weird interference problem

I have a motorcontroller and a motor and they sit next to each other. The motor shaft has a photointerrupter sensor and the sensor signal is connected to the motorcontroller. The motorcontroller has a feature where it will shuts off the motor whenever it senses a spike in the motor's RPM.

The problem that i'm having is that whenever I tap into the photointerrupter sensor using a wire, the motorcontroller shuts off the motor like there is an RPM spike. It acts the same way even if the wire i'm taping with is not connected to anything. My theory is that the wire acts as an antenna and it is outputing spikes created by the motor ecm. The motor controller mistaken these spikes as an increase in RPM and shuts off the motor.

What can I do to overcome this problem??

Hi david90!

I'm trying to understand your problem.
The photosensor gives pulses and you count the frequency using uc and decide rpm, right?
This connection is done permanently or is this the one you are tapping?

Giri

Probably the wire is picking up nois as you say. Try a shielded cable to the sensor and a filter on the pcb. This filter could be a low pass RC.
Another very probable cause is noise entering the board thru the power supply. Specially if the motor has brushes, it can generate a lot of electrical noise. In that case try filtering the motor leads.
Put the sensor input to ground and see if the problem persists. In that case the noise comes thru the power supply almost for sure.
hope this helps

vsgiri said:

Hi david90!

I'm trying to understand your problem.
The photosensor gives pulses and you count the frequency using uc and decide rpm, right?
This connection is done permanently or is this the one you are tapping?

Giri

Click to expand...

i'm tapping it with a wire that will be inputted into another uc to determine speed

Added after 4 minutes:

albert22 said:

Probably the wire is picking up nois as you say. Try a shielded cable to the sensor and a filter on the pcb.

Click to expand...

shield cable might work.

Where would I put the RC filter? if I tap the sensor with a wire, the motorcontroller is affected. So ur saying put an rc filter between the photointerrupter and the motorcontroller?

Sorry, I understood that you were working with the motor controller MCU.
When you say that you are connecting to a 2nd MCU I realize that you are trying to connect to an existing circuit. In that case the RC lowpass will not be a solution. Unless you put it on the motor controller board input (modifiying the board)
It seems that the impedance on the sensor circuit is very high. Study the datasheet of the photo sensor and the input circuit to evaluate the input impedance.
It might be necessary to insert a buffer next to the sensor.