[IAEP] [support-gang] Sensor Experts: I need help with my shopping list!

[forster at ozonline.com.au]

Caryl

> Temperature (thermister). Which of these will work best?
> www.allelectronics.com/make-a-store/category/770/Thermistors/1.html

You want the resistance at 20C, 70F to be very roughly mid way (in terms of voltage) in the measurement range of the XO. This gives the best range and accuracy.

http://wiki.sugarlabs.org/go/Activities/TurtleArt/Using_Turtle_Art_Sensors#Measuring_Temperature

has a thermistor which is 5000 ohms at 20C which is somewhere near mid range for an XO-1 (I dont remember exactly what mid range is but you can calculate it looking at the source code, 10kohm sticks in my memory but...). The XO-1.5 has a much better measurement range so choose on the basis of the XO-1

whether it is best to use positive or negative temperature coefficient, my choice of NTC was pretty much random. It might have had a difference on how simple the calibration function was but I didnt give this much thought

> Light Will this one work?
> http://www.allelectronics.com/make-a-store/item/PRE-24/CdS-PHOTOCELL-PHOTORESISTOR/1.html

Yes, looks OK, mid range light is within the XO's range

> There isn't a lot of info about the ones for measuring soil moisture (but there is interest in this one), water salinity, and the lemon battery (looks like a fun one). They must all use the 3.5mm plug, right? Can one sensor be made to use for all of these or do they need different components in the circuit? If so, what different things are needed? Would we need to insert a resistor for XO-1s with the lemon battery?
> 
> How about wire? Other than copper wire for the water salinity, what should I try to get for the rest? Gauge? Material, bare or covered?

Whenever you are terminating the plug with bare wires you should include a 680ohm series resistor to protect the XO-1
http://wiki.sugarlabs.org/go/Activities/TurtleArt/Using_Turtle_Art_Sensors#Specifications

When measuring moisture/salinity just experiment with 2 bits of copper wire from your junkbox. The length of wire you use can be determined by trial and error so they are mid measurement range. The measurements are uncalibrated, what matters is the relative measurements.

The lemon battery can use one of the copper wires above and any galvanised nail

> For the plugs, can we get cords with one plug at each end, split them and use for 2 sensors? Something like these? If we do that what other wire will I need to get?
> http://www.allelectronics.com/make-a-store/item/CB-389/12-CABLE-3.5-MM-STEREO-PLUGS-BOTH-ENDS/1.html

Yes, its a 3.5mm stereo phono plug. I have used bare plugs and soldered my leads. Some premade leads have a foil rather than a wire for ground that may be difficult to connect to. Note the previous warning about the series protection resistor for the XO-1.

Tony