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631  Using Arduino / Sensors / Re: Termostat problems on: July 14, 2012, 02:28:02 pm
You don't show how you are powering the system.  USB, battery and if the latter what voltage, type and size.

There is a possibility (remote ?) that the pot wires are inducing noise into your temperature sensor input circuit.  Perhaps placing 0.1 capacitors between ground and each of your 2 inputs will dampen any noise.
632  Using Arduino / General Electronics / Re: dumb question: diode and relay on: July 14, 2012, 02:45:20 am
The reed coil, unlike a conventional relay has no iron core so is of very low inductance.  It will still give a negative kick when de-energised but is very much less than that from a metal cored coil.  Having said that, it should still have a clamping diode fitted.
633  Using Arduino / Sensors / Re: Measuring gasoline level on: July 13, 2012, 04:59:53 am
Recover the float mechanism from a car (automobile ?) fuel tank and use that.  They are designed for use in a petrol vapour atmosphere.
634  Using Arduino / General Electronics / Re: 3 wire solenoid on: July 13, 2012, 04:56:48 am
The common connection (the one you call ground) goes to VCC and the two flying leads go to the collectors of the respective transistors.  The snubbing (or flywheel) diodes (2 required) are connected as Anodes to VCC end of solenoids (the common wire) and cathodes to each of the other solenoid connections
635  Using Arduino / General Electronics / Re: Transistor help on: July 12, 2012, 04:54:16 pm
That part is fine.
Arduino pin thru a 270 ohm resistor to the transistor base. Emitter to Gnd. Collector to Coil. Other side of coil to 12V.
Diode across the coil pins with Anode to transitor collector, emitter to 12V.

Yes, 300V to damage it. Likely fail when current exceeps 500mA first tho. Your 12V 33mA will be fine.

"Anode to transistor collector, emitter to 12V" should read "Anode to transistor collector, cathode to 12V" and the diode should be wired as close to the relay coil as possible.
636  Using Arduino / General Electronics / Re: Transistor help on: July 12, 2012, 04:40:56 pm
Unlike FETs, transistors are current control devices.  The specified gain of the transistor is 40, so for 33ma of collector current you will need approximately 0.7mA of base current.   With an arduio output voltage of 5 volts and a base-emitter junction voltage of 0.7, the base control resistor should be 4.3/0.7 Kohms.   That equates to 6 Kohms.   You need to ensure the transistor is driven hard on (to minimise heat generation) so I'd say something in the order of 2.7 Kohms will do. 

Whilst the suggested 270ohms is an order of magnitude lower it will do no harm to the circuit operation - just means the base current is a bit high (and energy wasteful)  The collector current will not rise above 33mA since it is determined by the load device (the relay)
637  Using Arduino / General Electronics / Re: 3 wire solenoid on: July 12, 2012, 04:28:41 pm
Your first diagram details the solenoid coils wired as 'common emitter' mode which means the maximum voltage across the coils will be 4.3 volts  (arduino output minus 0.7)

The second diagram shows common 'collector mode' and the solenoid coil will receive the full supply voltage minus 0.7

So to operate a twin-triggered solenoid you should reconfigure the first circuit to the common collector mode
638  Using Arduino / General Electronics / Re: Convert 6v to 5v to power stand alone Atmega328 on: July 12, 2012, 01:48:44 pm
So do the best of both worlds and run it off 3 x AA cells rated at 4.5 volts.  From the previous graph it would be capable of operating at virtually top speed.  If all you have is a 4-way AA cell holder you can install a dummy cell in place of cell No4.. This can be as simple as a piece of wood with a jumper wire to connect +ve and -ve poles together.  If you don't fancy wood then use a "dead" cell and again install a jumper wire or alternatively simply install a jumper between the contacts that would normally connect to the + and - ends of cell No4
639  Using Arduino / General Electronics / Re: Convert 6v to 5v to power stand alone Atmega328 on: July 12, 2012, 03:42:54 am
Or you could simply replace your 4 x 1.5 volt AA alkaline batteries with rechargeable NiMh or Nicad units which run at 1.2 volts, giving you 4.8 volts which will be sufficient to operate your processor directly.
640  Using Arduino / Project Guidance / Re: HV supply ideas on: July 11, 2012, 04:32:41 pm
Maybe a flux-capacitor might be worth considering.
641  Using Arduino / Sensors / Re: Termostat problems on: July 11, 2012, 03:31:52 pm
There appears nothing obviously wrong with your diagram - are you 100% certain your wiring is connected as shown. 
642  Using Arduino / General Electronics / Re: Alternate Wall Adapter? on: July 11, 2012, 03:26:16 pm
Yes - providing they are DC output and their off-load voltage doesn't exceed say 14 volts.  
Most cheap ones will fail the off-load limit :  I have a 12 volt one which exceeds 20 volts off-load !!
Certainly the arduino will always present a load which will make an unstabilised wall-wart output drop  to the arduino spec of 12 volts but without knowing by how much, it's safer to avoid anything that exceeds 14 volts.
643  Using Arduino / Sensors / Re: Termostat problems on: July 11, 2012, 11:52:43 am
we need a diagram of how you have wired things up
644  Using Arduino / Sensors / Re: measuring inclination on: July 10, 2012, 04:42:47 pm
If you are measuring humerus relative to scapula (humerus-scapula) you will get away with only three rotation sensors (X, Y, Z).   However if you are measuring relative to space (humerus-space) then you will also need to sense the scapula orientation relative to space (scapula-space) since any movement/rotation of it will need to be deducted from the humerus-space readings.
645  Using Arduino / General Electronics / Re: Measuring frequency with an DSO? on: July 10, 2012, 07:46:08 am
The datasheet for that 'scope specifies a timebase accuracy of 50ppm (0.005%).  The same timebase generator will be used to perform frequency measurement : just how concerned are you?
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