Add a button to control LED

You can control Red Pitaya’s peripherals via Web UI. In this tutorial will be shown how to turn on and off LED on Red Pitaya using parameters.


Requierment for manipulating leds using api is to first load fpga_0.94.bit fpga bitstream image. That can be done using next command line instruction: “cat /opt/redpitaya/fpga/fpga_0.94.bit > /dev/xdevcfg” Web UI

Let’s start with UI, in index.html file we have to add a button that will be used to control LED:

<button id='led_state'>Turn on</button>

and LED state label that will tell us if LED is On or Off.

< div id='led_off'>LED Off</div>
< div id='led_on'>LED On</div>


led_on div is not visible by default because when app starts all leds are off.

Also make some changes in style.css to set properties of these elements

#led_off {
    color: #F00;

#led_on {
    display: none;
    color: #0F0;

#led_state {
    margin-top: 20px;
    padding: 10px;

Then we have to add some logic in app.js, that will be executed when user clicks on the button with the mouse. This logic should change local led_state each time button is clicked and send current led_state value to backend so that Red Pitaya can update real LED state.

   APP.led_state = false;

   // program checks if led_state button was clicked
   $('#led_state').click(function() {

       // changes local led state
       if (APP.led_state == true){
           APP.led_state = false;
           APP.led_state = true;

       // sends current led state to backend
       var local = {};
       local['LED_STATE'] = { value: APP.led_state };
       APP.ws.send(JSON.stringify({ parameters: local }));

.. note::
   Parameter that transfers local LED state to Red Pitaya backend is called LED_STATE. You can change name of this
   parameter, but don’t forget to use the same name also in controller. Controller

After we send parameters we should read them in our controller. Controller source is located in


This global variable is our parameter, that we should read from server.

CBooleanParameter ledState("LED_STATE", CBaseParameter::RW, false, 0);

Parameter is a variable that connected with NGINX. Initialization has 4 arguments - parameter’s name, access mode, initial value, and FPGA update flag. Pay attention - name of parameter LED_STATE should be the same as in app.js and type(bool - CBooleanParameter, int - CIntParameter, etc…) too. This parameter updates in OnNewParams() function. This function is calling when new parameters arrived. In our case they will arrive each time you press the button in UI.

if (ledState.Value() == false)
    rp_DpinSetState(RP_LED0, RP_LOW);
    rp_DpinSetState(RP_LED0, RP_HIGH);

ledState.Update() - updates value of parameter. It takes value from NGINX by parameter’s name. That’s why names of parameters in controller and app.js should be the same. rp_DpinSetState - is a Red Pitaya API function, which sets state of some pin. Its’ arguments are rp_dpin_t pin and rp_pinState_t *state. In our program we control RP_LED0. There are 8 leds, thad we can control RP_LED0 - RP_LED7.

There are two states of a LED - RP_HIGH (turned on) and RP_LOW (turned off).

Don’t forget to init rpApp and release it in rp_app_init() and rp_app_exit().

More examples about RP APIs use can be found here.

Compile the controller, start app and try to push the button.