Each screen was made of wood with an 8" x 8" opening (shooting
area) and has circuitry in both the top and bottom panels (Figure 4).
An opening of this size should be no problem at say 10 ft. Just don't
get nervous when you shoot or you will be building another screen!
The three screens are mounted precisely one foot apart using right
angle brackets on a 12" x 30" wooden board.
The top panel uses a Chronograph_Photodiodes_IRleds PCB
populated with the four photodiodes D2-D5 positioned to sense the
illumination from below. No wire jumpers are needed since the board
puts them in parallel. This board is then wired to the Chronogrph_Sensor
PCB, which is populated with U1, U2 along with the Rs and Cs. Note
that this board has three wires soldered down to the bottom panel
Chronograph_Sensor board. The bottom panel uses the
Chronograph_Photodiodes_IRleds PCB populated with the four IRLEDs
(D6-D9) and is positioned to radiate up.
Four wire jumpers must also be soldered in to
form a series string of the IRLEDs. Originally, I
designed in 10 IRLEDs and through experimentation
found that four is sufficient. This board is then
wired to the Chronograph_Sensor_PCB board,
which is populated with U3 and R11 and has five
wires soldered to the eight pin male plugs which
connect to the µC unit. NV
1 phanderson.com LCD #117 Kit
3 onlinelabels.com #OL175WJ
4 Sherwin-Williams KRYLON Crystal Clear
An interesting and informative application of this chronograph
is to measure the velocity of a falling object due to the earth's
gravitational force. The instantaneous velocity Vi of a falling object
that has traveled a distance D is given by Vi = 2* g D where g is
the acceleration due to gravity and D is the distance traveled. So,
using our chronograph with the three screens in the horizontal
position and dropping say a marble from just above screen1, the
displayed Velocity_1 reading should be 8.025 and the Velocity_2
reading should be 11.349. Since the present code only displays
whole numbers, it should display 8 and 11, respectively. Note
that in this application Velocity2to3 should be greater than
Velocity1to2 since there is positive vertical accelleration (g).
■ FIGURE 4
June 2009 39