Milo,
Giving this problem a little more thought , I think the earlier replies have nailed down your problem.

Your original circuit shows the transistor bases connected directly to the Digout pins of the Arduino.  In other words there was NO control over the level of base current.  Yes your transistors would be fully turned on but the base-emitter current was being limited by the capability of the Arduino output circuit.

This would not only burn up the base-emitter junction but also potentially damage the Digout circuits since you were applying an effective short circuit to them.

Fitting resistors between the Digouts and the base connections will resolve the base current problems.  Assuming a transistor gain of say 50 (it could be 100+) and a coil demand of 80ma then you need around 1.5ma of base current so a resistor of 3k3 (3300 ohms) should be satisfactory.  If there isn't enough drive (transistor not turned fully on) you could reduce the resistor to 1k without damaging either transistor or Arduino

Jack

The 1N4001 suppression diodes have a peak inverse voltage of only 50 volts, which is a little low for your application.  You might well expect 100+ volts being generated by a 12 volt relay coil when it releases.   If you don't believe me try putting your fingers across the coil and then de-energising it !

It may be that the diodes have broken down and are passing current to the transistor when the relay is energised.  In other words the transistor is having to pass a lot more current than just the coil current.

I'd suggest you go for higher rated diodes, such as 1N4003 or 1N4004, they don't cost any more and certainly won't break down at the voltages expected

One thing you should do is check the transistor current when the relays are energised to determine if they are passing more than expected.

jack

Too complex for your need.  All you require is the 5.1volt zener and the 1k resistor feeding it.  I'd suggest you go for a 4.7volt zener to ensure you remain below the 5volt requirement.  This will produce a maximum of 10ma through the zener, 47mw of dissipation, so no problem with component heating
jack

How about sticking a 12 volt light bulb into the fluid.  Not only will you get a heating effect, but also a visual feedback of how much energy you're puting in.
Don't let the terminals contact the fluid since you will get an electrolysis reaction that might have an effect upon your experiment.  
There again many an accidental discovery has been found by unintentional events
jack

I saw an article some time back where the guy wanted the same type of data.  In this case he built a data recorder capsule and rather than transmit data back he used a radio locator beacon that activated itself when the balloon came down.  I presumed it burst at altitude and the data capsule "floated" down by parachute.
Once he'd found the unit he downloaded the data.
Of course this method is problematic since the device may be out of range, may land in water, may fail to come down within range etc etc
jack

If your resistor gets hot and you replace it with a constant current device, this device will also get hot since it will be doing exactly the same job as the resistor , the only difference being that it's an active control device as against a passive one.  You will therefore need to supply heat sinks for each controller.

You can group several LEDs in series and drive from a single constant current source.  Do not put the LEDs in parallel since there is no control over which LEDs the current flows through.

125 x 3 x .3 = 112 watts   wow !!

I'd suggest 20 strings of 6 LEDs per string, each string being driven by its own constant current controller from by a single 24 volt supply (rated at 10 amps or so)

Dropping say 6 to 8 volts at 0.3 amps, each controller will be dissipating around 2.5 watts so you should use devices rated for 5 watts minimum.  (Not good practice to drive components at 100% of rating)

jack

Surely the arduino can detect 0.3 volts as a HIGH if you input it as an analogue value and set an internal threshold to switch when voltage is at the level you desire.
jack

Twist the wires together, then any noise induced into the wiring is "common-mode" and in effect self cancels at the input terminals.  Also fit a low value capacitor across the input terminals where the wires connect to the circuit board (somewhere around 0.01 microfarad or less), this will dampen any remaining noise.  If the capacitor is too large then short presses of your switch may not be "seen" by the input circuit and the capacitors self inductance may not dampen noise sufficiently.
jack

This looks very reasonable and almost identical to the internals of an intrinsically safe solenoid valve's coil where a low input current (controlled by R1) is used to charge a capacitor which is then used to provide a "pulse" of current to operate the coil.

The inductance of your coil will limit the instantaneous current from the capacitor so you will probably find that R2 isn't needed.  In any case R2 reduces the efficiency of the circuit.

R1 is set to limit the current to the coil when the mosfet is turned on and the capacitor is discharged (to the steady state voltage across the coil)  Once you determine a value for R1 you can then calculate a value for C which will be large enough to operate the coil but low enough to give a recharge time suitable for your trigger rate.  The values of R1 and C are a balanced compromise between current limit and recharge time.

The diode D1 is best described as a transient suppressor and simply eliminates reverse polarity voltages getting to the mosfet and the arduino.  

jack

Grumpy,
Indeed they will.  I have some 24v AC heavy duty relays that operate quite satisfactorily at a maximum of 5 volts DC.  Push them any harder and the coil current gets excessive.  What I was getting at, is that you cannot simply substitute an AC relay or solenoid with a DC one of similar voltage.
jack

AC solenoids will generally NOT work of the same level of DC voltage.  They are very inductive devices and as such use their inductive reactance to control the coil current, just as DC solenoids use their resistance to do likewise.  If you measure the DC resistance of an AC solenoid coil you will find it has a low resistance and when you then do a Vsquared/R calculation to determine the coil wattage you will get an amazingly high value.  As such they tend to burn out quite quickly when connected to a DC supply.

On the other hand if you use a DC solenoid on an AC circuit, whilst you wont have problems with over current you may well find it doesn't pull in due to the inductive reactance or it buzzes like mad because the magnetic core slug is not designed for AC use

jack

If all you want to do is have a UPS from a pair of power supplies then couldn't you simply use a diode in each of the supply legs (In effect an OR circuit).  If one supply fails, its diode will become reverse biased and effectively isolate that supply from the load.  If you want to be able to monitor which supply is providing power (and it could be both via load sharing), simply monitor the voltage upstream of the diodes; the higher unit will be the supply source.
jack

Liverpool's nowhere near the north-west of the UK unless of course you don't consider Scotland part of the UK (U stands for United)

Jack

Should have made it clear the tube won't actually bend, but there are bending forces on the tube which are readily measurable by the sensors.
jack

As any politition will tell you "there is a third way"

That's to use a strain guage rosette system whereby three strain guages, placed 120 degrees apart around a circular column will respond to both wind speed and wind direction.

This is an idea I dreamed up when I was at university (40 years ago) but have never seen it in practice.

Basically get a (very) thin wall tube of stainless steel at least 100mm diameter by about 1 to 2 metres long.  Mount it vertically with a rigid base.

It would be preferable if the tube bottom was cut away to create three legs but these would have to be identical, otherwise system accuracy would be affected.   Fix the three strain guages near the base at excatly the same height and spaced 120 degrees apart.

The resultant strain figures will represent bending due to wind speed and the direction of bending will represent wind direction.

So, that's the idea, all you have to do is put it into practice

jack