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PCF8883 Proximity Sensor occasionally locks up

Question asked by Ari Zagnoev on Feb 22, 2017
Latest reply on Mar 8, 2017 by Ari Zagnoev

I notice that a similar question has been asked elsewhere in the forum. I have looked at the response from NXP but I am still having a problem and would appreciate some assistance.

 

Referring to the attached schematic PCF8883_typical_application.jpg

I am using the following component values:

Rf = 4K7 (I read in an application note that it should be under 5K)

Cf = 10pF

Rc = 75K   (this gives me 1.5V across Ccpc which is half the voltage of Vcc(integd) - I don't have a high impedance meter. I capture the voltage on an oscilloscope. The voltage does drop off rapidly but I am measuring the initial peak which for me is 1.5V. This is half Vcc(intregd) . My Vdd is 3V which is derived from a Linear regulator - no switched mode regulator)

 

I have the same problem irrespective of whether I use the filter Rf and Cf or not.

 

My sensing plate is rectangular with rounded corners. It measures approximately 30mm by 15mm. It is a thin steel plate covered with a thin sheet of Mylar. The user touches the Mylar. The sensor is connected to the resistor Rf via a free hanging wire which is about 15mm long. I prefer not to post photos as it is confidential. 

 

I am using it in push button mode. 

 

Most of the time it works well, but occasionally (maybe once every few minutes) it locks up in the off state even though I am still touching the sensing plate. To get it to work again I need to release and retouch the sensor plate a few times.

 

I have also seen it misbehaving in other ways but the main failure mode is the one I described above. 

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