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555 Timer Circuits - Changing Timing Rates

08/15/2013 2:02 PM

On an out of the box 555 timer circuit, adjustable timing from 0-90 seconds, would a change in the adjustment 500k potentiometer to 50k give me a 0-9 second rate? Also, any ideas on how to stop 555 based timers from shortening the time when re-triggering too quickly? I could post a high res photo of the timer if needed. Don't have a schematic, but the parts are clear.

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#1

Re: 555 Timer Circuits - Changing Timing Rates

08/15/2013 3:39 PM

Yes.

http://www.fairchildsemi.com/ds/LM/LM555.pdf

See figure 3

Cheaper to change the capacitor though.

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#2

Re: 555 Timer Circuits - Changing Timing Rates

08/15/2013 5:42 PM

Adjust the existing potentiometer to give you the required 9 second rate... then measure it's resistance! You will then have an answer.
Measurement is generally (IMO) vastly better than claculation!
Of course either may well give a value for a poptentiometer which isn't actually available in practice.

Welcome to the real world.
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#3
In reply to #2

Re: 555 Timer Circuits - Changing Timing Rates

08/15/2013 6:35 PM

Agree on the measurement part.

Unless you're buying mil-spec components, it just ain't gonna be calculable. Even if you are it will only be close.

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#10
In reply to #2

Re: 555 Timer Circuits - Changing Timing Rates

08/24/2013 8:59 AM

Measurement is generally (IMO) vastly better than claculation!

I wish I could give this proper attribution, but a friend of mine gave me this one from when he worked on Trek bicycles, iirc:

"One accurate measurement is worth one thousand expert opinions."

(on this forum, it may be preaching to the choir)

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#4

Re: 555 Timer Circuits - Changing Timing Rates

08/15/2013 6:41 PM

Great idea. Measure it out.

Anyone have a clue on making it more stable? Even if you get 10 seconds, unless thta cap is discharged, activating the circuit within a few seconds yields different results. Maybe 6 seconds.

Better yet, anyone know of a board with an LED readout that just less you enter in your time in ms?

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#5
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Re: 555 Timer Circuits - Changing Timing Rates

08/15/2013 6:47 PM

You're basically using a high falutin RC circuit.

If you want a highly accurate pulse, use a crystal oscillator, or sync to the 60 hz line power (50 hz in some places).

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#6
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Re: 555 Timer Circuits - Changing Timing Rates

08/16/2013 1:01 AM

To easily get repeatably, accurately adjustable times you'd be better to go digital - using e.g. a PIC microcontroller chip, or maybe an Arduino or a Raspberry Pi.

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#7

Re: 555 Timer Circuits - Changing Timing Rates

08/16/2013 8:01 AM

I would probably be correct if I said that the internet has more about the 555 (556) chip than any other device.

Its been around in various forms since 1971. Not many other chips (I know of none) are still being made 42 years later!!!

It has literally hundreds of web pages with tutorials and even software to allow various values to be keyed in and watch what happens....on screen on your PC or just online at a web page.

Its a great chip, though small cheap PICs are starting to make inroads into its use.

Start here:-

http://en.wikipedia.org/wiki/555_chip

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#8

Re: 555 Timer Circuits - Changing Timing Rates

08/16/2013 8:39 AM

Greenwich CT asked, "any ideas on how to stop 555 based timers from shortening the time when re-triggering too quickly?"

New trigger pulses occurring during a timer action are ignored, unless they come simultaneously with the end of the timer action. At the end of a timer output the internal flip-flop resets, and the timing capacitor is quickly discharged via pin 7. Using a smaller capacitor shortens this discharge time. So leaving your pot at 500k and reducing your capacitance by 10x would be a better way to change 90-second timing to 9 seconds.

If you use a CMOS version of the 555, such as Intersil's ICM7555 or NSC / TI's LMC555, you can increase the timing resistor to 5M or 10M, and reduce the capacitance 10x or 20x more, accordingly, further reducing your vulnerability to the problem. You can also use a fixed 1% timing resistor and add a trim-pot to pin 5 to calibrate the time despite your 5 or 10% timing-capacitor accuracy. This would allow for even higher-value resistors.

If you need to reduce the possibility of a shortened timing pulse to zero, you can add an OR gate (which is an AND function for inverted logic) to the input. You'd OR your negative trigger pulse with the 555 output, and present it to pin 2. The trick is to add an RC in the path from the output, delaying the logic low signal to the gate until the timing capacitor has been discharged. In the timing diagram, the start of a new 555 timing cycle is delayed a microsecond or so until the capacitor is discharged. Choose the RC delay value accordingly.

OR gates come four per package in a 74HC32, or you can get a single gate in a sot-23 package, e.g. the 74AHC1G32, only $0.28 at Digi-Key.

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#9

Re: 555 Timer Circuits - Changing Timing Rates

08/16/2013 9:23 AM

Theoretically yes, real world, no. As the timing capacitor ages plus any temperature changes will cause drift from your ideal values.

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Andy Germany (1); Captain Quirk (1); Del the cat (1); Greenwich CT (1); JohnDG (1); Randall (1); RufusVS (1); Winfield Hill (1); WJMFIRE (2)

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