If you ever wanted to know the velocity
of a projectile leaving your paint gun,
rifle, pistol, BB gun, slingshot, or arrow
then this is a project for you. This is a
great project for the marksmen or
hunter that loads his own shells.
Last year, my son bought a chronograph. I was so
intrigued by his demonstration as to how it worked,
that I began to gather information from the web and
finally designed my own. The info obtained pertaining
to IR LEDs, IR photodiodes, LCDs, EEPROMs, and RS-232
communications were most useful in the development
of this project.
This ballistic chronograph rivals expensive units with
some very impressive features:
By David Collins
All the modes are accessed via two pushbutton
switches (ENTER and END) located on the front panel.
In SHOOT mode, the two velocities are displayed in big
numbers and readable at greater than 10 feet. The
complete design is powered by a battery pack of eight
AA cells. The unit will display this voltage and beep if it
is less than 7. 5 volts.
➤ Measures velocities (ft/sec)
➤ Calculates the average
➤ Standard deviation
➤ Minimum and maximum velocities
➤ Saves data to EEPROM and displays on an LCD
There are two velocities measured for each shot as a
double check confirming the measurements. The majority
of chronographs on the market today only measure one
velocity. Only the very expensive ($700 to $800 range)
measure two velocities. This chronograph has a maximum
number of shots per series of 100 and you can record a
maximum of 10 series.
The modes of operation are:
➤ SHOOT — measures, displays, and saves the
velocities of each shot.
➤ RETRIEVE — displays the shot number, series number,
the two velocities of each shot along with min, max, avg,
and standard deviation for the series.
➤ SAVE TO PC — saves all the data mentioned in
RETRIEVE to a file on your PC via the PC serial port; use
the Hyper Terminial application to receive the data and
then copy it to your file (computers w/o a serial port
could use a serial to USB converter).
➤ DELETE A SERIES — deletes all the data in a
➤ DELETE ALL SERIES — deletes all data in the 10 series.
36 June 2009
THEORY OF OPERATION
This project has two major parts:
➤ The three screens
➤ The microcontroller (µC unit)
The screens consist of photodiodes and infrared LEDs.
The heart of the chronograph is the photodiode whose
reverse current is proportional to the amount of incident
light. Therefore, if you reverse bias the photodiode and
interrupt or diminish the amount of light as seen by the
photodiode, the reverse current will decrease. The momentary decrease in reverse current develops a change in
voltage across R10 (Figure 1). This change in voltage is very
small (approximately 12 mV), so the op-amps U1A and
U1B (LM224) amplify it and feed it to the comparator U1C,
producing a TTL level trigger pulse which is sent to the µC
for timing purposes. This trigger pulse is also sent to a
monostable multivibrator U1D and illuminates the green
LED for about one second. Not having a single shot digital
sampling scope, I added this during the development stage
in order to have visual indication of a detected pulse.
Screen: The term screen comes from the very
early chronographs where they used a metallic
screen material and sensed the change in resistance
as the projectile passed through. The chronographs
of today are mainly optical in design.