Re: Beam interruption / time measurement

On Sun, 16 Mar 2008 04:56:10 GMT, "Bob Monsen" <rcmonsen@xxxxxxxxx>
wrote:

"Bob Monsen" <rcmonsen@xxxxxxxxx> wrote in message
news:AH%Cj.14273$5K1.3954@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
"Robert Barr" <not@xxxxxxxxxxx> wrote in message
news:mnICj.1144$LV5.560@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
It's been many a year since I've messed with electronics -- back when a
nice color television used maybe a dozen 6GH8A vacuum tubes. (Good
riddance!)

Anyhow, what I have in mind is a low-cost timing setup, used to measure
the time between two events -- both of which would be the interruption of
a light beam. Extreme accuracy isn't necessary -- 0.01 seconds would be
fine, so no high-speed industrial stuff needed.

I want to measure the amount of time a motorcycle rider uses (without
touching a foot to the ground) to cover a fixed distance, riding as
slowly as possible, for perhaps 20 feet. In other words, a 'slow race'.
You'll see these at various biker events, and it's a whole lot tougher
than it looks. Here's the best I've seen in a long time:

http://www.youtube.com/watch?v=HrJvv_L8WBw

I'm thinking of two pair of tripods, one for the start line and one for
the end, with each pair holding an opto-electric device and a reflector.

The cheapest opto-electric devices I can think of are the safety units
that automatically reverse a garage door opener. I have a pair (from an
old dead opener), and they're pretty cheap on eBay. The problem with
these is, I can't find any source of information about how these operate.
Perhaps it's deliberately arcane knowledge, so that it's difficult to
defeat their purpose.

Does anyone have a link or source of info on how I could use these
critters to my purpose? Or, for that matter, a simpler, cheaper way to
do the job accurately? I've seen timing kits specially made for racing,
but they're asking about $600 and up. I'm thinking of a small fraction
of that.

I also found these, after about an hour of web searching:
http://www.goldmine-elec-products.com/prodinfo.asp?number=B00002&variation=

... but it's hard to tell how much useful information they'd come with.

I haven't decided on what to use for the actual timer & output, but that
might be the simplest part.

(One guy suggested pneumatic switches. Old-timers like me can remember
when gas stations had hoses draped across the pump lanes which, when run
over by a car tire, would ring a bell inside the station. They had these
antiquities called 'attendants' who would come out and put gas in your
car. Honest. Then they'd clean your windows. Nobody believes me, but
it was very common back when 6GH8A's were about $3.50 a copy...)


You can get cheapo laser pointers for about $3 on ebay. Get two. Have them
laying on the ground at the start and the finish, pointing into a little
elongated box with a photodiode at the end. Build a sensor driver that
gives you a pulse when the beam is interrupted by building this:


10 MEG
___
.----------|___|--------.
| |
| |
| |
| |\ |
o---------|-\ | |\
| | >----------'----------|-\
| .-|+/ | >------.-- A
Photo - | |/ 9V .----|+/ |
diode ^ | | | |/ |
| | .-. | ___ |
o-------' 10k | |<--'-----|___|----'
| | |
| adj '-' 100k
| for |
| sensitivity
| |
| |
GND ----o------------------------'
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


Slight correction. I built the circuit above and tried it out, and it works
ok, but the sense is inverted from what you want. So, use this instead:

Use a 9V battery, and connect the 9V to Vcc on the opamps

10 MEG
___
.----------|___|---.
| | 100k
| | ___
| | .--|___|----.
| |\ | 10k | |
o--------o|-\ | ___ | |\ |
| | >-----o---|___|--o----|+\ |
| .-|+/ | >---o---- Out
Photo - | |/ 9V .---o|-/
diode ^ | | | |/
| | .-. |
o-------' 100k | |<--'
| | |
| adj '-'
| for |
| sensitivity
| |
| |
GND ----o------------------------'
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

That way, you won't be wasting your battery powering the output except when
the
beam is broken. So, the battery will last longer.

The opamp used in leftmost part is a TLC251C (page 227 of AOE, 2nd
Edition, Hardcover) However, I suspect that an LM324 will work just as
well, and will be obtainable at RadioShack. The rightmost opamp is hooked
up as a comparator, and will compare the input voltage to the reference
provided by the potentiometer. You adjust the potentiometer so that the
output at A is off when the laser pointer is pointing at the photodiode,
but on when it is not. You may have to play with it onsite to make sure it
is right, so you can also add an LED to the output so you can see if it is
on or off without measuring it. The LED should go from A to a 1k resistor,
and the other side of the resistor should go to ground. If you have a
multimeter, you can use that to calibrate it onsite.

It will give you a positive pulse when the beam is interrupted. Try to buy
a photodiode that has good response at the laser frequency. However, if
you are just trying it out, you could probably just scrape the paint off
of any old diode, and give that a try. All diodes have a response to
light, which is why they paint them.

Put the photodiode at the end of a little 'tunnel' about 6 inches long to
prevent ambient light from affecting it. The smaller the opening, the more
likely it will be to work.

Tape it all to the ground with duct tape.

Once you have two of these, you can get pulses out of each output. For
each output, attach a twisted pair to the output and ground of the
circuit, and run it to a central place where you want to do the timing.
Put a 1k resistor between the output and the cable. On the other end, hook
up each of the twisted pairs to an optoisolator. This will give you two
open collector 'active low' signals.

1.5V
|
.-.
| | 1k
optoisolator | |
.------------. '-'
___ Twisted | | |
A -|___|---- ------|-. -|--------o--- B
1k \ / | | | |
\ / | | |/ |
\/ | V -> | |
/\ | - |> |
/ \ | | | |
GND ---------/ \-------|-' -|--------.
Pair '------------' |
|
|
|
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Now, you need a display. If you connect both of the outputs to the same
wire, you have a 'wired or' signal. Now, pull the wire up to some voltage,
and you'll get downwards pulses when the lasers are interrupted.

Connect this up with a 1nF capacitor to your sound input on your laptop,
through a 100k resistor:


1nF 100k
|| ___
B -----||----|___|--------o tip (left channel)
||
NC o ring (right channel)

GND ------------------------o sleeve

(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

(See http://en.wikipedia.org/wiki/Tip_ring_sleeve for info on cables)

You could also attach the 'start sensor' to tip, and the end sensor to
ring to get the different timing pulses on different stereo channels.

Then, download a copy of Audacity (freeware). You should be able to see
the pulses when you record from the line input, and measure the distance
between them very accurately.

If you have a Java hacker around, you could write a program that listens
to the audio port, measures the input, and reports the results. They could
make it as elaborate as they wanted, perhaps showing a moving clock until
the final pulse. It would be a simple program to write, I suspect.

I haven not built this circuit, so there may be issues I have not forseen.
Some of the more experienced guys may care to comment.

Have fun!

Regards,
Bob Monsen

If you want a digital readout without having to measure
pulse distances manually, check out the Frequency Counter option in
Daqarta. You can make it as big as you want, and it has a period mode
that will read out the time in msec directly. Its accuracy is
determined by the crystal that controls your sound card sample rate,
which is massive overkill for this application.

Best regards,


Bob Masta

DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!
.

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