Also make sure that you keep a safe distance from any
moving object that you are trying to “freeze” using the strobe
light. The idea is to have fun and not to get hurt.
The VEX pushbutton switch connected to IO12 is a good
example of a digital sensor that simulates our sense of touch,
and which has two states: ON = 0 or OFF = 1. It is used in this
experiment to turn the strobe light on or off when toggled.
The example provided with this article not only controls the
flash rate, but also doubles as a motor speed control or servo
position dial using Listing 1 as a starting point. Now that the
strobe light hardware is completed (including the strobe light
frequency setting dial as shown in Figure 5), we can
download the Easy C Professional firmware necessary to
control the strobe light using sensors. The sensors include a
potentiometer or a quadrature encoder which is used to vary
the strobe light flash rate (frequency). The VEX quadrature
optical encoder that is connected to interrupt 1 on the
Interrupts block and IO10 on the Digital Analog block provides
the frequency control in flashes per second. It is convenient to
attach a small VEX gear to the encoder shaft and use it to dial
a particular strobe light frequency. By making a small change
to the firmware, you can use the potentiometer connected to
IO11 instead of the optical encoder to control the strobe light.
Operating The Strobe Light
Operating the strobe light requires an alkaline nine volt
battery for maximum brightness. The VEX bumper switch
toggles the strobe light on or off. The optional toggle switch is
used to change to high power “party” mode by shorting the
resistor and allowing extra current to flow into the LED. Make
sure the LED is flashing and not on continuously, and
use this mode with caution since it can shorten the life
of the LED.
Use the potentiometer or quadrature encoder to
increase the flash rate by turning it clockwise (CW) or
decrease the flash rate by turning it counter-clockwise (CCW).
Note that the original VEX encoders did not provide the output
necessary to determine the direction. Only the newer
quadrature optical encoders have this capability.
FIGURE 4. The completed VEX strobe light using only a single bright white
LED flashing brilliantly, although it can easily be expanded to handle up to eight
flashing LEDs with enough light to brighten any party.
FIGURE 3. The single bright white LED
shown in this figure should be mounted so
that it is kept away from any flammable
materials since it can get very hot.
FIGURE 5. Shows the completed VEX strobe
light hardware that will be used to carry out
stroboscopic experiments including the strobe light
frequency dial attached to the VEX
SERVO 08.2011 65