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Basic Electronics » output pin is 2.5v when low

April 13, 2012
by RogerFL
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I have PB1 to the gate of a 2n7000 in the circuit like the tutorials here. Even when the output pin is low, I measure about 2.5v on it. When it goes high I measure about 5v.

The 2.5v allows small current flow between source and drain of MOSFET. I hear the fan on the drain spinning slowly. If low output were 0v like I expected it I think it would work fine and go all the way off.

Any ideas?

April 13, 2012
by Ralphxyz
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Do you have your USB connector plugged in? Try removing it.

The yellow wire on the USB energizes the ports on the mcu in fact you will @2.5v across the rails.

You can search the Nerdkit forum for long discussions about problems this causes.


April 13, 2012
by RogerFL
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Thanks Ralph.

Tried with it removed - still moves from 2.38v to 5v from low to high.

April 13, 2012
by haaser
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I am working on a similar project. Can you post a couple of pictures and your code so that we can review it to see if we can help?

April 14, 2012
by Rick_S
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You may have somehow damaged that port on your micro - you could try adding a 10K resistor from the pin to the GND bus on your board. That will act as a pull down and will ensure that when the pin is "off" it does in fact go low.


April 16, 2012
by RogerFL
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Rick, right on. I was needing to upgrade to 328p and when I did the port starting working fine.

Thank you all for the suggestions.

Ralph, I was not careful removing the USB-serial wires - it was still plugged into my mac. Thought I heard a little pop. Probably accidentally touched a power or transmit to ground. Anyway that USB-serial is trashed. Duh - lesson learned - unplug first.

Thanks again!

April 27, 2012
by BobaMosfet
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The main reason you power something OFF before connecting or disconnecting wires is because otherwise, an arc is created between atleast one lead and whatever it is being pulled away from-- which causes extremely high voltage for a very small period of time- but enough time to cause damage to particularly any FET based component.


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