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From: Fred Bloggs
X-Mailer: Mozilla 4.7 [en]C-CCK-MCD EBM-Compaq1 (Win95; U)
Subject: Re: 0-100s timer using 555 IC and panel LED
Date: Fri, 13 Sep 2002 11:47:38 GMT
NNTP-Posting-Date: Fri, 13 Sep 2002 04:47:38 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
Jeff Lawlis wrote:
> 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!
This is too cumbersome and inflexible for implementation with discrete
logic. Your time is much better invested in configuring any number of
PIC or microprocessor development kits for the job. You will still need
to design the semi-analog beam break detectors and a logic-OR into the
board. Take a look at any issue of Circuit Cellar for ideas- and pick
one with a high-level language compiler- preferably BASIC for the
ultimate in simplicity. The literature is rife with projects that can be
modified to fit your application. If you insist on a razzle-dazzle
4-digit LED display at each beam position, then drive these with the
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