9 Replies Latest reply on Apr 5, 2015 6:00 AM by user_14586677

    Connecting a potentiometer to a PSoC4 BLE pioneer kit

    ahmed.akl

       I've the PSoC 4 BLE pioneer kit. I want to use an external potentiometer to control the LED. My questions are:

         

      1- what are the pins that i should use to connect the potentiometer (i.e. VCC, GND). In other words, i want to use the power pins in the board instead of using an exernal power source

         

      2- If i want to use an external power source what should i do?

        • 1. Re: Connecting a potentiometer to a PSoC4 BLE pioneer kit
          user_1377889

          The interconnections of the BLE Kit are shown in the kit's documentation which you will find in

             

          C:\Program Files (x86)\Cypress\CY8CKIT-042-BLE Kit\1.0\Hardware\BLE Pioneer Board

             

          when you used the standard installation path. Have a look into the -pdf files.

             

           

             

          Applying a voltage to the pin that usually drives the LED might work, but could damage the PSoC when the pin is not in Hi-Z mode, an externally applied voltage might quite sure blow your BLE Kit.

             

          Controlling LED brightness is not just as easy as controlling a light bulb. LEDs should be driven at a constant current which easily works with a fixed resistor as (Vdd -VLED) / R shows, VLED depends on LED-type, typical 1.3V for red LEDs, 1.7 for green,, up to 2.7V for blue and whites.

             

          So another commonly approach used (ie. for car backlights, you might have seen this already) is to switch the LED on and off periodically with a high frequency (200Hz). The ratio between on and off time is named "Duty cycle" and can be between 0 and 100%, which is exactly how we want aN LED to be controlled. A Creator component featuring this is the "PWM" that has adjustable properties as "Period" which you should set in a way that together with your incoming clock you get a frequency of the above mentioned 200Hz The other property "Compare" you may vary between 0 and the period value to get the above named duty cycle of 0 to 100 per cent.

             

           

             

          The next problem is: How to "read" the potentiometer value?

             

          Well, here the ADC comes in handy.

             

          Connecting VDD and GND to the ends of your potentiometer will give an adjustable voltage on the wiper which can be transferred into a number by the ADC. There is (as usual) an example project for PSoC4 using the SAR ADC which you should study and compare with the datasheet. I am quite sure that there is an example for the PWM as well.

             

          Use the pins the build-process assignes, they will at least fit.

             

          When having any troubles, post your complete project here, so that we can have a look at.  To do so, use
          Creator->File->Create Workspace Bundle (minimal)
          and attach the resulting file.


          Happy coding
          Bob
           

          • 2. Re: Connecting a potentiometer to a PSoC4 BLE pioneer kit
            user_14586677

            Consider using the PriSM component for controlling LED brightness.

               

            It produces less EMI than a standard PWM approach.

               

             

               

             

               

             

               

             

               

             

               

            To compute the series limiting R for LED Rled = (Vdd - Vled) / Iled

               

             

               

            If using off board LED then use alternative connection, you get more drive

               

            current and better Iled control.

               

             

               

               

             

               

            Regards, Dana.

            • 3. Re: Connecting a potentiometer to a PSoC4 BLE pioneer kit
              ahmed.akl

               Thanks Bob & Dana for the valiable information

              • 4. Re: Connecting a potentiometer to a PSoC4 BLE pioneer kit
                user_14586677

                Gad to have been of assistance.

                   

                 

                   

                Dana.

                • 5. Re: Connecting a potentiometer to a PSoC4 BLE pioneer kit
                  ahmed.akl

                   I know that my question is naive, but i don't want to damage my board due to a wrong connection or a missing information from my side.

                     

                  According to the kit's schematics, I can connect the potentiometer to VCC and GND of J1, then connect the wiber to any input pin such as p2[5] in J2.

                     

                  Am I right?

                  • 6. Re: Connecting a potentiometer to a PSoC4 BLE pioneer kit
                    user_1377889

                    That is quite correct. With PSoC4 you always should use Port2 pins for analog inputs, since only those are routed to the SAR ADC. Remember that there is a current floating through the potentiometer which should be limited to something like 1-2mA that turns out to be a yalue of 2k ohm.

                       

                     

                       

                    Bob

                    • 7. Re: Connecting a potentiometer to a PSoC4 BLE pioneer kit
                      ahmed.akl

                       So this means that i need to connect a resistor (i.e. 2K ohm) between the potentiometer and the VCC (as in attached figure) to limit the current entering the pin when it is connected to the VCC through the potentiometer.

                         

                      Right??

                      • 8. Re: Connecting a potentiometer to a PSoC4 BLE pioneer kit
                        user_1377889

                        N o, what I wanted to express is that you should use a 1-2k ohm potentiometer.

                           

                         

                           

                        Bob

                        • 9. Re: Connecting a potentiometer to a PSoC4 BLE pioneer kit
                          user_14586677

                          Your pot Resistance is govered by your allowed Pdiss on low

                             

                          end and leakage current on high end. So for a 3.3V system a

                             

                          2K pot would have a Pdiss of ~ 5.4 mW, not a big deal generally

                             

                          speaking, unless battery operated, then wasting power directly

                             

                          impacts battery lifetime.

                             

                           

                             

                          On the high side you do not want pot IR drop to create errors at

                             

                          A/D input. So you start by assigning an allowed error for A/D signal

                             

                          path, then calculate if leakage of A/D input would exceed your pot

                             

                          IR drop equivalent error. Note there are AC considerations as well

                             

                          if response time an issue, eg. its not just a DC consideration.

                             

                           

                             

                          Point of all this is in product design your calulate to confirm the

                             

                          decisions you make are good engineering practice.

                             

                           

                             

                          Regards, Dana.