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1156  Forum 2005-2010 (read only) / Interfacing / Re: Transistor behaviour on: December 29, 2010, 05:47:50 pm
Without the collector connected the base-emitter junction conducts to light the LED, the emitter having about 2 volts on it since the two resistors form a potential divider.

However when you connect the collector and then switch on the base-emitter junction, the collector-emitter becomes "connected" and because the collector is at GND potential, so is the emitter.  Hence the LED has GND at both terminals.  (In actual fact the emitter will be at about 0.7volts but this won't be enough to illuminate the LED)

jack
1157  Forum 2005-2010 (read only) / Interfacing / Re: Motor driver not working, digital HIGH only 0.5 V on: December 19, 2010, 03:23:38 pm
I see this is your second post on the subject.

Without a circuit of what's in the green box you really are shooting in the dark

Suggestion :-

Disconnect motors, disconnect all arduino side connections, connect supply and grnd to green box, couple green box ground to arduino grnd.  Now  set one arduino output high. Connect In1 to this pin, does it go low.  Record results.  Repeat tests for In2, En and Se in turn using the same arduino pin.

What are the values of the resistors within the box connected to In1 and In2.   If you have a test meter what is the resistance from both sides of these resistors to the grnd terminal.

Would it be possible for you to get a high resolution photograph of the circuit board foil side and the same of the component side.  If so, also advise what each resistor value is.  With this information it MIGHT be possible to create a circuit diagram.

jack
1158  Forum 2005-2010 (read only) / Interfacing / Re: Motor driver not working, digital HIGH only 0.5 V on: December 19, 2010, 12:15:50 pm
It appears that your device is overloading the arduino outputs.  I take it the SE connection is the "sense" input to the L6203.  If so then why do you have it connected to an arduino pin (input or output ? you don't say which).  It surely should be getting some form of current feedback signal from the motor driver.

What make of module is containing the L6203,  do you have a circuit diagram of it and how you have connected it to the arduino.

jack
1159  Forum 2005-2010 (read only) / Interfacing / Re: Wire-wrap current limit on: December 29, 2010, 10:23:47 am
Not quite relevant but we used Rochester (RIS) emergency shutdown panels in the middle east back in the late 70s and these used wire wrap panels as the "programme".  Between each shelf of logic gates sat a "programme" rack and on this was contained all the interlinking connectors.  To programme the system you simply wrapped the desired I/O together.   If you needed to change the logic you simply pulled out the interface module, altered where the wires run and plugged the module back in.

Great and simple to understand and work on.

The pins were square so's the wires would be "bitten" into so cutting the insulation as you wound on the wire.  Fortunately the arduino pins are also square.

Happy days, apart from the sand and lack of "beverage"

jack
1160  Forum 2005-2010 (read only) / Interfacing / Re: ADC Mystery on: December 28, 2010, 04:42:53 am
This is only a philosophical guess but here goes.

I'd have thought that an ADC must always have a +-1 bit resolution about a centre point.  There is no such thing as a single absolute or exact value in life, only an approximate measure of a variable and the mathematics of determining analogue values within an ADC must always have some degree of digital "dither".  No two things are ever identical (only so near or like each other that neither the eye nor measurement can detect difference) If your ADC was only a 4-bit device there would still be +-1 bit resolution (there can be nothing less than 1) but the hysteresis would be so much larger.

With respect to filtering, if the measurement device's input is of high impedance then the capacitors are effectively charging to the peak values seen, however short they may be, so giving a higher figure than the unfiltered input will show (an average of any noisy signal perhaps)

Surely a simpler way of "stabilising" the reading would be to take an average of several readings and use the averaged value for display.

jack
1161  Forum 2005-2010 (read only) / Interfacing / Re: Any ideas to measure Frequency of a AC source? on: December 26, 2010, 07:22:40 am
What range of frequencies are you wanting to look at.  If it's quite a narrow band, say +-50% around a centre frequency and the waveform is sinusoidal then you could simply use an RC series circuit and measure the voltages developed across the R (Vr) and the C (Vc).  Since you are using a series circuit, the current Irc through both components is the same.  Vr = IrcR   Vc = IrcXc    Note however that Vr and Vc are out of phase with each other.
Google RC series circuit and you'll find the necessary explanations of how to compute C.

Jack
1162  Forum 2005-2010 (read only) / Interfacing / Re: Arduino + air flow sensor on: November 04, 2010, 05:34:45 pm
Try a small cermet microphone.  The sound of air inhalation and exhalation without apparent snoring will amaze you.  The sounds of each are quite different and the arduino can easily determine which.
jack
1163  Forum 2005-2010 (read only) / Interfacing / Re: Detect snow in rain gauge on: November 28, 2010, 02:45:13 pm
Or simply purchase something like one of these and try it

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=4889790&cm_vc=av_uk#header

The snow should reflect the infrared.

