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Topic: Diode/ frequency signal interuption (Read 1 time) previous topic - next topic


Jun 29, 2017, 11:27 pm Last Edit: Jun 30, 2017, 12:21 am by vipergt
Do you guys know of anything we can use to prevent back feeding current besides a diode?

We have the signal acceptor(frequency in hz) on an arduino uno that requires a lift with a 4.7K resistor to 5v to bring the signal up from 0V but the signal source also needs to feed into a computer and we don't want the voltage lifted signal to go to computer (not sure if it will fry). We tried putting a diode in which blocked the 5v back flow to the  computer but the arduino no longer got a proper signal and didn't read correctly as I assume the diode is somewhat rectifying the signal. So we just need a circuit or component that will prevent the back flow and let the signal run clean through. Thanks in advanced for the help!!


Jun 29, 2017, 11:55 pm Last Edit: Jun 29, 2017, 11:57 pm by Wawa
Not enough info for a proper answer.
Most Arduino variants use backflow protection.
A Nano uses a simple schottky diode, and an Uno/Mega uses a p-channel fet.
Schematic diagrams can be found on this site.


What are the voltage extremes of the incoming signal?  What is it coming from?  Is it a logic signal or analog?

The Arduino is running at 5V right?

The computer is using what voltage?  What is it?  What interface?

A series resistor can limit current to safe levels and act as extra protection
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]



As others have said "... not enough info.."

However if I understand your description the diode is anode to the computer and cathode to the signal source and arduino.

Is it possible the computer has a "lift" resistor?  It would cause the signal source to have to over come it.

I would consider all combinations the circuit might generate and look a the effect of each.

Please do not PM me with thread based messages.  If your thoughts are worth responding,  the group should benefit from your insight.


there are 3 files attached one is the raw out put signal on a scope, another is the 5v pullup on the scope and another wonderful microsoft paint diagram of the whole works, i think drew the diode in correctly. on the project i just connected the diode to the ardiuno side of things (and 5v pullup) powered it up and then tested with my multimeter on the sensor side and flipped it to the direction where the meter read 0v and not 5v. looking at the scope out put (found online same sensor similar project) it occurred to me that i might not need to worry about the 5v pull up. and looks to me (not sure) that the sensor in its raw form outputs a 0-9.86v squareish wave and when the pull up resistor is attached it some how brings the signal down to a 0-2.28v square wave??? also mentioned is that the sensor is an open collector output not exactly sure what that means. and my interpretations of the scope readings are from my freshman year of high school so feel free to correct me if im wrong.


Jun 30, 2017, 04:32 am Last Edit: Jun 30, 2017, 04:33 am by DVDdoug
I don't know what's going on...

An open collector is supposed  to draw current through the pull-up resistor (when the output goes low, "overpowering" the pull-up).    And you shouldn't be getting 9V unless you've got 9V through a pull-up...     And, without something pulling-up an open-collector it won't go up.


here is the last scope reading with the 5v pull up, if that is how an open collector works its very well possible that the computer (ecu) has a pull up built in, i will test the input on the computer and get back, hopefully the computer pull up isnt over 5v or i suppose i will need to play with resistors to get the voltage back down to 5 for the ardino to measure.


A safe way is a (1N4148) diode between Arduino pin and device output, with the cathode (ring) to the device.
With internal pull up on the pin (or external 1k pull up in noisy environment).

Note that there is no "0-4.5volt output" on an Arduino. It is 5volt PWM.


If there's any risk of a device outputing more than 5V, do not connect it directly to an
Arduino pin, add a series resistor to protect the pin.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


ok thanks guys ill try a couple things and get back to you

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