XO-1.75 microphone socket

[forster at ozonline.com.au]

Scott

Thanks for your comments. As this is now more a discussion on  
education, I have copied to IAEP.

Background for the IAEP list:

Suggested experiments for using TurtleArt with external sensors are at  
http://wiki.sugarlabs.org/go/Activities/TurtleArt/Using_Turtle_Art_Sensors

The allowable input voltage on the XO1 microphone socket is ?0.5V <Vin  
<5V. The published specifications of the XO1.5 are ?0.5V < Vin <3.5V  
but the actual safe input is ?9V < Vin <9V.

The question is whether ?One value is better suited to personal  
tinkering, the other to widespread propagation?

I am interested in getting feedback from teachers on this question.

> Why not use 1.5V alkaline cells?
Alkaline cells exceed the published specifications of the XO1.5 (and  
can damage the XO1.0)

> Or measure voltage from a lemon  battery?
Though quite safe, the lemon battery (0.9V) exceeds the published  
specifications

> "measuring AC amps" example should be moved to a separate "only with  
> supervision and adequate care" page
This generates typically 30mV, 1/300 of the safe input of the XO1.5  
and is quite safe.

> Maybe also the "Generating electricity" example also, although it
would take a lot of turns of wire, a pretty strong magnet, and very
vigorous motion to exceed 9V

This generates 10?s of microvolts, a safety factor of 1,000,000

> maybe also the "burglar alarm", depending on whether the XO-1 is  
> always safe if its inputs are shorted

My understanding is that this is safe.

I do have some doubts on including information on measuring mains  
power.  
http://wiki.sugarlabs.org/go/Activities/TurtleArt/Using_Turtle_Art_Sensors#Measuring_power
There is the risk that this might encourage students into unsafe  
areas. I was encouraged by  
https://docs.google.com/present/view?id=df7px97w_54fs9nh9fj (in  
Spanish) which suggests using a tong ammeter to measure power in mains  
circuits to measure carbon footprint. The TurtleArt sensor is  
superior, it can log over an extended period, costs about $1 and can  
be constructed with village technology. The tong ammeter costs about  
$300 and takes instantaneous readings.

Tony

On Tue, Nov 16, 2010 at 11:38 AM, Walter Bender  
<walter.bender at gmail.com> wrote:
> On Tue, Nov 16, 2010 at 11:19 AM, C. Scott Ananian <cscott at laptop.org> wrote:
>> On Mon, Nov 15, 2010 at 4:19 PM, ?<forster at ozonline.com.au> wrote:
>>>> Just make sure you keep in mind the difference between the specification
and what is likely to be acceptable. ?One value is better suited to personal
tinkering, the other to widespread propagation.
>>>
>>> Good point. As background to my questions Turtle Art 103 now supports
sensor input.
>>> http://wiki.sugarlabs.org/go/Activities/TurtleArt/Using_Turtle_Art_Sensors  
>>> As the specification stands, no teacher is going to conduct science
experiments with external voltages. If the specification is changed to +-9v
then they will have the confidence to conduct experiments with caution.
>>
>> Why not use 1.5V alkaline cells? ?Or measure voltage from a lemon  
>> battery? ?I can think of any number of safe experiments.
>
> Why not indeed. See  
> http://wiki.sugarlabs.org/go/Activities/TurtleArt/Using_Turtle_Art_Sensors#Lemon_battery

Right.  You can also do quite a lot of useful experiments using the
built-in 2V bias.   I'm not sure exactly what the problem is -- maybe
Tony can offer a more precise objection?  Perhaps he feels there needs
to be more guidance given on how teachers can construct "fool proof"
sensor experiments that are minimally dangerous to hardware, even if
the teacher isn't watching every kid every minute?
   --scott

ps. my rule of thumb would be "don't involve an external battery more
powerful than a lemon"  -- as I understand the protection circuits,
there's no way you can connect the internal bias voltage coming from
the microphone jack in a way that would damage the input.  You can
construct a lot of experiments (ie, most on the Turtle Art Sensors
page) which are thus "guaranteed safe, even if kids make mistakes" on
an XO-1.5/XO-1.75.  But the "measuring AC amps" example should be
moved to a separate "only with supervision and adequate care" page.
(Maybe also the "Generating electricity" example also, although it
would take a lot of turns of wire, a pretty strong magnet, and very
vigorous motion to exceed 9V.)  And the "Lemon battery" and
"generating electricity" examples should additionally be warned
against on the XO-1 --- maybe also the "burglar alarm", depending on
whether the XO-1 is always safe if its inputs are shorted.