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From: firstname.lastname@example.org (Rob Paisley)
Subject: Re: 0-100s timer using 555 IC and panel LED
Date: 13 Sep 2002 06:40:00 -0700
NNTP-Posting-Date: 13 Sep 2002 13:40:00 GMT
email@example.com (Jeff Lawlis) wrote in message news:...
> I am a Physics teacher with a basic understanding of electronic
> circuits. I would like to build an elevated ramp with adjustable
> slope, whereby an object would be allowed to roll down the slope. At
> 5 places along the ramp, I would like to incorporate phototransistor -
> LED (focused) pairs, such that when the light beam is interrupted, it
> activates a relay. The main goal would be to time the object at
> different locations, in order to caluculate the velocity at each
> position. I would like to use four digit LED diplays, displaying time
> to the nearest hundredth of a second.
> My questions are as follows:
> a) I was thinking of using a 555 timer in astable mode,
> operating at a frequency of 1000 Hz. This would be coupled to
> appropriate counters and IC chips to convert to 7-segment LED display.
> What is the "real" accuracy of such a circuit in this mode?
> b) Can I use the output of one 555 timer with 4-5 counter
> circuits (one for each group of LED displays)?
> c) Once you remove the input from a counter chip, does that
> "freeze" its output? I need a way of freezing the time at each
> location, once the light beam is interrupted.
> I know this is an ambitious project for me, but the kids would love
> it, I can guarantee!
If I remember my high school physics, the actual time for this sort
of experiment is not too important the relative time is what counts.
You could set the oscillator to give approximately 1000 pulses in
the time it takes the object to reach the bottom of the slope. (Set
the needed pulse rate by running the experiment.)
Then have the input to the counters switched off as the object
breaks each of the light beams. An AND or NAND gate with a latch on
the sensor input would work.
You will also need a sensor and gate to start the counters when the
object is released.
You can drive any number of counters from the output of the 555 and
The more basic problem is obtaining suitable, low cost counters.
After that its just a matter of designing the circuit.
If real times are needed for the experiment a better clock system
will be required and displays that can show the time in hundredths of
Some sort of electronic stop watch/timer with electronic START/STOP
inputs would be ideal as they could be put to use elsewhere in the
class for individual work when not running this experiment. (Check
with the PhysEd department to see if they have heard of such a thing.)
Again, the timers would all be started with one sensor and then
stopped with separate sensors. A common reset would also be good.
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