# Need help understanding timer circuit?

Discussion in 'The Projects Forum' started by UsernameMD, Sep 12, 2014.

Sep 12, 2014
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Hi,

As the title suggests I need help understanding the timer circuit in the schematic attached.

I built this project in college last semester, but did not design it.

It is a circuit designed to measure a users reaction time to an event.

The user presses the set button and after some delay the LEDs start displaying the count. The object is to stop the count as quick as possible so as to measure the users reaction time. The LEDs are then reset by pressing the reset button, and the LEDS will go to '000'

Just wondering how this works, Digital is not a strong point of mine, and I am trying to change that.

Thanks

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2. ### shteii01 AAC Fanatic!

Feb 19, 2010
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555 provides the clock signal.

Frequency of 1 Hz.
1 Hz=1/Time
Time=1/1 Hz
Time=1 second
So the counter counts at 1 second interval. You start at 0, after 1 second that count is changed to 1, after one second the count is changed to 2, after one second the count is changed to 3, and so on.

Sep 12, 2014
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Sorry I should have been more specific, I am very weak at Digital and trying to improve. Here is a picture of the 555 area of the board.

Why are there two resistors and a capacitor there?

Also does this mean the 555 is in astable operation (I am just comparing with the datasheet now)? If so what exactly does astable mean?

Thanks so much for your help, I apologise for the silly questions.

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Mar 24, 2008
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5. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Two resistors and one capacitor are used to determine the time period. This way you the user say: I want square wave with 2 second period, 50% duty cycle (1 second high, 1 second low). So you pick a capacitor, you pick one resistor, solve the equations to find the second resistor. Build the circuit. Done. Here is 555 calculator and a little explanation:
http://www.csgnetwork.com/ne555timer2calc.html

The datasheet should have all the relevant info, including sample circuits of 555, including explanations.

Sep 12, 2014
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Thanks guys, I will look into both these things and post back in a while when I know a little more.

7. ### Dodgydave AAC Fanatic!

Jun 22, 2012
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Looks like you missed off the 1khz clock,for the reaction time, the 555 provides a 1 second enable pulse to set the reaction timer going,as soon as the person stops the timer,the counter will display it in milliseconds (1000)= second.