■ FIGURE 2.
By the time you finish, you will definitely know how to
solder surface-mount components. There are 196 small
photodiodes and 49 small LEDs. I would build the target
first, then prepare the small laser printed circuit board
(PCB).
On Target
The target measures 3. 5” x 3. 5” and uses photodiodes
to detect the laser. The laser has a 6 mm spot at 40
meters. In a nutshell, a 1/2” square uses four photodiodes
which are placed apart and electrically connected in
parallel. An LED is located in the center. A grid is set up
using seven rows x seven columns, or 49 sets of diodes.
The 3/16” clear red plastic allows the red laser to
penetrate and blocks most of the ambient light. The .01
mm thick white translucent piece of plastic causes the
laser to diffuse and trigger the photodiodes (see Figure 1).
A Microchip PIC16F917 is a 40-pin microcontroller. It
has 25 inputs and output ports. Sixteen of these ports are
used to detect the laser pulse by multiplexing the
photodiodes. Once the hit is determined, they turn on an
LED indicating where the hit was. Eight of the other ports
are used to multiplex a three-digit display. The seven-segment display shows the accumulated points.
When light strikes a photodiode, it conducts
electricity. The collectors are biased to Vcc using a 3.3K
SIP (Single Inline Package). The 3.3K SIP used in this
project has eight pins and seven 3. 3 resistors (bussed), all
connected to a single source. There are seven rows and
seven columns as mentioned previously. The outputs of
the rows are multiplexed using port B and the collectors
are multiplexed to ground using port A. In the center of
each square is an LED whose polarity is reversed across
the photodiodes. The microcontroller
runs at 20 MHz and scans the 49
squares in 68 microseconds. When a
hit is detected, the microcontroller
stops the scanning. It turns on the LED
in the square where the hit was
detected by reversing the polarity of
the column and row. It determines the
points in the area of the hit and adds
them to the score. The hit and score
stay lit for five seconds and then the
unit starts scanning again. The score
can be reset by pushing a reset button.
This is connected to the MCLR (master
reset) of the microcontroller.
It can be powered by four AA, C,
or D batteries, or a five volt power
supply can be used. A jack is provided
for the power input.
The three seven-segment displays
are common anode and are multiplexed using three PNP
transistors. The grounding of the cathodes is performed
via eight 470 ohm resistors going to port D. The 470 ohm
resistors are in a network which is similar to a SIP;
however, they are contained in a DIP (Dual Inline Pin) and
are not connected to each other. There is a DIP switch
which can change the display for testing, slow fire, rapid
fire, or your own program. (Up to eight programs are
available that will be discussed later.)
Building the Target
Because of the small surface-mount photodiodes and
LEDs, use a small tip soldering iron and magnifier (see
Figure 2). Place the four feet on the bottom side of the
board next to the mounting holes. This will prevent
scratching the traces and keep the board from sliding
when soldering.
Solder the photodiodes and the LEDs to the board.
The photodiode’s marker dots (collector) face to the left;
the LED cathodes all face down and are also marked with
■ FIGURE 3. The target printed circuit board.
May 2010 31
