# Use a 555 timer as a stopwatch and display in seconds

Discussion in 'The Projects Forum' started by bambicomp, Jan 15, 2009.

1. ### bambicomp Thread Starter Member

Sep 23, 2008
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0
Hello

I am creating a light gate circuit. Bascially I now have one signal that will pulse high and low. I intend to use this signal to enable / disable a 555 timer.

I wish to use the timer as a clock to count how long the signal is on for. How do I do this?

WHat do I connect the output of the timer circuit to in order to have it displayed on a clock to show the time in seconds/ milliseconds.

Also is the accuracy of the 555 controlled by the RC circuit?

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
It's more like the inaccuracy. 555's are very useful, but precise timing is not part of the many functions.

The 555 can generate a train of pulses with your input as a trigger. The question is: how were you intending on accomplishing the clock function? That is, accumulating the pulses and displaying them? Also, what duration of time do you need to measure, and with what precision?

3. ### bambicomp Thread Starter Member

Sep 23, 2008
15
0
What else could I use for precise timing?

The 555 can generate a train of pulses with your input as a trigger. The question is: how were you intending on accomplishing the clock function?
That was what I was hoping to find out here, but had some sort of bought clock/ display in mind. Any other ideas?

Also, what duration of time do you need to measure, and with what precision? Duration would be maximum of 30 seconds I suppose and accuracy to one decimal point. Doesnt need to be overly accurate.

Looking for a simple solution if possible thats why I figured on using the 555 timer.

Thanks for any help

4. ### mik3 Senior Member

Feb 4, 2008
4,846
63
You can set up the 555 as an astable oscillator with a frequency of 1 Hz. The 555 output will drive the clock of a counter IC which will count up and display the result on a seven segment display. Your signal, which is going to be measured, will be applied on the reset pin of the 555. When it will go high the 555 will be enabled and it will start pulsing the counter and the counter will increment every one second. When you signal will go low the 555 will stop pulsing and the counter will stop counting up. The value on the seven segment display will be the time the signal was high. The accuracy will be approximately +/- one second. For more accuracy you need a more complex circuit. If you want to measure the time your signal is low with the same circuit there is a modification which I can tell you if you like.

5. ### bambicomp Thread Starter Member

Sep 23, 2008
15
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Mik3 - Can you tell me a suitable counter IC that I could use.

Also is + /- 1 second the best accuracy I could get. Is there any way of improving the accuracy by playing around with the 555 timer additional circuitry? Or even making it less accurate so that I can show how to improve / worsen the accuracy of the circuit.

6. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Check out the CD4060 IC. It can do a crystal oscillator on board, though I hear it is kinda tricky. With a 32768 crystal it puts out 2Hz, a fairly precision signal at that. The data sheet shows the schematics.

If you need milliseconds then think a 100Khz crystal oscillator, and divide it by 100.

7. ### mik3 Senior Member

Feb 4, 2008
4,846
63
You can set the 555 frequency to 10 Hz, then connect it to the clock of a counter which has a divide by 10 output and then connect the divide by 10 output to the clock of another counter which will count up. In this way the accuracy will increase to +/- 0.1 sec.