is located for easy sighting (Figure 4).
Guides and Diffusers
Cut two 2” x 8” x 1 mm Styrene for the diffusers. Cut
four 3/8” lengths of 3/16” Styrene tubing for the diffusers.
Ream them with a 1/8” drill to clean out the burrs. Glue
the four 3/8” tubes to the center line of the Styrene strips
1/4” from each end (see Figure 5). For safety reasons, I
chose to use wooden dowels for the supports instead of
1/8” welding wires or 1/8” aluminum tubing. Cut four 8”
lengths of the 1/8” wooden dowel.
DO NOT USE THE CHRONOGRAPH IN THE
PRESENCE OF FLUORESCENT LIGHTS! THEY WILL
TRIGGER THE SENSORS AT 120 TIMES PER SECOND!
I have added a switch and a program so that you can
see how much light is falling on each sensor. To get to this
mode, hold down S1 and plug in the cable. Switch the
toggle switch to the voltage position.
Make sure you have at least .02 volts before setting
the pots. Don’t forget to change the toggle switch.
1. You should be in a sunlit room. The diffusers do not
need to be on.
2. Hold down S2 (right switch) and plug in the power.
The unit will default to “TEST.”
3. Turn the sensor 1 pot CCW to decrease the
sensitivity. Turn until the right LED turns off. Using your
hand, pass over sensor 1 about two inches. The LED
should turn on and off.
4. Do the same for sensor 2.
Now, let’s take an object with a known velocity.
Gravity is pretty constant. (I know, it does vary with
altitude.) An object accelerates at a rate of 9.80665
meters per second.
Using the formula:
13” = .3302 meters, which gives us 1.27
meters per second or ≅ 4. 17 foot per
Mount the black box on a tripod using
its mount. Turn the unit pointing down with
the cable coming out the top. Add the
Plug the cable in the display unit; it
S2 FOR STATS
If it shows the FPS display, press S2 to clear. A lit LED
indicates that a sensor has been tripped. There should be
no LEDs on prior to shooting. If there is, clear everything
by pressing SW2. If the LEDs are persistently on, check for
the presence of fluorescent lights or adjust the sensitivity.
Drop a marble or object through the diffusers. The
display should show (depending on the height of the
object above the first diffuser):
FPS ≅ 4. 2 #1
MPS ≅ 1.3
Pressing S1 will place #1 in the storage register; return
to “SHOOTING.” Do this 10 times storing each result.
When you reach the end of file, it will let you know so
you can review the stats by pressing S1. It will first display
the average, then the maximum with its shot number, and
then the minimum with its shot number.
Each storage register will be displayed until you reach
10. It will give you a choice to repeat or to erase. Pressing
S2 will just return you to “SHOOTING” and not store the
1. Make sure you have a good back stop.
2. Use safety glasses and hearing protection.
3. Always keep the gun pointing down range; pushing
the buttons and reading the display can be a distraction
when attempting good gun control.
4. Keep in mind that some gun sights add two inches
above the gun barrel. Sight the chronograph using the
5. Consider using the diffuser when shooting to block
the sun from shining directly into the sensors.
6. You may want to have a pad and pencil for notes,
especially if storing different gun parameters.
7. If there is no reading, the first thing to check is that
the toggle switch is not in its voltage position.
Aim your shot down the center of the black box
about 2”- 4” above the top. The chronograph should be at
22 March 2017
■ FIGURE 5. Chronograph diffuser.
√2gd Velocity average = 2