Roger Recasens

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6 years, 10 months ago.

Voltage Analog input (FRDM KL25Z)? 3,3V or 5V?

Hi,

I read in the manual of FRDM-KL25Z that it can support a maximum voltatge of 3.3V but I have a force sensor (FlexiForce A201) that works with 5V (the output sensor is 0-5V).

The circuit consists of two resistors connected in serie. The first (R1) is a resistance of the sensor and the second (R2) is connected to GND, and it forms a voltage divider. Between the two resistors (Vout) is connected the AnalogIn pin, as you can see in the picture:/media/uploads/RogerRF/divisor_de_tensio.jpg

The function analogIn read() returns 0.0 to 1.0, which 0.0 means 0V and 1.0 means 3.3V.

But, R1 is a variable resistor (the minimum resistance is 3k ohm) and the R2 is 10K ohm. The Vout would be maximum 3.85V, and read() should return 1.0, but it is impossible to reach this value. Itis working if R2 is 100K or more, and then the Vout is 4.85V (the read returns 1).

My questions are: The maximum voltage than the FRDM-KL25Z can read is 3.3V or 5V? Did I do some wrong?

Thank you!

Question relating to:

Last updated: 30 Jun 2020
The FRDM-KL25Z is an ultra-low-cost development platform for Kinetis L Series KL1x (KL14/15) and KL2x (KL24/25) MCUs built on ARM® Cortex™-M0+ processor. Features include easy access to MCU I/O, battery-ready, …

Apologies for being late to add a comment on this. The specification for the input resistance to the ADC pin (Radin) is specified to be typically 2k in with a max of 5k in. This will significantly affect the output of your network, and may not be very stable with temperature. Ideally, you should buffer the output of your network to provide a low impedance input to the ADC. Also, be careful not to exceed the voltage input specifications.

posted by Alan Kitchen 16 Jan 2018

1 Answer

Andrea Corrado
6 years, 10 months ago.

Hi Roger :)

Thanks for the question, a couple of things.

1) The nominal input voltage is 3.3V with a maximum of 3.6V. 2) You should not be feeding 5V directly to the GPIO!

You need to use a voltage divider to scale your output from 0 to 3.3V to get useable sensor data, its unclear already if you have done this, is the following circuit the sensor or your own voltage divider?

Its just a matter of testing and determining, when should your max force give 3.3V and when min force should give 0V, scale your resistance accordingly.

Regards,

Andrea, team mbed

Accepted Answer

Thank you!

posted by Roger Recasens 09 Jun 2017