# Voltage level test



## geosman (Sep 15, 2007)

I put 8 LEDs on a PCB and connected the LEDs from pins 2-9 on the connector and to a 1K resistor which goes to ground. This was to test some software. Now that phase is OK
I have to read the the status bit serially 8 times and use each value to sum up the value on data pins. Can I safely place a 1.5 v battery in parallel with one of the LED/resisters in order to change the 255 value I currently get? If so would I connect the negative side of the battery to ground?


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## gcavan (Aug 13, 2009)

You won't damage the LED/resistor; 1.5 mA will not likely even light the LED. Without knowing what is on the other side of "pins 2-9" I really cannot say anything about it. You need to look at what's there and determine whether it will accept an input voltage, and what it considers a logic high or low. In any case, a correctly sized buffer resistor should be used.


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## Done_Fishin (Oct 10, 2006)

I'm guessing that we are talking parallel port and data bits 0 thru 7 on pins 2 thru 9.

the port uses digital signals at either 0 or 5 Volt levels 
I am assuming that when you send the appropriate 0 or 1 to the output you are illuminating the LED and limiting the current through the LED with the 1KΩ resistor.

I can't quite see the purpose of adding a 1.5Volt battery across a LED since there will automatically be a finite leakage through the LED depending upon the individual LED's characteristics. Whilst it won't necessarily do any harm to either the LED or the port it's not a good idea to try to "charge" battery cells even with such a small current as will be produced by sending a 1 to the output and sinking the current through the 1kΩ. If anything should go wrong you could well end up placing a 5Volt output direct to a 1.5Volt battery which may well damage your parallel port buffer.

please explain more clearly what you are trying to do & why so that we can guide you better. You may well want to consider using optocouplers as a buffer between your experimental work and your computer to limit any possible damage that might occur in the event that Murphy decides to strike .. Remember that Murphy's Law states quite clearly that " if anything can go wrong it will!"


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## geosman (Sep 15, 2007)

Each LED/resistor looks like this on the parallel port data pins 2-9:
pin o-----------|>|---/\/\/\----------o Ground pin 25.
-----------x-----------------------y
X should appear between pin and led y between resistor and ground.
My recollection is that the parallel port is 12v not 5v but haven't rechecked its been awhile since I played here.
In either case a +1.5 at x and the neg end of battery at y on any one of the pins would act as a hi/lo (depending on batt. orientation) and thus change the reading I get when I do the reading of the status pin 8 times. I know the status pin reads that way because I use it in my seismic amplifier. What I don't know is if I am going to change the value it reads when I put the battery as stated above. The objective is to get any reading except 255 which is what I am currently getting. That should show that it is working and not just letting me see a default value. Hope I am making sense this time.


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## Done_Fishin (Oct 10, 2006)

12V is only found on serial ports .. Parallel ports are TTL (5V)

1.5 volts is not sufficient to warrant being read as a 1 unless you might be using 3.3V logic and even then I think it would be classed as in "no mans land" an area that can be read as either a zero or a 1 as the circuit thinks fit.


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