# Creating a 5V 20Hz pulse

Discussion in 'The Projects Forum' started by iONic, Oct 17, 2010.

1. ### iONic Thread Starter AAC Fanatic!

Nov 16, 2007
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I'm looking for a simple solution for making a 5V pulse at a frequency anywhere between 20Hz and 40Hz with 12VDC as a source. Also... a 50% duty cycle.

Looking for the most compact solution...
Perhaps a single IC with minimal external components can achieve this also.

Thanks

Last edited: Oct 17, 2010

Aug 6, 2010
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3. ### tom66 Senior Member

May 9, 2009
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What kind of accuracy are you looking for?

For high accuracy (say ±0.001%) you can use a quartz crystal (32.768 kHz) and feed it into a binary ripple counter (divide by 16,384) to get 32 Hz.

But if not, then a 555 timer as an astable is the next best solution. T = 1.1RC, so an R = 1k and a C of 3.3µF would get you 36.3ms nominal, which is 28 Hz.

4. ### iONic Thread Starter AAC Fanatic!

Nov 16, 2007
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Yes, I was just playing with some 555 calculations. But from a 12V source I would also need a 5V voltage regulator triggered from a transistor on pin 3 of the 555. The part count starts to get large.

Accuracy is a non issue, that's why I said between 20Hz - 40Hz. Not sure the exact value, but in the end if I'm shooting for 25Hz and I get 22Hz or 27Hz it would ne a non-issue.

Do they make a single IC that could give a 5V TTL output as this would avoid additional parts with voltage regulation.

Apr 5, 2008
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6. ### iONic Thread Starter AAC Fanatic!

Nov 16, 2007
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Yes, the Zener is a better option than the 78L05 VReg.

Thanks

7. ### tom66 Senior Member

May 9, 2009
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A cheaper alternative is to use a 10k resistor and a signal diode (reverse polarity) to 5V. This will clip the signal to 0V to 5.7V which is fine for most applications, use a Schottky (slightly more expensive) to clip it to 5.3V.

8. ### SgtWookie Expert

Jul 17, 2007
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If you need a 50% duty cycle, you'll need a slightly more complex circuit.

See the attached; output frequency is ~27Hz; however your mileage will vary with the accuracy of the timing capacitor.

Note that C1 and C2 are required; these are stated in the datasheets as minimums. C2 can be larger if desired.

If you want the frequency output to be adjustable, you could use a 20k potentiometer wired as a rheostat in series with R2, and reduce R2 to ~14k.

Don't just use a pot for R2 however, as you will burn it up if you happen to adjust it too low in resistance.

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9. ### iONic Thread Starter AAC Fanatic!

Nov 16, 2007
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Thanks Sgt.
The ultimate objective is to drive some LED's so that they do not appear to be turning on/off but rather to sort of pulsate, thus the exact duty-cycly and freq. are not hyper-critical. Not really knowing how it will look until it is visible leads me to think that some possible variation of duty-cycle and freq. would be helpfull during the design. Once I've determined the effect I want, fixed component values will be used.