# Timing Circuits

Thread Starter

#### bambicomp

Joined Sep 23, 2008
15
Hello I wish to set up a timing circuit. The circuit will basically time between two signals. When the first signal goes high (i.e 5V) the clock will start. When the second signal goes high(again 5V) the clock will stop. How do I implement this. The time between the two signals will prob be a maximum of about 30 seconds.

I have read a few places about flip flops and using the set / reset signals. Is this the way to go and can someone help me out with this part and also how to set up the clock etc as I really need this urgently.

Thanks for any help.

Thread Starter

#### bambicomp

Joined Sep 23, 2008
15
Basically what I have at the moment is two sensors (laser light gates). The output of bothe the circuits is 0V when the beam is in tact. When the first beam is broken it jumps to 5V momentary and then back to zero. When the second beam is broken it does the same.

So I want to take the two signals and form a clock and counter basically. How do I do this. Can anyone show me a circuit diagram?

#### SgtWookie

Joined Jul 17, 2007
22,230
How accurate do you want this 30 second timer to be? Just seconds?
Or perhaps 1/10 second, or 1/100 second? Maybe 1/1000 second?

#### veritas

Joined Feb 7, 2008
167
Thread Starter

#### bambicomp

Joined Sep 23, 2008
15
Would there be much of a difference in the level of circuitry needed to display the time in 1/10 of seconds as opposed to seconds?

Also what would the astable multivibrator with the RS flip-flop give me? Why would I use it?

Can anyone explain in a block diagram exactly what I need and I try and take it section at a time?

Am I right in saying that for the first section I need:
To use a flip flop with my two signals going in and then the output will give me the length of time it is on. So that this can be fed to some enable of another circuitry?

Is that correct?

#### SgtWookie

Joined Jul 17, 2007
22,230
You'll need a 3-digit BCD (binary coded decimal) counter to display the seconds and 1/10 seconds. A CMOS 4553 (CD4553B, M14553)) is such a counter.

You'll also need a method to drive an LED display. A CMOS 4543 (CD4543B, M14543) are such drivers that conveniently interface with a 4553 counter.

See the attached for an example of 4553/4543 CMOS IC's connected to a 3-digit LED display.

However, you still require a source for an accurate 1/10 second clock pulse. In order to approach anything accurate, you will need a crystal oscillator; they are hundreds of times more accurate and stable than a simple RC timer.

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#### veritas

Joined Feb 7, 2008
167
A flip-flop would tell you when to count units of time. The first signal would set it, and the second would reset it.

A multivibrator is a quick and dirty way of implementing a clock for a timer. I would say that you could easily get one to be within a .1 second error margin after 30 seconds, but as SgtWookie stated, with a longer period of time or a more precise application, a crystal oscillator would definitely be more accurate.

If you use the flip-flop to enable/disable your timer, you can then capture the time elapsed between your two triggers with a BCD counter.

Thread Starter

#### bambicomp

Joined Sep 23, 2008
15
Right I have the flip flop idea. And seem to have that up and running.

Next stage. You mentioned better timing accuracy with a crystal. How would I implement that? Without using the multivibrator.

#### SgtWookie

Joined Jul 17, 2007
22,230
A CMOS 4060 IC (MC14060, CD4060B) 14-stage binary counter/oscillator with a 327.68kHz crystal and supporting components (a couple of small capacitors, a couple of resistors) can produce an accurate 10Hz clock signal.

Datasheet for the MC14060 is here: http://www.onsemi.com/pub/Collateral/MC14060-D.PDF

#### beenthere

Joined Apr 20, 2004
15,819
Missed a decimal place - that's 32.786KHz.

Thread Starter

#### bambicomp

Joined Sep 23, 2008
15
Could I use a normal 2 pin crystal in any way so that I could show the effect of putting in different frequecy crystals and the resulting accuracy of the system?

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