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976  Using Arduino / General Electronics / Re: electric fence batteries - possible use ? on: September 03, 2011, 01:11:55 pm
The catch is that these batteries are not rechargeable so it makes for an expensive one-time use.  Follow the previous recommendation and go for a switching regulator.  That is by far the most efficient way to go. 

Say your 5 volt load is a maximum of 200ma, then power requirement is 1 watt, which is  24watthours per day.   So total requirement over 10 days is 240watthours.  At 12 volts, that equates to around 20AH.  Say you limit depth of discharge to 50%, to prevent irreversible battery damage, then you need a battery of at least 40AH.
977  Using Arduino / General Electronics / Re: monitoring 120/240 vac on: August 17, 2011, 06:56:25 am
Your main problem of using a simple resistor divider network is that your circuit will be either at near ground or near mains voltage, depending upon the "polarity" of your input wiring.  Hook it up wrong and your whole arduino circuit is potentially lethal. 
978  Using Arduino / Project Guidance / Re: Decoding phase shifted signal in 198kHz LW radio carrier on: August 17, 2011, 05:44:15 am
It should be a pretty reliable service - it is said that if the UK Trident fleet cannot receive Radio 4 on 198 long wave for three consecutive days, they are to follow their orders.   smiley-sad

I'd have thought that sort of frequency far too high for submerged reception.  Perhaps somewhere around 16kHz might be a more realistic figure.  Yes I know that's within the audio band but it doesn't preclude wireless transmission - you "just" need a few gizmos and a suitably tuned aerial
979  Using Arduino / Motors, Mechanics, and Power / Re: Small motor sourcing? on: August 11, 2011, 10:18:47 am
Electric razor, even smaller than a toothbrush.  Yes it's a vibrator but all that is is an off-balance weight that can easily be removed.
980  Using Arduino / Project Guidance / Re: Earthing a metal case on: August 02, 2011, 03:06:18 am
If you are connected to a laptop (whether on battery or mains adapter) the USB shielding is floating from "true" electrical earth so there is no need to earth the casing, other than via the USB cable shield as Mike says.  However if you are using a mains powered workstation then the casing of that machine and possibly the USB shield will be connected to "true" electrical earth.  If you then also connect your metal enclosure by a separate earth bond to the mains ground you could possibly create an earth circulating current into the USB shield which may induce noise into the USB data. So better to simply use the USB shield as a single earth link as per the laptop system.
981  Using Arduino / Project Guidance / Re: Submergible/Floating tanks controlled by Arduino. on: July 31, 2011, 04:46:36 pm
The "open" bottom concept (whether large or relatively small holes) is the same as generally used in submarine design.  The only problem is, if the tank turns upside down then injected air (to raise the tank) will simply bleed out the upturned open base.  You might be better to keep the open bottom concept but fit a bladder (balloon) into the base of the tank such that expanding the bladder to raise the tank displaces water through the open bottom but, in the event of capsize, the air supply still acts against the bladder and does not leak out the base.
jack
982  Using Arduino / Motors, Mechanics, and Power / Re: Remote Actuated Fuel Gas Valves on: July 28, 2011, 05:19:18 am
Taking Lefty's idea a stage further, have the ball valve lever pulled closed by a strong spring.  The lever itself could be held in the open position by a trigger/latch mechanism operated either by a small solenoid or an RC servo motor.

Nice to see you understand the techniques of Ex systems - at least you are not shooting in the dark.

jack
983  Using Arduino / Motors, Mechanics, and Power / Re: Remote Actuated Fuel Gas Valves on: July 27, 2011, 02:40:55 am
Out of interest, what "standard of safety" are you interested in :

a) reliability of closure
b) reliability of opening
c) tightness of closure
d) suitability for use in an explosive atmosphere (now that's a stumbling block)
e) prevention of leakage from valve body

Important aspect to consider are :

1) is the device to be used within enclosed spaces
2) is the device to be used in the presence of members of the public
3) are you adverse to taking risk

If any of the above 3 are "yes" then I'd suggest another project.  Hydrogen gas and insurance litigation are mutually exclusive subjects

Making something "safe" is a long way from making something that works with minimal risk.

