A ballistic chronograph is used to detect the speed of a
projectile shot through a sensor port. The simplest type of chronograph
design is based on finding the average speed of the projectile between
two ports. This is as easy as dividing the distance between the two
sensor ports with
time taken to travel between the ports. In practice this is done
with by simply
taking the time from sensor A triggers until sensor B triggers,
assuming this to be the time the projectile spent traveling from sensor
A to B. The distance between two sensors is assumed to be a constant
value known with great accuracy. The speed can then be determined by
the simple formula V = S / T. I've made two versions of the chronograph
using different display technology. One uses a vacuum fluorescent
display, while the other uses a more modern LED display, both of which
were found in microwave ovens. The two displays I used differ from
regular 7-segment LED modules by being divided into grids, rather than
individual digit modules. A grid is basically a single 7-segment
display unit used to show one digit. This means that the dot points and
hour/minute separator are actually contained on their own grid, instead
of being connected to each digit. The most recent model uses the LED
display, and is programmed using WinAVR. The basic hardware and
firmware is the same in both versions, with the only difference being
in how the display is driven.
The
basic principle behind driving both the VFD and LED displays is
multiplexing. Since the
number of I/O pins is limited, you have to get creative in order to
light up the entire display. Initially, it would appear to take 4
digits multiplied by 7 segments = 28 I/O lines to drive a four digit
display. 32 lines if you want a dot point! However due to something
known as persistence of vision, not all of the digits need to be on all
of the time for the display to appear fully lit. This leaves
room for switching one digit on at a time, and therby sharing the same
data lines. Suddenly you only need 7 + 4 I/O lines to drive an entire
four digit
display! Since the displays I used were taken from microwave ovens,
they were complete modules instead of individual digits. The dot
points were shared by the a,b,c,d and g segments, so a separate
dataline for the dot was not required. Instead another anode controller
was needed. Changing the 7segment.h library for regular displays would
only require that you alter how the dot is drawn, and reduce the number
of grids to four. The basic hardware setup can remain the same.
In this design, a digit is lit by switching on a 2N3906 which provides
current to the segment anodes in the digit. The ATtiny2313 then looks
up which pattern to use, and inverts it. A dark segment is held high,
and a bright segment is held low. The digit is held on for 1ms, before
moving on to the next digit. Since there are 5 grids on this display, each digit is only for 20% of the time.
The
display was mounted slightly inside the case to provide shielding from
direct sunlight, doing so improves the contrast greatly. The sensor
gate used to measure projectile speed isn't very high tech, and
consists of two photogates mounted in some PP pipe. The LM393 circuit
uses roughly 1000ns to respond to a
signal change, and may present a significant source of error. If each
comparator has an equal delay nothing is added to the error. If on the
other hand, one comparator is faster than the other the difference will
between the two will add to the total measurement error. I haven't
looked into measuring the response time of the detector circuit, since
the worst case total error (assuming one sensor triggers early, and the
other late for example) is only 0,01% at 100m/s. Firing into a tube and
hoping to break the beam isn't easy, even with a large projectile. For
a future sensor gate, I've thought of making a setup similar to the one
used in commercial chronographs. The idea is to use an IR LED to
illuminate a strip high above the photodiode. The photodiode is of the
narrow angle type, sensing in a long, thin line. The detection
circuitry could consist of a high-pass filter and discrete amplifier,
hopefully reducing delays. If anyone has made something clever like
this, let me know!
Previous VFD Version
This project started with me finding two
identical VFD displays while
gutting a bunch of microwave ovens. Since both of the microwave
oven display boards had a vacuum fluorescent display and mains
transformer designed for driving the VFD they were ideal for a project.
I was keen on using them for something, but without any immediate use I
just put them in storage where they stayed for a year. Then after
learning how to program in assembly for AVR and acquiring a discounted
STK500 through my university, I thought I could put together an
accurate speedtrap for an upcoming coil gun project. Note: This project has been superceded. If you want to use a VFD, take a look at my UV timer box
to see the proper way of doing it. Using the circuit presented here
does not drive the display with enough current, causing it to be very
dim.
