From: John Popelish
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Subject: Re: 0-100s timer using 555 IC and panel LED
Date: Wed, 11 Sep 2002 01:43:20 GMT
NNTP-Posting-Date: Tue, 10 Sep 2002 21:43:20 EDT
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?
The accuracy depends on the specific timing components used to make
the circuit, as well as a lot of other harder to quantify variables,
like how much noise there is on the supply voltage. By the time you
have gone through the effort to make the rest of this system, I think
the extra effort required to add a crystal oscillator and count down
chain is not very much larger. I would go with a commercial
oscillator module (Digikey has lots of choices for $3 to $4). This
would give you an absolute accuracy that adds no significant error to
the light beam break mechanism.
> b) Can I use the output of one 555 timer with 4-5 counter
> circuits (one for each group of LED displays)?
Yes, with one gotcha. The 555 is notorious for producing about a 50
ns glitch on the negative ot positive output transition. You may need
to add a glitch filter (a series resistor at minimum, or an RC at
most) to keep this lousy transition from double clocking your
> 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.
You will need gating logic to start the counters at some precise event
(probably the first beam break), stop them at some specific event
(subsequent breaks) and reset them at a specific event ( a push
button, perhaps). The start stop function can be made by using a flip
flop along with a gate to freeze the clock signal. The reset should
also reset all the start stop flip flops to the stop state.
> I know this is an ambitious project for me, but the kids would love
> it, I can guarantee!
I built something very much like this to time pinewood derby races.
Sorry to say, I didn't keep the schematic diagram.