the hardware

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

The control board consists of three sections. The first is a simple voltage regulator to supply 5V to the microcontroller. The second is the microcontroller itself and the third is the eight constant-current drivers for the LED strings. The high intensity LEDs used in the helmet operate at maximum efficiency with a forward current of 20mA. The classic constant-current source circuit used in the design gives an output current of 0.7V/R which is 21mA for a 33R resistor. Should you only require 10mA LED current then the 33R resistors should be changed to 68R.

stripboard construction

This circuit is ideally suited to being built on stripboard but, should you wish to use a printed circuit board, a set of Gerber files for a single sided board can be found lower down the page.

Firstly prepare the stripboard as shown in the picture. Please note that this layout is the prototype and the extreme top and bottom strips are not required. In the prototype they carried the battery supply voltage, which is not actually required to feed the LED strings as it is wired directly inside the helmet.

The picture on the right shows the finished board mounted inside a helmet. The wire links (red) to the top and bottom strips are not needed as the pins on these rails are not connected. The IC socket, pins and screw terminal should be fitted first, followed by the resistors and capacitors. The IC socket is only soldered to the stripboard by the end and centre pins on each side. The centre pins are the power supply to the IC and must be linked to the rails running under the socket. Next, fit the wire links to the constant-current drivers. These pass through the holes adjacent to the appropriate pins of the IC socket and are then bent across and soldered directly to the pins. This is done in the area where the copper strips have been completely removed. Finally fit the 16 transistors, taking care to get the orientation correct.

Now it's time to test the board. A visual examination, to look for solder bridges and dry joints, is always a good idea before applying power to a new board. If the board looks OK then apply 12V to the screw terminals and check the power pins on the IC socket for the presence of 5V. Now connect a surplus LED between the 12V supply and any one of the output terminals. The constant-current driver can be tested by connecting a short piece of wire between the 5V pin and the appropriate output pin on the IC socket. Test all of the outputs using the same method. When you are happy that all the output drivers are working correctly, insert a programmed PIC16F88 into the IC socket and switch on.

printed circuit board

Here is a zip file containing the Gerber files for a single sided printed circuit board.

bill of materials

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

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