From: John Fields
Organization: Austin Instruments,Inc.
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Subject: Re: 0-100s timer using 555 IC and panel LED
NNTP-Posting-Date: Fri, 13 Sep 2002 08:44:55 CDT
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Date: Fri, 13 Sep 2002 13:44:55 GMT
Fred Bloggs wrote:
> Jeff Lawlis wrote:
> > Hello,
> > 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
If by implementation with "discrete logic" you mean building the
required functions with discrete transistors, resistors, diodes, etc., I
agree. However, these days we have available a nice assortment of
prepackaged, dedicated logic functions of varying complexity called
"integrated circuits" which are available as SSI, (small scale
integration) LSI, (large scale integration) MSI, (large scale
integration) and VLSI. (very large scale integration)
Using these integrated circuits ("chips", in the vernacular) it is
possible to build many useful and interesting devices without resorting
to the use of microprocessors or microcontrollers with their very steep
learning curves. The device Mr. Lawlis described seems to be such a
device, and his current level of expertise does not seem to be at a
plane which would allow him to easily embrace a microcontroller
solution. He does, however, state that he has a basic understanding of
electronic circuits and with a little guidance he should be able to get
his project up and running with very few of the delays he would
encounter using a microcontroller.
> Your time is much better invested in configuring any number of
> PIC or microprocessor development kits for the job.
Hmm... Methinks Mr. Lawlis is a better judge of how his time should be
invested than are you. Or, perhaps, you think you're qualified to
dictate what his curriculum should and shouldn't encompass and how he
should spend his every waking hour?
> 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
> development board.
Twaddle. If you have something other than lip service to contribute (a
_working_ circuit design,say, with some source code to defend your
position since that seems to be your bent) then do it and spare poor Mr.
Lawlis the wild goose chase.
Tell you what, Bloggsy, you submit a µC solution complete with hardware
and software and I'll submit a pure hardware solution and let's see
which way Mr. Lawlis wants to go. Since the weekend's almost here, how
about a deadline around midnight on Sunday with all documentation posted
Professional circuit designer