VFDs are most common is Stereos, Microwave ovens and VCRs. They are
actually tube devices and require a filament voltage and "high" anode
voltage to function. Most commonly the filament runs at around 200mA or
so, while the anode voltage required is between 15 and 30V. The
filament produces a cloud of electrons like in any tube, and the
voltage difference between the filament and anode accelerate electrons
to the anode segments. Holding some segments at 0V and other at the
anode voltage allow one to create digits. A VFD has several "grids" as
well, which function as a switch between the anode and filament. To
activate any of the segments in a VFD one or more of the grids must be
brought to the anode voltage. This allows the electrons to flow from
the hot filament to the anode segments, exciting the phosphors and
giving light. So to write a digit to the display a grid must be
selected, and then the correct anode segments must be selected. The
pinout of a VFD is usually easy to find. The filaments will have pins
on each end of the display, and the resistance across them should be 10
ohms or so. The grids can be found simply by looking at the grid and
following the lead from under the glass out the the pins. The anode
segments will have a schematic on the back, or they can be determined
empirically. Given the
strange voltage requirements of a VFD, a level-shifter must be used
before
interfacing with 5V logic chips. This can be built by simply using a
few transistors and resistors.
So after some unsoldering, examination of the circuit board and probing
I discovered the pin-out of the transformer and VFD. Shortly afterwards
I had a 23V and 5V voltage source with filament supply, along with
a 5V TTL compatible VFD. The next step was to make the actual
detectors, which ended up just consisting of an IR LED/phototransistor
pair and comparator setup. Each pair was mounted exactly 15cm from
eachother in a 3cm diameter PVC tube. The IR LED and phototransistor
are placed on the same side of the tube, so that a passing object
increases the amount of IR detected by the phototransistor. Once the
amount of detected light passes a certain threshold the sensor will
comparator wil trigger.
After discovering the difficulty of
something as simple as division in assembly, I chose to program in
Basic instead given some past experience with basic for PICs. MikroBasic
Pro from
mikroElektronika was used for the programming.
The program itself is rather simple, with the most complex and process
consuming portion being multiplexing the display. The program
continuously takes the value in the speed variable and converts it to
single digits. The digits are converted to a corresponding output
pattern which is sent to the segment control pins (abcdefg) and the
correct grid is chosen. The digits are then sent individually to
the VFD, lighting one digit place at a time in quick succession, which
is
known as multiplexing. Due to presistance of vision the digits appear
to be in one place the entire time. Interrupts on INT0 and 1 start and
stop the timer respectively. The INT1 interrupt also takes care of
calculating the
speed. Timer 0 is used and setup for Compare Match mode at 240, which
gives are period of 83.3kHz. This results in 12µs increments in
the counter value. On top of this, timer0 increments every 50ns amd is
used to create a small decimal value when done timing, giving 50ns
resolution. This is done by taking the value in the timer's register
after it has finished counting, since this value will not have been
sufficent to trigger an overflow, but still represents the recorded
time. Currently it is coded it to read speeds from as low as
0.19m/s
up to 99.99 m/s before the number of digits on the display run out. The
precent error from 0.19 to 99.99 m/s should never exceed 0.01%. The
chronograph
starts up with 0.00 on the display, and will show the speed of a
projectile immediately after detecting a shot. The last measured speed
will stay on screen until the next shot, giving good time to read it.
The program assumes the sensors are spaced 15.00 cm apart. How
accurately you can manage to space the sensors will have an impact on
the error of the measured speed.
I left a little 4-pin connector sticking
out the side for quick sensor replacement. That way mechanical or audio
sensors, or another type of optical sensor can be
swapped in place on the fly. The inputs are TLL compatible at 5V, and
are trigger on a rising edge. The source code, schematic and a little
calculator are provided for download.
Modifying the code for regular LED 7-segment displays should be very
easy using MikroBasic
Pro. Remember to cut out the grid and segment code used for writing to
the dot if you chose to do this! The circuit required for driving a LED
display compared to the VFD
will be significantly simpler too. But hey, you can't spell obsolete
without 1337.
Disclaimer:
I do not take responsibility for any injury, death, hurt ego, or other
forms of personal damage which may result from recreating these
experiments. Projects are merely presented as a source of inspiration,
and should only be conducted by responsible individuals, or under the
supervision of responsible individuals. It is your own life, so proceed
at your own risk! All projects are for noncommercial use only.
This work is licensed under a
Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
