I am still working on an arduino based IR-Photodiode Matrix, and i have run into a rather bad problem . -Basically i have one master multiplexer that controls 5 other multiplexers which in turn power 8 photodiodes sequentially , and i then get the data from those photodiodes through a single data line to the Arduino. -Now the thing is, i am having some huge "interference" from the normally NOT powered multiplexers/photodiodes that i did not notice before : ie out of 1023, i get a value of almost 300 on those elements that are not powered! I even checked with a voltmeter, and the normally unpowered multiplexer actually put out almost 2V on their pins! -Following the info in the datasheet i grounded the E1 and E2 pins on the 74HC238s
-I can see different possible sources for this problem: *i "cheated" in my wiring by connecting the VCC pin to the Enable input pin of the slave since i assumed (wrongly it seems) that an unpowered multiplexer should not be active *similarly i haven't put a pull down resistor on the slave multiplexers *I have checked each and everyone of my connections and i see no problem, although i might have missed something...
For additional info : here is the datasheet : http://www.nxp.com/acrobat/datasheets/74HC_HCT238_3.pdf And a simple schematic i drew up for the project: http://www.kaosat.net/wordpress/wp-content/uploads/2009/05/photodiode-matrix-schematic
More info about the project and some pictures here : http://www.kaosat.net/?p=452
Any help or tips would be very appreciated! Thanks a lot in advance!