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991  Using Arduino / General Electronics / Re: Controlling 12V @ 12A with an arduino. on: July 02, 2011, 02:48:08 am
Using the emitter follower configuration, your relays will never see more than the voltage at the transistor bases.  As Lefty says, the relays need to be in the feed from the +12 to the collectors and you also need snubbing diodes across their coils to prevent damage to the transistors.  At 12 amps your relay contacts are going to experience a bit of arcing so ensure they are conservatively rated (say 20 to 30A rating).   I like the way you have configured your reverse switching, since accidentally switching on both inputs simply stops the motor.  Many such circuit endeavours actually short circuit the supply in the event of switching both together.
992  Using Arduino / General Electronics / Re: battery back-up circuit ICL7673 on: June 30, 2011, 07:17:32 am
Why not simply use a couple of diodes acting as an OR gate.  Whichever system (primary supply or back-up battery) is the higher voltage will feed your load with the diodes preventing back-flow into the lower system voltage.

993  Using Arduino / General Electronics / Re: Parallel Voltage Regulators on: June 30, 2011, 03:09:17 am
1) You cannot simply parallel regulators

2) If you cannot buy a heat sink then make one by bolting the regulator to a LARGE lump of aluminium

3) If your 5 volt load is reasonably constant, you can dump some of the excess 7 volts across a power resistor before the regulator.  Say the load is 1 amp, then dump 4 of the 7 volts across a 3.9ohm, 10watt resistor.

4) Install a small PC fan to keep the regulator cool

5) You can boost regulator output by using its output to drive a power transistor, but this is somewhat overcomplicating the issue
994  Using Arduino / General Electronics / Re: Resistor placement. on: June 26, 2011, 05:50:57 am
If you want to minimise component count, place your three LEDs in series and use one resistor to feed the chain.  Because LEDs are "current" driven devices such an arrangement is sound providing the rated currents of the LEDs are identical.  You might see minor voltage variations across each LED but that's of no concern in such a circuit.
995  Using Arduino / General Electronics / Re: Monitoring it's own current draw? on: June 25, 2011, 07:34:33 am
See attached for a hall sensor with 250mA fullscale range
996  Using Arduino / General Electronics / Re: Monitoring it's own current draw? on: June 24, 2011, 10:18:57 am
Or better still use a hall sensor powered by the system voltage which gives an isolated 0-5 volts output
997  Using Arduino / Project Guidance / Re: Pneumatic Controller on: June 23, 2011, 03:17:47 pm
I hate to be negative but to use compressed air to drive a bicycle will need lots of high pressure air.  Certainly you could get an air-motor to provide sufficient power to drive a bicycle but duration of supply may well be a problem.
How do you propose to generate this, compressed storage or petrol driven compressor.  Either way you will find a diving type air tank pressurised to 200 bar won't last very long or a petrol driven compressor will be a bulky and heavy item.   
If you want to use air as the motive force you might want to look at using lots of low pressure air, ie a propeller driving like a "pusher" aeroplane, directly driven by a petrol motor.
998  Using Arduino / General Electronics / Re: Start Up and Shut Down Motorcycle Engine Using Transistor as Switch on: June 23, 2011, 07:19:21 am
How do you propose to detect that the engine has started so that you can stop cranking the starter motor.  I also think you'll find the starter load will be in the many 10s of amps so would suggest you use a correctly rated relay for the main driver contact and use the arduino to control the relay (via a suitable transistor driver)
999  Using Arduino / Project Guidance / Re: Breath Pressure Sensor on: June 22, 2011, 04:37:57 pm
The human lung can only produce about 1 to 2 psi so if you use a 100psi sensor you'll only be using around 2% of its range.  This means you'll only get around 2% of the 5 volt output range (0.1 volts).  This will severely limit the resolution of your intended application.

May I suggest you conduct an experiment by making a u-tube out of 1/4" bore clear plastic tube with each side around 4 feet long.  Fill with water so that each side contains around 2 feet.  Now blow down one side as hard as you can and get a colleague to note how high the water moves up one side.  The pressure you blow is the difference between the water levels in each side of the tubes.  If you're lucky you might get a difference of around 1.5 feet.   To convert this to psi,  2.3 feet of water equates to 1 psi.  You will be surprised at high feeble your lungs are.

Select a device that has a lower range, say 0-5psig or 0-5psi differential (often referred to as DP)

1000  Using Arduino / Project Guidance / Re: Breath Pressure Sensor on: June 22, 2011, 02:55:20 pm
The 4115 is an absolute type sensor so is unsuitable for your application

Go for the 5500.  The term differential means that the sense element looks at the difference between the HP and LP ports (there are two to see in the photo)  As stated above, leave the LP one disconnected and connect your blow-tube to the HP port.
1001  Using Arduino / Project Guidance / Re: Breath Pressure Sensor on: June 22, 2011, 02:43:23 pm
Commonly available pressure sensors are generally referenced to atmospheric pressure, which is 0psig or 14.7psia.  "g" stands for gauge (referenced to atmospheric pressure) and "a" stands for absolute (referenced to a total vacuum).   You can of course get ones referenced to absolute zero but these are generally limited to specialised industrial applications and are therefore quite expensive.

Therefore if yours is capable of going to 16 PSI then that'll be 16 above atmospheric  16psig or 30.7psia

Putting it into context, blowing to 16psi above atmospheric is about, or above, the limit of the average human.

Edit:  Whilst writing the above your note re the actual sensor came in.  This is a differential sensor so with the LP port left open to atmosphere the measured range is 0-75psig.   If you connected the LP port to an absolute vacuum, the range would still be 75psi but the measured range would be 0-75psia or -14.7 to +60.3psig
1002  Using Arduino / Project Guidance / Re: "YOUR BOAT IS SINKING" SMS FROM IPHONE CONTROLLED BY ARDUINO on: June 22, 2011, 07:31:09 am
Another case of using technology for the sake of it, rather than curing the original problem.
Better to spend some money on recaulking the boat to keep the water out which will make it safe and reliable.  There is never an adequate excuse for tolerating a leaky boat. 
Then as suggested buy a reliable self monitoring bilge pump. 
Once the boat has properly wetted the planks will swell and become tighter than they will be if they've been allowed to dry out.
1003  Using Arduino / General Electronics / Re: Differential line drivers for 100m twisted pair? on: June 20, 2011, 09:53:37 am
It seems strange that RS485/422 chips are the only things commonly available.

Why should it seem strange when that's exactly what you're trying to replicate
1004  Using Arduino / General Electronics / Re: Weather proofing electronics on: June 20, 2011, 09:49:12 am
a vacuum cleaner may be good enough to get the air bubbles out.  Also warming epoxy type fillers with a hair drier reduces their viscosity.
1005  Using Arduino / Project Guidance / Re: Super simple RPM monitor. on: June 19, 2011, 05:49:14 am
Glue some magnets to your drive wheels and fix a reed switch to the chassis.  As each magnet passes the reed a pulse is generated.  Cheap and simple.  One magnet will give one pulse per rev and 4 magnets will give 4 pulses etc

Then you simply feed the left and right pulses into a couple of dig-ins and compare the frequencies.  It matters not if the pulses are out of sync since it's frequency you're interested in, not phase difference.

Of course the higher the pulse rate the better the resolution, so if your motors are geared down you might be better looking at motor shaft speeds rather than drive wheels. 
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