Jack
1164  Forum 2005-2010 (read only) / Interfacing / Re: Transformers on: December 23, 2010, 03:23:34 am
The  design of a transformer entails ensuring there is sufficient iron core top prevent magnetic saturation.  This is based on so many square millimetres per watt of power transformed.  The mains side of a transformer has sufficient turns to create sufficient impedance (inductive reactance, not DC resistance) and and this is based upon so many turns per volt applied (based on having an unsaturated core)

If you reverse primary and secondary and try applying mains to a low voltage secondary, you have sufficient iron for the wattage to be transmitted - that hasn't changed, but you have insufficient number of turns for the voltage being applied.

Reversal is often done with 230 to 110 transformers that are specifically designed to work either way, but they are wound to ensure there are sufficient number of turns, whichever side is chosen as the primary.

jack
1165  Forum 2005-2010 (read only) / Interfacing / Re: High Voltage on: December 24, 2010, 04:47:03 am
I don't care what your circuit simulator says, there is NO way you are going to get 40kv across a 100ohm resistor from that circuit.

If it was capable of generating 40kv the instantaneous wattage W=VxV/R    So would be 16E6watts  Thats 16megawatts.  Think about it !!

If you want to play with high voltage ringing circuits then google tesla coil and you'll find loads of methods of generating high voltages

jack
1166  Forum 2005-2010 (read only) / Interfacing / Re: Pressure transmitter 4-20mA with arduino on: December 22, 2010, 05:41:00 am
Frode,

The accuracy of your primary element (the sensor) is specified by the manufacturer in their data sheet as 0.25% of full scale.  The term "full scale" is important since, let us say the range is 0-400bar, then the accuracy is 0.25% of this, which is +- 1bar.   If your measurement is only 10 bar on a 400bar unit the true value could be anywhere between 9 bar and 11bar, which is actually 10% of the measured value.  Whilst this doesn't sound very good, think of it as an analogue guage ranged 0-400bar.  How good do you think your eye and judgement are when the gauge is measuring only 10 bar.

Where most folk fall down is in the 250ohm resistor.  You need to get a precision unit typically 0.1% or better, which you will not find in your local junk shop.  If you have any friends in industry they might be able to "release" one for you.  

The arduino ADC is, I think, a 10-bit device which gives a resolution of 1 part in 1000 or 0.1%.  

All of these errors need to be considered as accumulative.  They may well cancel, but maybe not.  So at best your system accuracy will be somewhere in the order of 0.5%.   Repeatability in the short term should be somewhere around 0.3% but the sensor has a poorish thermal stability so if the ambient changes from say 0degC to 30degC you might , at best, expect system accuracy to be no better than 1%.

I'm sure the mathematicians will challenge these numbers, but I'm equally sure you will appreciate that "exactness" is an impossible target to achieve.  Life is a compromise whereby "near enough" has to be "good enough".  (Unless of course, you have unlimited funds.

Jack
1167  Forum 2005-2010 (read only) / Interfacing / Re: Pressure transmitter 4-20mA with arduino on: December 21, 2010, 04:05:18 pm
Yeh I know that but was just advising about the elevated zero in case it was not immediately appreciated by Frode.  Old enough to go back to the good old days 3-15psi then 10 to 50ma and finally 4-20.  Couldn't be bothered with today's virtual signals all running down the same wire and dependant upon multiple single points of failure.
1168  Forum 2005-2010 (read only) / Interfacing / Re: Pressure transmitter 4-20mA with arduino on: December 21, 2010, 03:31:30 pm
In a word  Perfect

Bear in mind that the bottom 20% of the 0-5volt range is "lost" sonce the scale range starts at 4ma which produces 1 volt across the 250ohm resistor.

Yes, it is a pain but that's tranducers for you.

jack
1169  Forum 2005-2010 (read only) / Interfacing / Re: Pressure transmitter 4-20mA with arduino on: December 21, 2010, 02:18:44 pm
You have not defined exactly which model of 3100 transducer you are intending to use.  You mention 4-20ma but I believe that is because you are unaware that the three wire device is voltage output, whereas the 4-20 device is a 2-wire device.  You talk about 4-20 (2-wires) but your drawing shows 3-wires (voltage output)

Until you define exactly which you intend to use, we might be giving false guidance.

We will give you the correct answer to your question but only when you give sufficient information on which to base the answer.

kind regards

jack
1170  Forum 2005-2010 (read only) / Interfacing / Re: Getting odd readings from hacked digital scale. on: November 29, 2010, 04:16:49 am
You may be failing to appreciate that strain guages are dual action.  That is, as well as pushing down on them you can also pull up on them.  In effect their full range is from minus-weight to plus-weight.  So in effect, in a body mass device you are spanning over only half the weight range.

Also because the op-amps can only span from 0 volts to +5 volts they need to be set to mid range at zero load to match the strain guage capability.

You have proven that you can vary gain by twiddling the adjustable pot.  What you now need is a zero off-set adjustment so that you effectively blank out the -ve range capability of the strain guage and the mid-voltage offset of the op-amp.

There again I may be talking a load of bull.

jack
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