Simple LED circuit that identifies when a 3.3V power line is being drawn by a connector.

Thread Starter

DTT

Joined Jul 30, 2020
2
Hi all,

I am a student struggling with a simple circuit for a self project. I have an edge connector mounted on a backplane PCB. Another PCB then is inserted into the connector mounted on the first board. Essentially, I would like to create a circuit that turns on an LED when the board is inserted into the connector (a "plugged in" indicator). A 3.3V trace is connected between the two boards when the board is inserted.

So my (very simple) circuits question is how can I create a circuit that turns on an LED when the 3.3V line is connected upon insertion? I am not too concerned with individual values of any resistors or mosfets (I can find this out on my own). Just was wondering if anyone is familiar with what is used in consumer products to indicate via LED when something is plugged in.

Thank you for any suggestions!
 

Bernard

Joined Aug 7, 2008
5,558
Where is the monitoring LED located, mother board ,daughter board, or elsewhere? If on the mother board and a spare pin is available fold back + 3.3V on daughter board to the spare pin, assuming a common, ground, is also available. On mother, connect a low voltage LED & resistor
between common & spare pin.
 

Thread Starter

DTT

Joined Jul 30, 2020
2
Where is the monitoring LED located, mother board ,daughter board, or elsewhere? If on the mother board and a spare pin is available fold back + 3.3V on daughter board to the spare pin, assuming a common, ground, is also available. On mother, connect a low voltage LED & resistor
between common & spare pin.
The monitoring LED is located on the backplane, so the mother board in this case. Spare pins are indeed available (this would be the simplest case in which it is essentially a switch that either breaks or completes a trace), but I wanted to avoid using additional pins if there was a way I could use the existing 3.3V pin.
 

MisterBill2

Joined Jan 23, 2018
6,330
The only way that makes any sense is to put the LED on the PCB that is being plugged in, but then it will only indicate that the voltage is present, not how much current is being drawn.
And this would be a rather unusual application, what is the intended purpose of this function??
 

MisterBill2

Joined Jan 23, 2018
6,330
By the way, HOW MUCH current was never mentioned, and that matters a bit. The two normal ways to detect current are either with a series resistance, and measure the voltage that the current develops across that series resistance, or else to detect the magnetic field developed by the current to be sensed.
The series resistance method is electrically more straight forward, but it must have that series resistance added, and that can be a big problem, in addition to causing some undesired voltage drop. And if the backplane is adequately designed then adding a series resistor will be a logisticly miserable project.
So one more question now is what is the intended purpose of detecting the current? There probably is an alternate scheme that can deliver the same results.
 
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