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1066  Using Arduino / General Electronics / Re: Measuring current on: March 22, 2011, 03:48:48 am
Think low tech
Put a black bag over it
1067  Using Arduino / Motors, Mechanics, and Power / Re: Stopping stepper if an obstacle is detected ? on: March 20, 2011, 11:26:46 am
Correct me if I'm wrong but isn't the current in a stepper motor independent of the load on the shaft.  After all, all you are doing is providing a rotating magnetic field which is being followed by a permanent magnet. The strength of the field and the magnet is what determines the output torque and all you are effectively doing is switching field coils on or off to produce a fixed field "rotational effect"  

Edit :  I suppose at high speed (pulse rate) there is some back emf induced into the coils by the rotor which will tend to reduce the coil current but at low rotation speeds or when simply jogging this effect must be minimal so coil resistance (or rather reactance) will determine current irrespective of whether the rotor is free to rotate or jammed.  There may however be some "signal" induced into the coil (perturbation on the pulse shape) as the rotor moves which might be "seen" by by an intelligent monitoring system.

jack
1068  Using Arduino / General Electronics / Re: Measuring current on: March 20, 2011, 11:20:15 am
I think you might find a hall effect sensor meets your needs.  These can measure both AC and DC  current and give a DC voltage output which is proportional to the current.  In effect your arduino would be reading a simple DC analogue input as frequently (or infrequently) as you desire.

http://sensing.honeywell.com/index.cfm?Ntt=csla&x=9&y=11&Ntk=si_all_products&ci_id=154286&la_id=1
gives you a starting point.  Generally these require around 12 volts DC to excite them and give an output signal of around 50mV per amp.

jack
1069  Using Arduino / General Electronics / Re: Overheating on: March 16, 2011, 03:31:11 am
Simply because what's left inside the casing after overheating isn't what was put in to do the job.  ie they're cooked.  Consider it a bit like baking a cake.  Once it's been in the oven it is no longer eggs, flour, water etc
1070  Using Arduino / General Electronics / Re: Forward Voltage Drop on: March 16, 2011, 03:25:29 am
Because the LED is effectively a forward biased diode who's current is limited only by its own internal resistance.  IF the CR2032 could supply sufficient current the LED would self destruct.  As to the suggestion that the battery internal resistance will limit the current, that is so but is far from good design practice.
1071  Using Arduino / General Electronics / Re: Forward Voltage Drop on: March 15, 2011, 05:28:07 pm
Your CR2032 won't last long without some form of current limiting device; the simplest of which is a resistor in series with the LED.  I'd suggest you consider a 33ohm for a start point. 
If your multimeter fuse is blown and you really want to measure current you can do a "get-you-by" temporary fix by placing a piece of cooking foil around the fuse.  This offers NO protection to your meter so you do at your own risk.  In the good old days we used the foil liner from cigarette packets when such disgraceful practice was required.
1072  Using Arduino / General Electronics / Re: Transistors? on: March 15, 2011, 05:21:40 pm
The valves that Mike was referring to are what our colonial cousins on the west side of the pond would call a faucet

jack

1073  Using Arduino / Motors, Mechanics, and Power / Re: Building a Winch. Stepper motor or regular DC motor. on: March 15, 2011, 01:26:20 pm
If the two motors are pulling in the same direction then you could use a single motor by feeding the rope from the winch drum over a pulley assembly at the terminal location then run the rope back to the location of the winch where it can be anchored. 
1074  Using Arduino / General Electronics / Re: Electric water/pipe valve? on: March 15, 2011, 07:15:44 am
If you want a low pressure "solenoid valve" then go for a motorised central heating zone valve.  These operate from mains AC but more importantly are readily available and reasonably cheap.  In fact you should be able to find one at your local scrap dealer for pennies.

Alternatively have you considered simply turning off the pump
1075  Using Arduino / General Electronics / Re: Electric water/pipe valve? on: March 15, 2011, 05:57:19 am
More information required :

a) temperature of water
b) pressure across valve when closed
c) flow rate through valve when open
d) power supply used to operate valve (low voltage 6/12/24DC or mains 110/230AC)

1076  Using Arduino / General Electronics / Re: Voltage divider 101 help, any suggestions? on: March 14, 2011, 05:38:14 pm
What happens to your voltages when you disconnect the connector to the podbreakout device
1077  Using Arduino / General Electronics / Re: true analog out on: March 14, 2011, 04:35:21 pm
How much current do you need to supply to the 0-10volt control system.
If very low (say 20ma or less)then a simple potentiometer will suffice (using say a 500ohm pot) with some slight non-linearity
If reasonably low (say less than 100ma) then a simple op-amp with a potentiometer input will do
If higher than 100ma then you need a regulator chip such as an LM317.  These have a minimum output of around 1.5 volts but if you need to get down to zero then a couple of diodes wired in series will get rid of this
Using an arduino to provide a signal such as you require with very low change requirements is an absolute waste of technology.
jack
1078  Using Arduino / General Electronics / Re: Voltage divider 101 help, any suggestions? on: March 14, 2011, 04:26:16 pm
Have you connected the free end of the green wire to ground (or where).  The schematic you used is correct (using your resistor values) so there is no reason why you aren't getting the correct voltage unless a) it's not breadboarded correctly b) not wired correctly or c) component values are not as stated.  Since you can measure the voltage (by a meter I hope) then you can use the same meter to verify the component values.  Commercial resistors are almost 100% correct (within their tolerance band) but it is possible that the 100k or the 68k are faulty.
jack
1079  Using Arduino / General Electronics / Re: Do I need to use a resistor for a small speaker? on: March 14, 2011, 02:04:32 pm
On the basis that the arduino can happily supply around 25ma at 5 volts, the output power is around 100mW.  This can be heard on a small speaker OK but certainly won't blow your ears.  The speaker circuit impedance should be around 200 ohms (5000/25).  Most small speakers are about 8 ohms so you are going to need a series resistor of around 192 ohms (let's say 180)  since the speaker will now only "see" 8/180 of the 200mW, ie around 8mW, you'll be lucky to hear anything.

Better to use an op-amp to act as an amplifier between the arduino and the speaker, as well as permitting the speaker to be connected directly to the amplifier (without a series resistor).

jack
1080  Using Arduino / General Electronics / Re: measuring very low AC voltage on: March 14, 2011, 04:42:23 am
Clearly you need what is referred to as a "precision rectifier" circuit.  In these a simple opamp has a feedback loop which takes into account the normal forward voltage loss across the diodes.

Try googling "precision rectifier" or go directly to   http://sound.westhost.com/appnotes/an001.htm for a write-up on the subject

jack
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