984  Using Arduino / Motors, Mechanics, and Power / Re: Splitting a single power supply usefully? on: July 24, 2011, 01:41:24 pm
You seem to have ignored my cautionary note re the wall-wart regulation of its 12 volt output.  Before connecting your micro use a digital volt-meter to confirm the specified output of the wall-wart really is 12 volts (with no external load applied).  I have several of these cheap power supplies and some output as much as 19 volts with no load !!
985  Using Arduino / Motors, Mechanics, and Power / Re: Splitting a single power supply usefully? on: July 24, 2011, 04:06:49 am
In reply to your specific question :

You consider the arduino regulator supplyy and the solenoid supply as two separate loads wired in parallel on the 12 volt system - not in series as you suggested.

Be aware that most cheap wall-warts provide pretty rough rectification and somewhat more that their specified output voltage at low loads, so it would be advisable to fit a large smoothing capacitor across the 12 volt output, say 1000microfarad rated at about 35 volts.

You feed 12 volts to your regulator circuit, using suitable local capacitors as specified in the regulator data sheet and from it you feed the regulated DC to the arduino.

You also feed 12 volts to your transistor + solenoid circuits taking care to ensure you fit flywheel diodes across the solenoid coils.

The 12 volt negative line is considered as a common ground and connects to all systems : regulator, arduino and either transistors or solenoids (depending on whether you are doing high end or low end switching)
986  Using Arduino / General Electronics / Re: Use charger as DC supply to Arduino on: July 22, 2011, 05:15:48 am
Looks very much like a "constant current" charger which means it will adjust the output voltage to keep the current flowing.  If you have it charging the battery and feeding the arduino, then, if the battery becomes disconnected, the charger will increase its output voltage in an endeavour to keep the current flowing.  As it's rated to charge 12 volt battery systems this implies it will be capable of increasing the output voltage above 12 volts, which may endanger your arduino.

So, not to be recommended.
987  Using Arduino / Project Guidance / Re: detecting a coil on: July 20, 2011, 02:36:19 am
When the PWM value is zero you are in a "don't care" status since it is irrelevant whether the coil is connected or not.  As soon as you have a PWM value greater than zero you can monitor the current and hence determine whether or not the coil is connected.  Simples
988  Using Arduino / Motors, Mechanics, and Power / Re: Brushless motor to be employed as passive load - electrical engineering on: July 19, 2011, 01:59:06 pm
Most readily available DC motors (to use as a generator) use fixed field permanent magnets, so you need to ensure whatever you use has wound field coils.  Similarly if you get hold of a DC generator you need to ensure it's not a PMG type.  (The PM stands for Permanent Magnet)  You ideally need a variable DC power supply, but it all depends on what the generator requires.  You might get away with a couple of car batteries and a suitable rheostat (a potentiometer used as a 2-terminal variable resistor).  50watt halogen bulbs make good field control resistors, you can switch these in series and/or parallel to give various currents. However these get very hot (most of the 50 watts) so you need to ensure they are kept off flammable surfaces.   Until you find a generator it's impossible to specify exactly what you need.   I used a large battery charger capable of providing 20 amps with 2-volt switched steps from 2 to 36 volts that I built when I was an apprentice - and it's still going strong some 50 years later.  This also had a series rheostat in its output leg so as well as stepped voltages I could also trim the output current. 
989  Using Arduino / Motors, Mechanics, and Power / Re: Brushless motor to be employed as passive load - electrical engineering on: July 19, 2011, 09:20:56 am
A single resistor of a value to suit the rated generator voltage and amps at full engine revs is all you need.  The variation in capability is accomplished by varying the field excitation.  Yes a generator output is proportional to speed at each an every value of field flux : if there is no flux then there is no output.  So varying the flux at each and every speed will vary the output voltage and hence the absorbed power into the fixed resistor.  You simply need to measure generator output voltage and amps to give output power to the resistor.  On the basis that field flux is independent of engine speed it is a reasonable approximation to assume that electrical power output is the shaft power input.
990  Using Arduino / Motors, Mechanics, and Power / Re: Brushless motor to be employed as passive load - electrical engineering on: July 19, 2011, 03:48:26 am
I've done something sightly similar in the past but to test the electrical characteristics of a large DC motor (acting as your engine).  I used another large DC wound-field motor acting as a generator to apply the brake load .  The armature of the generator was connected to a large (low) resistance capable of absorbing the full rated power of the "engine".  Power (watts) = V*V/R so knowing the power of the engine and the voltage capability of the brake generator it's easy enough to work out the required resistor.  Resistors are generally power rated to operate in free air so one way of increasing the resistor power rating is to immerse it in a bucket of running water.    By varying the field current to the generator you vary the output voltage and hence vary the braking load applied to the engine.

By this means it is possible to perform a complete performance test of the engine at all speeds and torque loads, including if required the ability to start the engine.
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