Start with a cheap industrial hardhat and end up with a WWRY helmet to "Blow Your Mind"!!!!




A set of four "Mind Control" helmets was required for Arran High School's production of WWRY in June 2010. An extensive web search failed to reveal a source from whence they could be purchased or hired, so it was out with the design tools and down to work. As a retired electronics engineer, it was a fairly simple task for me; but others might find it more difficult, so I thought I would share the process with you.

The aim was to produce a stand-alone helmet with flickering lights which could be controlled by the cast member wearing it or another cast member standing immediately behind. These requirements meant that the helmets would need a battery supply and an easily accessible switch.

The starting point is a standard industrial hard hat. There is plenty of space inside, between the soft plastic internal dome and the hard outer, for a small printed circuit board containing the electronics. After some experimentation I decided that 32 high-intensity 5mm LEDs would probably be enough and they could be connected in eight chains of four. Each chain is driven by a constant current sink on the control board and the eight chains each flicker with a slightly different pattern. A constant current drive is used for the LEDs so that they will maintain the same intensity until the batteries are finally exhausted. The flickering is controlled by a PIC16F88. The details of the control board can be found on the Hardware page. The Software page has a description of the software as well as a listing of the source code and a hex file to allow you to program your own devices. You will need to build one control board for each helmet.

A chain of four high-intensity LEDs has a forward voltage drop of around eight volts. Eight AA alkaline cells in series will give a battery supply of twelve volts allowing the batteries to be drained almost to exhaustion. This can also be conveniently split into two sets of four, thus distributing the weight evenly. The battery holders are mounted on the outside of the helmets towards the rear and have covers stuck on with double-sided tape.

The maximum current drain when the helmet is switched on is about 165mA. Good quality alkaline AA cells have a capacity of 2600mAh, so a new set of batteries will give in the region of 12-14 hours operation, which should be enough for a week-long show run.

construction

A mouse click on any of the images in this section will open a larger version in a new window.

First remove the internal dome then cut off the helmet peak and clean up the cut edge with a file or a Stanley knife. The next step is drill all the holes for the LEDs. Plastic LED clips were used, but these are actually more expensive than the LEDs going into them and you may wish to just hold the LEDs in place with some glue or silicon sealant. The plastic clips require a hole diameter of 6.3mm (1/4") but, if the LEDs are being located directly, a 5mm hole is fine. The picture on the right shows where the LEDs are located. The thickness of the helmet material varies quite considerably and care needs to be taken to ensure drilling in the thinner areas so that the LEDs can protrude sufficiently on the outside. Also drill a 6.5mm hole at the rear for the switch.


An area towards the rear, on each side of the helmet, should be left clear to allow space for the battery holders to be mounted. Once the holes are all drilled and any swarf cleaned away, the helmet can be painted if desired. Plasti-Kote spray paint was used to produce the gold effect, but it doesn't adhere well to the hard plastic of the helmets and is difficult to touch up. A brushed silver, gold or aluminium paint may be better, but it would be wise to experiment first using the peaks removed earlier as test pieces.

When the paint is hard, the LEDs should be mounted. This is the most tedious part of the construction process and care must be taken to avoid damage to the paint finish. The picture of the finished helmet shows some paint damage, but this wasn't visible to the audience and didn't detract from the effectiveness of the effect.






The LEDs are connected as sets of four in series as shown in the block diagram on the left. The next picture shows how they are wired together.




This picture shows how the LEDs are wired. It also shows that not all of the LED clips needed to be fitted with rings. Those which did, are in the thinner areas of the outer shell. Although the visual effect is less random, it is easiest to keep the LEDs in each string adjacent, so that the component legs may simply be bent over and soldered together. Spreading them around the helmet means a lot more wiring. Regardless of the placing, they must always be connected with the anode of the first to the positive supply; cathode to anode along the string with the last cathode connected to a pin on the driver board. The inside view shows that several pairs of adjacent strings have their anodes commoned to simplify the supply wiring. Each string comprised a pair of colours and the colours were distributed all across the helmet.


Once all the LEDs are secure and connected in strings, the control board should be mounted. A strip of 6mm plywood was used as a base secured with two 3.5mm x 8mm screws from the outside. The head of one of these screws can be seen in the picture of the finished helmet. The control board should be securely attached to this mounting strip.

Connect a jumper wire between the two battery holders, so that they are in series. Connect the negative battery supply to the appropriate terminal on the control board, the positive battery supply to the switch and the switched supply to the control board and first anode of each LED string.

Finally, wire up the cathodes of the LED strings to the pins on the control board.

Now it's time to put in the batteries and check it works! As the helmets may be subjected to high g-forces when they are inevitably dropped, it is wise to secure the batteries in the holders with a strip of gaffa tape. Then stick the covers on top of the gaffa tape with a strip of double-sided tape. The covers can be cut from empty margarine tubs and painted to match the helmet.


Now there's just one more thing to do. Cut off the rear neck strap section from the inside dome and put the dome back into the helmet. It is best to fit this backwards as the padded section, which would normally be against the forehead, will cover the switch terminals.



bill of materials


The part numbers in the bill of materials were valid at the time of writing (June 2010) but prices may have changed or components become obsolete since then.



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