From: Spehro Pefhany
Subject: Re: how to reduce noise from 555 timer...???
X-Newsreader: Forte Agent 1.92/32.572
Date: Fri, 20 Dec 2002 13:37:37 GMT
NNTP-Posting-Date: Fri, 20 Dec 2002 08:37:37 EST
On Fri, 20 Dec 2002 14:10:17 +0800, the renowned "Laichh"
>I am designing a weighting machine using Strain gauge, the design look like
>Strain Gauge -> wheatstone bridge -> Instrumentation Amplifier (MAX4460,
>gain=10,000) -> Analog-2-Digital convertor (ADC0801, Vref=2.5V) -> Shift
>register (74LS574, CLK=3Hz from 555 timer) -> X-OR Array (determind input &
>output of 74LS574 is equal) (Vcc=+5V)
>everything goes fine until I implement a 555 timer to generate a 3Hz pulse
>(RA = RB = 1.6k ohm, C = 100 micro Farad) to trigger a 74LS574 8-bits
>D-flip-flop, when the 555 timer generated pulse, the output of my MAX4460
>will swinging same freaquency as 555 timer at 3Hz, I check the current drain
>by the circuit, when the 555 output a HIGH, the current is about 20mA, and
>LOW will be about 16mA. I did try different combination of 555 construction,
>RA = RB = 16k ohm, C=10 micro Farad, and this doesn't help. I suspected this
>meight be wheatstone bridge problem, I try to use a 4.7V zener Diode to make
>the bridge stable, and it doesn't help at all. Can somebody please help me
>to solve this problem...??? I have to done my project before Christmas......
As I think Win mentioned, use a CMOS 555 which doesn't crowbar the
supply, and try increasing the impedance of your timing components by
10:1 (10uF electrolytic, 16K + 16K).
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