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Chip question

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limlik

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Sorry if this is the wrong place, but I have a question. Say I have an output from something that transmits in certain pulses. (They pulse at certain rates depending on what is going on) How could I go about making it to were the pulse rate is slightly less when a certain trigger happens? But not by changing the source. Source of output >>>> Signal >>>> Pulse change/trigger detector >>>> thing receiving signal. I have everything but that one segment figured out.
 
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Perhaps a monostable multivibrator might work. Pulse hits mono. mv, which only outputs one frequency no matter what the input. Well, to a certaing point. Anyway, check out multivibrators, they can be easily made from the old 555 chip.
Check out this site >
Or this one > **broken link removed**
or this one > 555 and 556 Timer Circuits

Maybe one of the EXPERTS could help.
 
limlik,

I think what you need is a "missing pulse detector" circuit. This circuit outputs a signal if the incoming pulse train misses a pulse or the pulse rates drops below the specified level. This can be done with LM555;s or other many IC's.

Google: missing pulse detector

Ken
 
Thanks for the replies, but I think I phrased part of this badly. What I have a a signal that runs at a certain pulse rate depending on what the control is set at. (This will never be messed with) What I need to for somewhere along there is something that will slow this pulse down a little, like if the pulse length is 1.30, when triggered it would drop it to 1.25, or if it were 1.45, it would drop it to 1.40.
 
So the pulse period (time between rising edges) is fixed (mostly), but the width of the pulse varies?

When a pulse arrives at the "magic box", it puts out a pulse that is shorter, by some known factor, than the input pulse width?

Is the length of the time between rising and falling edges of the pulse always shorter than the time between the falling edge and the rising edge of the next pulse (i.e. <50% duty cycle)?

Sometimes it helps us if you can explain "the problem", as well a "your solution".

Ken
 
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Do you always want to shorten the pulse width by a fixed amount, or is there an algorithm that the reduction is relative to the incoming pulse width?

OK, the answer is a PIC microcontroller...simple...I think.

PIC measures the incoming pulse width W, stores the value W, subtracts X from W, outputs a pulse of W-X width. You would always be one pulse behind, but for RC servos that should be irrelevant.

Ken
 
That is how I have been doing it using a basic stamp 2, but I was curious if there was a way to do this type of thing using other methods. Thanks though :) but if there is a way to take a fixed amount off with out a micro controller that would be great. Like always take .05 ms off.
 
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YES!...I think.

The servo pulse triggers a monostable that goes low for 0.05 mS. The output of the monostable is AND'ed with the original pulse. The output of the AND gate is a pulse 0.05mS shorter than the original.

Does this seem right?

Ken
 
Oh, ok so it would go low and subtract from the servo which went high so it takes that much off. Right?
 
That's my thought!

Ken
 
Awesome, I will see if it works when I have the chance. Gotta run to Radioshack or something sometime soon. Thanks
 
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Only problem (yeah, sure! ;) ) I can see is: will the AND gate output a very brief pulse at the start of the servo pulse, before the monostable's output goes low? This may be insignificant. Or, might be solved by stacking unused AND gates between the servo pulse line and the summing AND gate input, as a delay line.

As an aside, was the Basic Stamp a bit pricey for this application, or were there other reasons?

Ken
 
The price is part of the issues but I also need to have it small size and weight. Going to make both and see which is better.
 
Since you're experienced with the Ba$ic $tamp, you might consider an 8-Pin PICAXE-08M µC. BASIC programming is almost identical to the Stamp. The chips are about $3.50. Editor and manuals are free. Two resistors and a serial cable to program, like the Stamp. It can be run at 8MHz with an internal oscillator...no external resonators required...so I think it's fast enough. That would cut the size and cost.



Ken
 
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Cool, I will check this out, realized the other method might not work. I forgot that the amount that the pulse needs to be altered is dependent on what it is at at the time.
 
Did you have a lookup table in the Stamp to do this?

Ken
 
do you know if you can make the monostable go to certain settings depending on what the pulse length of the signal to the servo is or is that only able to be done with a coded chip?
 
do you know if you can make the monostable go to certain settings depending on what the pulse length of the signal to the servo is or is that only able to be done with a coded chip?

That would require the monostable to see into the future or store information from the past. ;) The first is not possible, and the second says µC. ;)
Ken
 
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