From: Paul Burke
Subject: Re: I/O logic problem
Date: Fri, 06 Dec 2002 08:42:32 +0000
Organization: Scazon Systems
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> According to my knowlegde, normally, we input 4V or above for 5V system, it
> is High. 0.5V or below, it is Low.
For HCT high is 2.0V or above, low is 0.8V or below.
> I use a MCU, according to the datasheet, Input High is 0.8VDD minimum, Input
> Low is 0.2VDD maximum.
What voltage is this thing running on? Whatever you use, theses are the
signal levels you need to work with.
I use a IR sensor series with a resister. When IR
> applied on the sensor,
> current flow through the sensor and resister. So, by detecting the voltage
> drop at the resister, we can detected IR.
> VDD-----R----------IR detector----GND
> However, my problem is that: IR source which I want to detect is too small
> compared working environment. IR in the working enviroment cause the IR
> decter "semi-on", so, the voltage Vg is about 4V (off my IR source).
> When i turn on my IR source, it is 3.5V.
You need an amplifier circuit to remove the background IR. This is
usually done by a low- pass filtered version of the signal to the non-
inverting input, and the unfiltered signal to the inverting input. You
only see rapid changes at the output. Use a Schmitt buffer to interface
this to your MCU.