Hi All,

I have a general question.

I have a PCB that has 3 wires connected to it: VCC, GND, and INPUT3. The INPUT3 connects to a GPIO pin. The wires are always connected, but sometimes we receive pulses on the input, which I believe is due to the wire behaving like an antenna, picking up some noise. However, since there is a pull-up resistor (R62), should this still be happening?

 

You need a pull-up resistor at INPUT3.

 

Input 3 looks more like an output.

 

Input 3 looks more like an output.

At a node, an output is the same thing as the input.

 

There is a inte

You need a pull-up resistor at INPUT3.

There is an internal pull up, but I guess i will also add a physical one

 

if Q16 is off, then its collector circuit is high impedance and that could be a problem (sensitive to noise). if the device connected to it is also high impedance (or has weak pull up resistor), then this is more than likely. you can add low value pullup (such as 1k) and see if the problem goes away.

 

I have a PCB that has 3 wires connected to it: VCC, GND, and INPUT3.

I see for 4 wires.
What is INPUT Control 3 connected to?

 

Are the symbols indicating the circuit common, actually connected to earth GND as shown?

 

There is a pull up resistor on the Base of Q15. It almost turns on Q15. Very little noise will turn it on.
You need a pull-down resistor on Q15Base. 4.7k to 47k something in that range.

 

With the circuit fragment shown in post #1, and the description telling us that a pulse appears on a fragment of that circuit, it is not possible to provide a USEFUL guess as to the problem, other than to let you know that it originates in an area that we are not shown. That much is totally clear to me.

 

There is a pull up resistor on the Base of Q15. It almost turns on Q15. Very little noise will turn it on.
You need a pull-down resistor on Q15Base. 4.7k to 47k something in that range.
View attachment 331699

Thanks

Yes i agree, i had kept a pull up originally in case my input was to be be open collector.

 

i had kept a pull up originally in case my input was to be be open collector.

In that case put the resistor on the output of the open collector, not on the base of the receiver transistor. (move the pull up resistor to the other end of R59)

 

Thanks @ronsimpson

I am also thinking about removing the transistor and simply connecting the signal to GPIO, i can do the level conversion in my code.

Ideally in the both pull up and pulldown. Pull up if the sensor is open collector, and pull down is sensor is disconnected but i have extension wire connected to my PCB.

 

Looking at you schematic, when the signal is open the GPIO is about 1.6V and when signal is 0V GPIO is about 0.3V. (math in my head, I may be off some)

 

@ronsimpson

Yes correct, i forgot to change my values

 

Looking at you schematic, when the signal is open the GPIO is about 1.6V and when signal is 0V GPIO is about 0.3V. (math in my head, I may be off some)

What about if there is voltage on signal say 3.3-4V? would i need to change anything?

 

At a node, an output is the same thing as the input.

The tag:V-out DC DC is referencing the voltage source for the pull up resistor. More common would be just "V+" possibly with a voltage following.

 

I can not change that voltage

 

I did not suggest changing the voltage. I commented that possibly that is what another saw as indicating that it looked like an output.
As for a pulse appearing at the input, is it a pulse that should be appearing, or one that should not appear?? Is that wire routed near some noise source? Should that wire be shielded to prevent noise pickup??

 

@MisterBill2

I will be not be able to change the voltage, therefore i have redesigne the circuit below. If i am correct with this design, my Sensor Puslse can be any Voltage, Open Collector and GND, and the pulse on my GPIO would either be 3.3V or GND. Is this a correct design?