Fahrenheit or Celsius.
• Have an option to
report the minimum and
• Drive several bright red
and white LEDs.
• Operate from a 3. 7 volt
lithium-ion battery or
any other power source
that can supply three to
• Run continuously for at
least two hours.
• Have a pushbutton
switch to set options and to select the flashing
• Accommodate at least four operating modes.
• Be small enough to be mounted on or inside of the
vents of a helmet or in a small enclosure.
• Be light enough to be unnoticed by the user.
■ PHOTO 3. Note the
three distinct LEDs.
■ PHOTO 2. White LEDs.
The three white LEDs that I placed on the front of the
helmet are 1/2 watt devices that are unusually bright (see
Photo 2). Each 5 mm LED enclosure contains three
discrete LEDs. Photo 3 was taken through a red filter and
shows one of the LEDs operating at a very low voltage.
Because the intensity of the LED’s light has been
dramatically diminished, you can clearly see the three
points of light from each of the LEDs and the wires that
are connected to them.
These LEDs provide a very bright light and each one
only draws about 60 ma; they can be clearly seen in
Photo 1 which shows the front of the helmet.
The red LEDs for the rear are also rated at 1/2 watt.
They are 10 mm units that throw a bright directed beam.
One red LED faces straight back with the other two being
aimed a bit to the sides. This arrangement gives maximum
visibility. The vent to the right in Photo 4 houses the
circuit board and the sensor.
from its output pins. A transistor is inserted between the
microcontroller’s output pins and the LEDs to provide the
necessary current. Any general-purpose NPN transistor
should work. My first design used two 2N2222 transistors.
I could have gotten away with using only one transistor
but decided that it would be simpler to use one transistor
to drive the three front LEDs and another to drive the
three rear LEDs. This effectively doubled the current
handling capacity and saved me from experimenting with
series resistors that were needed to match the different
LEDs so that they could be operated by a single transistor.
This arrangement worked quite well and was used for
several months. I continued to experiment with the circuit
and discovered that I could improve battery life and LED
brightness by replacing the 2N2222s with N-channel
MOSFETs. The 2N2222 transistors have a fairly high
internal resistance and dropped the voltage to the LEDs by
well over 0.5 volts. MOSFETs, on the other hand, have a
very low internal resistance and there was very little
voltage drop through them. I was pleased to find that the
MOSFETs worked well when substituted for the 2N2222s
without any changes to the circuitry (see Figure 1).
A PIC12F683 ties the hardware together by reading
The Temperature Sensor
■ PHOTO 4. Helmet back.
Temperature readings come from a Dallas
Semiconductor DS18B20. This device uses a 1-
wire protocol to communicate with the
microcontroller and is factory calibrated to an
accuracy of ± 0.5 degrees C. Routines to read
these sensors are readily available on the web
and I had no difficulty interfacing it to the PIC.
Getting Power to the LEDs
The LEDs that I am using draw much more
current than the microcontroller can supply
August 2010 33