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From: dansimon@REMOVExmission.com (Daniel Simon)
Subject: Re: 0-100s timer using 555 IC and panel LED
Date: Mon, 16 Sep 2002 18:52:41 GMT
Organization: XMission http://www.xmission.com/
NNTP-Posting-Date: Mon, 16 Sep 2002 18:50:25 +0000 (UTC)
X-Newsreader: Forte Free Agent 1.1/32.230
On 10 Sep 2002 11:27:42 -0700, firstname.lastname@example.org (Jeff
> 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!
When I took physics in the prehistoric 70's we attached a
strip of paper to the falling object which would pulll the
paper thru a gizmo that made periodic marks on the paper. we
would measure the increasing gaps in the marks and derive
You could probably solve your problem with a general
purpose frequency/period counter with start and stop inputs.
Construct your apparatus with multiple sensors and repeat
the experiment once for each sensor position (as Mr.Pinnel
suggests). you could build a sensor selector switch into the
ramp to make this quicker.
A good counter is handy for all timing measurements large
and small and should be standard equipment in a physics lab.
I have seen tons of quite servicable ones on eBay and at
surplus instrument sellers for giveaway prices.
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