CHAPTER 10 ■ LEDS
Forward Voltage Drop
The voltage value displayed on the meter is called the forward voltage drop. That is, if the electricity is
moving in the correct direction (forward), then this is how much voltage the LED consumes (drops).
Obviously, voltage usage is valuable information for any component you want to use in your robot. This
gives you an idea of minimum battery voltage requirements and how many you can string together for a
given battery.
When purchasing LEDs, beware of those listed as having forward voltages (Vf) above 4 V. For example,
some brighter LEDs contain multiple LED chips in a single package, and therefore require double (or higher)
voltages. Advanced robot circuits operate at 5 V or lower and many microcontrollers won’t guarantee
their outputs will always exceed 4.5 V. This makes it impossible to illuminate 12 V LEDs without additional
circuitry.
Variety Pack
Order some assorted packages of LEDs (see Table 10-2). A mixed bulk bag is an inexpensive way to become
familiar with sizes, colors, shapes, and lenses. Also, because you may damage some LEDs in your early
experiments, it’s more comforting to blow a $0.09 red than a $2.99 ultra blue.
Before buying a large quantity of a single kind of LED (as opposed to a variety pack), it’s best to begin
by ordering only a few of that type to insure the overall light output and quality meet your needs. You’d be
surprised how off-color and spotty some LEDs can be.
Brightening Your Way
Sandwich, the line-following robot, lights up with eleven different LEDs (see Figure 10-18). Three green
LEDs and three yellow LEDs indicate the brightest path. Three red LEDs sit inside the motor tube to
illuminate it from the inside, purely for fun. Two white LEDs act as headlights, for following lines at night or
through tunnels.
Table 10-2. LED Assortments
Supplier Part Number Price Number of LEDs
Electronic Goldmine GP27 $2.49 50 assorted (although some surface-mount)
Electronic Goldmine GP36 $2.49 15 uniquely shaped assorted
Jameco 18041 $7.95 100 standard assorted
Electronix Express 32LEDGRAB $6.40 100 variety assorted grab bag
Electronix Express 3200LPK $12.95 75 variety assorted
Electronix Express 3200LPKD $19.00 105 deluxe assorted