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The construction a single needle instrument.

After making the first instrument I found out it is very interesting and satisfying work and it taste to more. In my previous article I described the construction of a dual needle instrument based on stepper motors. This single needle instrument will be driven by a air-core. The communications part will be the same.

What is a air-core and where to find it.
Most of us will think they never heard or seen these things, but this isn't the case at all, every one who drives a car comes in contact with it ever day although under a hidden form. It is hidden in the cluster of the car. Most instruments like the fuel indicator, motor temperature and even the RPM meter are driven by these small and very stable air-cores.
The ai-core is a more complex form of magnetic coil.
A air-core is build up out of a small magnet that is surrounded by two coils


By putting power on one of the two coils the magnetic field changes and the centre magnet is forced to move. When we now send a controlled magnetic wave (sine, cosine) trough the two coils we can manage the exact position of the needle with very high precision. This is the analogue approach.
You can achieve the same result in a digital way by using pulse width modulation. The trick is to switch the power on and off so fast that only the average of the power counts as the input power.
An example to make this clear.

On 20% of the time: Average voltage = 0.20 * 5 volt = 1 volt


On 80% of the time: average voltage = 0.80 * 5 volt = 4 volt


This last method will be used in the instrument. Because we will again use a PIC, it is obvious to use the digital approach.
Where to find these air-cores. Like mentioned above, in cars. I went to a garage and asked if they didn't have an old cluster somewhere laying around ready to scrap. Without any problem they gave me one.
I took out the three air cores. After a closer look I remarked that these air-core had only three connections in stead of four. They have put the two Ground wires together and formed one connection. I couldn't use them this way according the schematic I needed 4 connections. I modified them by de-soldering the common ground wires and soldering this to a separate connector. By doing this I made two separate coils. This is what we needed.
Before connecting the air-core to the electronics you can make a simple test to see if it's working well. First you take an ohmmeter and check if the two coils are well separated from one another. Then you check the resistance value of both coils. They should have approximately the same value. If all this is OK then you can connect one coil to a 5v power supply and now the air-core should move. You can repeat this with the second coil. You will notice that the needle is moving very vigorously. After this test you can be sure that this item is working perfect.


The Housing:

The housing of this instrument is build up like the previously build two needle instrument. It exist out of aluminium plat of 1.25 mm, acrylic lens of 3 mm and 6mm, studding of M3 and different tubes from 4mm diameter



The electronics:

The whole thing is made out of a connection chip (MAX489CPD), a PIC (16F628-20), a volt regulator (7805) and a coil driver (L293D). Add to this the necessary resistors, condensers an a crystal and the whole thing is ready for use. All this can be soldered on to a small print board of 82mm on 82 mm. Add to this a 15 pins connecter and the job is done.


PIC program and Visual Basic program:

All this is done on the same way as the previous instrument. Only the firmware of the PIC is not so elaborate as the stepper motor instrument.

The end result looks like this.


The hydraulic gauge of the flaps together with the hydraulic gauge of the gear will be mounted on the right side of the cockpit. This will make the cockpit even more complete.

With this type of instrument you can also make all kinds of one needle instruments as long as they don't move more then 320°. So it is perfect for a airspeed indicator, vertical speed indicator and so on.

We can now say that we can virtually build all types of instruments based on these two examples. Of course only for those who have two right hands. Certainly a good alternative, it is much cheaper than the stuff you can bay on the market and much more versatile because you can design your own faceplate. Of course programming it is a bit more difficult.


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© Verley Jan 2007-2017 original text from 2006 translation Jan Verley