Author Topic: Holding Low Signal  (Read 1909 times)

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Offline frank26080115Topic starter

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Holding Low Signal
« on: January 14, 2008, 05:36:58 PM »
I need to hold a low pulse signal for just 5ms, what capacitor to 5V do I need? If it can't be done with a capacitor then forget about it.

Vishay IR receivers from Goldmine Electronics do not output continually, they only output a signal for a few ms at a time.

Offline SmAsH

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Re: Holding Low Signal
« Reply #1 on: January 14, 2008, 07:39:10 PM »
hmm...thats quite a puzzle have you tried researching it on the web?
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Offline frank26080115Topic starter

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Re: Holding Low Signal
« Reply #2 on: January 14, 2008, 08:45:34 PM »
I know how to retain a high signal with a cap and resistor, but i'm not sure if the opposite could be done

Offline SmAsH

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Re: Holding Low Signal
« Reply #3 on: January 14, 2008, 08:54:14 PM »
yea thats easy but ive never had to hold a low signal :-\
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Offline paulstreats

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Re: Holding Low Signal
« Reply #4 on: January 15, 2008, 05:59:57 AM »
Am i right in thinking that the high pulse is created elsewhere?
If so then all you need is for the line to go low when its not high?
a simple pull down resistor should work in this case

Offline frank26080115Topic starter

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Re: Holding Low Signal
« Reply #5 on: January 15, 2008, 03:42:15 PM »
the line is usually high, the output pin is push-pull, never floating, when there is a 38khz modulated infrared signal detected, the pin outputs low, when no infrared is detected, the pin defaults to high. but the low signal only lasts for a small time and defaults to high until the infrared signal disappears for 2ms and reappears

Code: [Select]
infrared:
______/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\
output:
------\________________/-------------------------------------------------------------------------------------

so my first instinct is to pulse the IR in bursts

Code: [Select]
______/\/\/\/\/\/\/\/\/\/\______________/\/\/\/\/\/\/\/\/\/\______________/\/\/\/\/\/\/\/\/\/\_________

------\________________/-----------------\________________/-----------------\________________/----------

But I'd like it to be


Code: [Select]
______/\/\/\/\/\/\/\/\/\/\______________/\/\/\/\/\/\/\/\/\/\______________/\/\/\/\/\/\/\/\/\/\__________

------\__________________________________________________________________________________________________
« Last Edit: January 15, 2008, 03:45:54 PM by frank26080115 »

Offline dunk

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Re: Holding Low Signal
« Reply #6 on: January 15, 2008, 05:16:53 PM »
if you are looking for a low effort and relatively low cost way of doing this
there's a range of chips designed for remote controls etc.
you can use them with IR like you are doing or RF units.
they allow you to have several different buttons at the TX end control the logic of several different pins at the RX end.

there's lots of them out there but here's an example:
http://www.rfsolutions.co.uk/acatalog/Holtek_Encryption_Chipset.html

your right though, you can probably achieve what you want by smoothing the output at the RX end with a capacitor which will give you an output voltage somewhere between your logic voltage and 0V.
you could then feed that that into the collector on a transistor to bring it back up to your logic voltage.

i'm afraid i don't know how to pick the correct capacitor for tuning this sort of circuit. not really my aria. i usually do everything with microcontrollers...

dunk.

Offline frank26080115Topic starter

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Re: Holding Low Signal
« Reply #7 on: January 15, 2008, 08:43:22 PM »
I actually just need this for object detection, without the fuss of anything analog.

i'd try the transistor idea but I'm low on board area, at most I want to do is to add a single cap right beside it or else I'd just have a software loop looking after it

maybe even pin change interrupt but that might not be optimal
« Last Edit: January 15, 2008, 08:48:42 PM by frank26080115 »

Offline dunk

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Re: Holding Low Signal
« Reply #8 on: January 16, 2008, 03:12:23 AM »
o. if your using a microcontroller then i don't see why you have a problem.
just pulse the output quicker than 5ms and look for the pulses in software.
if it's for object detection this actually makes it easier as you can count how many pulses you are seeing every second and this gives you some idea how close an object is. (the more reflected pulses the closer the object.)

dunk.

 


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