Subject: [TCRG] RSOH May-4-2001
Date: Sat, 5 May 2001 21:11:14 -0500 (CDT)
From: Alan Kilian
To: tcrobots@orbis.net (Twin Cities Robots Group)
Strobes, strobes, nothin' but strobes.
A couple of weeks ago, I rediscovered some fast strobes I have
that seem like a fun thing to play with trying high-speed photography.
See the last RSOH (Or so)
About 11 years ago, I modified a regular camera strobe, built a
sound activated trigger for it, and took some really cool photos
of breaking light bulbs, breaking balloons, and tossing things into
a fish tank, so I wanted to be able to duplicate those photos before
trying to do some REAL fast moving objects like maybe bullets.
So, I started with an electret microphone from Radio Shack, and an
LM741 OpAmp, and built an amplifier with a gain of something like
twenty.
This is the first time I have tried to actually design an OpAmp
circuit instead of just assembling one from a schematic. It's
tougher than I thought. I got a signal from the microphone, sent
it through a capacitor to block out the DC part of the signal, and
then into the inverting input of the OpAmp. I set the non-inverting
input to ground, and connected two resistors to set the gain.
HEY, I got a signal out! But not a nice signal, so I tried some
different things: Changing the capacitor value, changing the
microphone pullup resistor etc. After my lips got tired of
whistling to make a signal, I disconnected the microphone, and
connected up a signal generator. Then I could just trace things
with the scope. Things were not as obvious as the seemed. I had a
signal coming out of the signal generator, and a signal coming out
of the OpAmp, but there was no signal at the OpAmp's input!!!
So, I thought that I had just made an oscillator, but the output
signal changed frequency to match the signal generator's frequency
as I changed it.
So I upped the signal generator's voltage output. I finally got
to where I was pushing something like 12 Volts peak-to-peak into
the OpAmp, and the poor thing was slamming from rail-to-rail
trying to make a 200 Volt output, and then I could see a small
signal going into the OpAmp input. What's going on???
Well, as you have heard, I am in this U of MN Mechanical Engineering
class "System dynamics and control", and we have studied OpAmps
a little, so I made some diagrams, and ran through how an OpAmp
works, and HEY!!! Of course there's no signal at the OpAmp's input!!
There isn't supposed to be one there. It turns out that an OpAmp
has a gigantic amount of gain. Maybe 100,000 times the input would
come out of the output if you let it, and the feedback resistors
try to make sure that the input signal is always at zero volts.
(It gets a bit complicated, so I'm REALLY simplifying it for this Email.
don't get on my back because it really isn't quite like this.)
Anyway, the signal at an OpAmp's input is incredibly tiny UNLESS
you're abusing it like I was, and then you can see some leftover
signal that the OpAmp couldn't quite get rid of.
So, if you can see a signal at the input of an OpAmp, things are not
working right.
After all that I had an amplifier. It didn't work all that great,
If you touch things wrong, it goes into oscillations, and just
produces a tone output. I'll work on it some more to try and get
a better one next week.
So, I needed some way to look at this signal, clap my hands, and have
the strobe fire. The strobe has an optocoupled trigger, so if I can
turn on an LED, I can trigger the strobe.
Normally, I would have built a comparator, and take the output of that
and drive the optocoupler's LED.
But that would be to straightforward for us.
Jeff and Brynn came over, and things got really complicated.
What Jeff suggested is to use the oscilloscope's triggering circuitry,
and tap into that output and drive the optocoupler's LED.
Huh?
Of course Jeff just knows things. There is a connector on the back of
the scope (Who ever looks back there?) That wiggles whenever the scope
triggers. Hey, cool!!! We had to build a buffer for it before it had
enough oomph to drive the LED, but we were in business. We could
clap or whatever, and the strobe would flash.
So, next thing is to see if there is some way to use a digital camera
to see if we could go film less.
The idea is this: set things up, turn off the lights, push the camera
shutter button, and then while the camera is exposing the CCD, make a
sound. We didn't know how long the CCD would be exposing itself (Woo Hoo)
but it was worth a try.
It worked!! Even the cheap $175 digital cameras Jeff and I have worked.
The camera kind of goes "crick" and then you hit whatever.
Oh, man, we need to go back in time a few hours. Before we got the
sound trigger working, we experimented with a "contact" trigger.
That's just making sure that the object you want to take a photo of
causes two pieces of wire to touch when it's in the right position.
I tied a piece of solder to a quarter and connected it to one lead of
the strobe. I made a little loop of solder about 1 Inch above the
table, and connected it to the other strobe lead. Now whenever the
quarter touched the loop of solder above the table, the strobe would
flash. So if we drop the quarter, we should be able to capture it in
mid-air just as it touched the other loop of solder.
We tried this with Jeff's camera and my camera and it worked both
times. Then Brynn setup the mega camera. I don't really know what
kind of camera it is, but it sure is nice. He can TELL the camera
how long to hold the shutter open and everything. So he should be
able to post the images of the falling quarter.
Ok, back to the present. (This morning) We had the sound trigger
going, and we did the classic milk-drop photo. Thank you Doc Edgerton.
It worked great as you can see when Brynn and Jeff get the photos up.
Then we built an analog delay using a 74LS123 dual one-shot, and
played with that. You can see all the photos.
The point is that it WORKS with digital cameras!!! Yowee!
So, I'll try to fix the amplifier, add a sensitivity knob, build the
comparator and delay with knobs, get the whole thing into a metal
case with batteries, and then really start to have some fun.
-Alan
