From: Terry Pinnell
Subject: Re: 0-100s timer using 555 IC and panel LED
Date: Fri, 13 Sep 2002 18:14:33 +0100
NNTP-Posting-Date: Fri, 13 Sep 2002 17:14:39 +0000 (UTC)
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firstname.lastname@example.org (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.
Are the relays essential? Obviously they wouldn't be necessary for
freezing the respective counters (and arguably could detract from
accuracy), but maybe one of your other goals requires them?
>The main goal would be to time the object at
>different locations, in order to caluculate the velocity at each
Nice word that - it deserves to exist! (Maybe a verb defining the
combined actions of calculating and then double-checking?)
>I would like to use four digit LED diplays, displaying time
>to the nearest hundredth of a second.
That squares with your subject header, which puzzled me because 99.99
secs seems to imply a very long ramp!
> 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?
My data sheet for the LM555 from National Semiconductor includes this
on page 3 of 12 under 'Timing Error, Astable':
Initial accuracy 2.25%
Drift with temperature 150 ppm/deg C
Accuracy over temperature 3.0%
Drift with supply 0.3%/V
(all for Rt of 1k to 100k and Ct of 0.1uF).
> b) Can I use the output of one 555 timer with 4-5 counter
>circuits (one for each group of LED displays)?
Yes, you could freeze each counter module in parallel, using the 555's
output (or its inverse if appropriate).
> 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.
That would be possible by several methods. It rather depends on what
sort of 4-digit count-and-display sections you intend to use. It might
be a bit tedious to build all 5 of them from say the sort of old TTL
chips I have to hand (such as 7490, 74141, 7447, etc). I think I'd
look for ready-made modules.
BTW, what approach do you plan for the 'start point'? Will that simply
be the first of your five sensors, with the object (ball?) moving very
slowly? Or do you want to start strictly at 't=0'?
Also, I was wondering whether it would be feasible to simplify
considerably by using just *one* sensor (and hence *one*
counter/display module), re-positioning the sensor several times and
repeating for that ramp angle?
Another radically different approach would be to take the sensor
outputs to a PC via its parallel port, with the potential advantages
of displaying the calculations etc.
Hobbyist, West Sussex, UK