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: Tue, 10 Sep 2002 17:23:06 CDT
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Date: Tue, 10 Sep 2002 22:23:06 GMT
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!
a) The accuracy of the 555 "out of the box" is about +/-5%, but you've
got to add the contributions from the timing components which, for +/-
5% each, will bring the accuracy to +/-15%. If you have access to a
frequency counter you can trim the frequency and get a pretty good
output, since the 555 tolerates supply and temperature variations pretty
The way I would approach it would be to use a single BCD counter chain
with four sets of seven segment decoder-drivers like 4511's on the
counter outputs. The 4511's have latched inputs, so as the ball
interrupts the beams as it traverses the ramp I would have the beam
breakings latch the counter's output into the display drivers. At the
top of the ramp I'd also have another opto which would clear/reset the
counter chain and the displays while the ball was held over it and then
allow the counter to count and when it was released and the ball started
rolling down the ramp.
Professional circuit designer