying (for recording time course with Arduino).
- Some sensors should require amplification.
- Easy breadboarding. Students won’t have time to do a lot of soldering, particularly of SMD parts. We should do one or two labs where they do some soldering, but if we use SMD parts for other labs, we’ll need to design and assemble breakout boards for them.
|thermistor||temperature||resistance||NTCLE413E2103F520L||35¢||non-linear, very sensitive, slowly time varying, no amplification needed|
|RTD||temperature||resistance||480-2017-ND||$1.94||slowly time varying, only 0.4% change/degree, similar to high-precision (which cost over $10 each)|
|temp sensor||temperature||voltage||MCP9700-E/TO-ND||25¢||slowly time varying, not very accurate, linear, not much challenge for circuitry|
|electret mic||sound||current||102-1721-ND||75¢||rapidly time varying, can be amplified or not|
|potentiometer||angle||resistance||987-1277-ND||66¢||slowly time varying, hard to connect to mechanically?|
|potentiometer||angle||resistance||3382H-1-252||$2.23||slowly time varying, easier to incorporate into a goniometer?, lead in for servos|
|breathalyzer||alcohol||resistance||605-00011-ND||$4.50||fun for students? needs humidity and temperature correction.|
|pH probe||pH||voltage||SeroSystems pH probe||$21||Too expensive and needs temperature correction.|
|gel electrodes||EKG||voltage||SilveRest||22¢||(cheaper without snap), very low voltages and 60Hz noise requires good amplifier, time varying|
|gel electrodes||GSR||resistance||SilveRest||22¢||(cheaper without snap), slowly time varying|
|CdS photoresistor||visible light||resistance||PDV-P8104-ND||80¢|
|ambient light sensor||visible light||current||1080-1019-ND||46¢|
|reflectance sensor||reflectance||current||QRE1113-ND||83¢||LED + phototransistor in same package, 5mm sensing distance|
|IR emitter||754-1600-ND||19¢||IR emitter and red LED needed for pulse oximeter|
|red LED 660nm||754-1218-ND ?||9¢||IR emitter and red LED needed for pulse oximeter|
|green LED||754-1591-ND||9¢||green LED needed for simple pulse sensor|
|conductance cell||salinity||bulk conductance||?||?||May require student design and construction. Cheap with Al foil, expensive with Ag/AgCl electrodes.|
|touch sensor||touch||capacitance||?||?||May require student design and construction. Al foil and plastic wrap?|
Richard Hughey had an interesting idea for an air-flow sensor. Use one of the siren whistles for kids that cost about 50¢ each in quantity. They have a plastic turbine in them, and we could use an LED and light sensor (perhaps one of the reflectance sensors) to get a pulse stream from the turning of the turbine, timing the pulses with an Arduino.
I just noticed that I don’t have any switches, accelerometers, magnetic sensors, … on that list. A breath switch might be a useful sensor for those thinking of the rehabilitation concentration. A tilt switch is also a reasonable option. A lot of the sensors I have looked at are too expensive for their value in the course, though it might be worth spending a little lecture time going over how they work.