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1291  Using Arduino / Sensors / Re: Detecting a high speed object with a phototransistor. on: March 09, 2013, 12:16:05 am

Lots of discussion on that projectile sensor if you dig around a bit.
1292  Using Arduino / Project Guidance / Re: Measuring the weight of a beehive on: March 08, 2013, 01:52:11 am
I'm not sure a "balloon" type system would work quite as expected. When weight is added to the hive the balloon will flatten out and result in a non-linear pressure reading at your manometer?

Using the example of a car tire, pressure measured in PSI, the footprint (in square inches) of the tire multiplied by the inflation pressure equals the weight supported by the tire (an "ideal" tire, ignoring sidewall stiffness, etc.). When weight is added the footprint can increase and the pressure can stay near or at the original pressure.

I might be totally off my rocker.

1293  Using Arduino / Motors, Mechanics, and Power / Re: Relay shield on: March 07, 2013, 11:13:16 am
What type of light (voltage / amps / AC or DC)?
1294  Using Arduino / Project Guidance / Re: Load sensor amplifier on: March 07, 2013, 01:01:57 am
I'd be interested in your results, Lefty. Seems too good to be true.
1295  Using Arduino / General Electronics / Re: 109.3035 ohm resistor on: March 05, 2013, 06:34:54 pm
Well, I was implying that if it's spec'd at 109.3035R at 75F then you have to know when it's 75F if you expect it to be at 109.3035R (a circular problem).

As it turns out these types of resistor are accurate to 10 ppm from 15C to 50C so just having it at "room temperature" would be sufficient.
1296  Using Arduino / General Electronics / Re: 109.3035 ohm resistor on: March 05, 2013, 04:13:59 pm
The resistance of a PT100 sensor at 75 degrees Fahrenheit is exactly 109.3035 ohms.

But how do I calibrate my thermometer to 75F?
1297  Using Arduino / General Electronics / Re: Digital pots like AD5206 on: March 05, 2013, 04:07:51 pm
It's a resistor ladder where each output can be switched on. Pretty straightforward to build one with discrete components.
1298  Using Arduino / Project Guidance / Re: Trouble aligning laser diodes on: March 05, 2013, 03:58:51 pm
If you tap a hole in the side then you can insert a screw to press against the diode and get horizontal adjustment. If you tap a second hole at 90 degrees to the first then you can get vertical adjustment. That second hole isn't possible when you have the diodes in a row of course; you could either make two holders in the shape of a square each holding four diodes or one large holder in the shape of swastika.
1299  Using Arduino / Project Guidance / Re: Load sensor amplifier on: March 05, 2013, 03:46:12 pm
What you'll find is that when the cell is at rest it will report some non-zero voltage (load cells are never perfectly balanced), and then you'll want to get the largest voltage swing possible from that point when load is applied. You'll be doing a lot of fiddling with the gain resistor when you're setting this up and will want to have 500R or 100R or maybe even 10R trimmers on hand to do that.
1300  Using Arduino / Project Guidance / Re: bare bones and cheap cnc first major arduino project on: March 03, 2013, 12:42:49 am
I am aware that these motors can be very slow. something like 1/64 gear reduction
Yes, they are much too slow, and they have very poor repeatability. Don't even consider them.
1301  Using Arduino / Sensors / Re: Analog pressure gauge / potentiometer coupling? on: March 02, 2013, 01:58:56 pm
You could use a hall or optical sensor inside the gauge.
The gauge has a simple construction and it probably wont have enough power to turn a pot.

A more pro solution would be a magnetic encoder IC (e.g. AS5040). That would then just require a magnet attached to the center pin on the dial.
1302  Using Arduino / Project Guidance / Re: R2D2 Project - Am I buying the right battery? on: March 02, 2013, 01:16:20 pm
The charger looks a little weak. 500ma max and it doesn't appear to have any cell balancing features; just a straight 500ma charge?

This charger requires an external AC adapter (wall wart) but generally a much better charger.
1303  Topics / Product Design / Re: Moving from the uno to programming AVRs. Please recommend on: March 02, 2013, 02:07:36 am
What are the "4-8K variety"?

I've used ATtiny85, ATTiny84, and ATMega328 and always with ArduinoISP. Never had any problems.
1304  Using Arduino / Motors, Mechanics, and Power / Re: Help with stepper motors? on: March 02, 2013, 02:01:57 am
Pinouts for the SN754410 are the same as the L293. Given a choice between the two the SN754410 is preferred as it has slightly higher power ratings.

In my experience the biggest problem with these chips is having sufficient/clean power. If your supply is noisy or cheap you'll have problems.
1305  Using Arduino / Sensors / Re: IR receiver based distance sensor on: March 01, 2013, 11:53:54 am
It seems the IR receiver circuitry kind of adapts or something... I guess that's caused by the circuitry inside the IR receiver.

Yes, IR receivers automatically adjust their sensitivity. If you look at your datasheet it will refer to this as "automatic gain control" or "AGC" in the block diagram. You can compare this to Vishay part TSSP4038 which does not adjust its sensitivity (it's "fixed gain").

TSSP4P38 is what you're looking for if you want to do distance detection. This is $.95 at Digikey.

See also for a nice matrix list of different IR receiver types.
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