> For the complete documentation index, see [llms.txt](https://dev.ubidots.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://dev.ubidots.com/dashboards-and-widgets/html-canvas/examples/create-an-lcd-screen-with-the-html-canvas.md).

# Create an LCD screen with the HTML Canvas

This example shows how to create an LCD screen on a dashboard using the HTML Canvas widget.

#### HTML:

```html
<div id="image_background">
<p>Last value:</p>
<span id="image_background_text">No data</span>
</div>
```

#### CSS:

```css
@import url('https://fonts.googleapis.com/css?family=VT323');

#image_background {
background: url('https://i.imgur.com/uuYygjz.png') 0 / contain no-repeat;
font-family: "VT323";
font-size: 25px;
letter-spacing: 10.5px;
height: 220px;
width: 450px;
}

#image_background p, #image_background_text {
margin: 0;
left: 75px;
position: absolute;
}

#image_background p {
top: 88px;
font-size: 25px !important;
}

#image_background_text {
top: 121px;
}

```

#### JavaScript:

In the following code, set `VAR_ID` to the variable ID to display:

```javascript
var bg = document.getElementById("image_background");
var text = document.getElementById("image_background_text");
var lastValue = null;
var TOKEN = ubidots.token;
var VAR_ID = "variable-id";

setInterval(function () { getLastValue(VAR_ID, TOKEN); }, 2000);

function makeHttpRequest(method, url, headers, payload) {
  return new Promise(function (resolve, reject) {
    var options = {
      method: method,
      headers: headers,
    };

    if (payload) {
      options.body = JSON.stringify(payload);
    }

    fetch(url, options)
      .then(function (response) {
        if (!response.ok) {
          throw new Error(response.statusText);
        }
        return response.json();
      })
      .then(function (data) {
        resolve(data);
      })
      .catch(function (error) {
        reject(error);
      });
  });
}

function changeImage(value) {
  var choose = "normal";
  var background = {
    blue: "https://i.imgur.com/ZE0W7Yx.png",
    normal: "https://i.imgur.com/uuYygjz.png",
    yellow: "https://i.imgur.com/RHvDkUE.png",
  };
  if (value <= 30) {
    choose = "blue";
    bg.style.color = "white";
  } else if (value > 30 && value <= 70) {
    choose = "yellow";
    bg.style.color = "white";
  }
  bg.style.background = "url(" + background[choose] + ") 0 / contain no-repeat";
}

function getLastValue(variableId, token) {
  var url =
    "https://industrial.api.ubidots.com/api/v1.6/variables/" +
    variableId +
    "/values";
  var headers = {
    "X-Auth-Token": token,
    "Content-Type": "application/json",
  };

  makeHttpRequest("GET", url + "?page_size=1", headers)
    .then(function (res) {
      if (lastValue === null || res.results[0].value !== lastValue) {
        text.textContent = res.results[0].value;
        lastValue = res.results[0].value;
        changeImage(lastValue);
      }
    })
    .catch(function (error) {
      console.error(error);
    });
}
```

Once you save the settings, it looks like this:

<figure><img src="https://884329393-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-MhzNRg0B4ECiNXc093G%2Fuploads%2FP5ivCSYYeRcdB3DNGmVn%2Fimage.png?alt=media&amp;token=d270fea4-9111-481c-a401-014ed2c65d49" alt="" width="563"><figcaption></figcaption></figure>

### Adjusting the screen's contrast

Update the HTML to:

```html
<div id="image_background">

<img id="display_background" src="https://i.imgur.com/meoKhMG.png" />

<p>Last value:</p>
<span id="image_background_text">No data</span>
</div>
```

Update the CSS to:

```css
@import url('https://fonts.googleapis.com/css?family=VT323');

#image_background {
background: url('https://i.imgur.com/ehNdUxY.png') 0 / contain no-repeat;
font-family: "VT323";
font-size: 25px;
letter-spacing: 10.5px;
height: 220px;
width: 450px;
}

#image_background p, #image_background_text {
margin: 0;
left: 75px;
position: absolute;
}

#image_background p {
top: 88px;
font-size: 25px !important;
}

#image_background_text {
top: 121px;
}

#display_background {
height: 94px;
position: absolute;
top: 70px;
left: 52px;
pointer-events: none;
}
```

Update the JavaScript to:

```javascript
var bg = document.getElementById("display_background");
var text = document.getElementById("image_background_text");
var lastValue = null;
var lastContrast = null;
var TOKEN = ubidots.token;
var VAR_ID_LV = "id-of-variable-to-be-displayed";
var VAR_ID_CONTRAST = "id-of-contrast-variable";
var scale = d3.scaleLinear().range([0, 4]).domain([0, 1023]);


setInterval(function () {
    // Get LCD Screen last value
    getLastValue(VAR_ID_LV, TOKEN, function (res) {
      var value = null;
      try {
        value = res.results[0].value;
      } catch (e) {
        console.log("No data");
      }
      if (
        (lastValue === null || value !== lastValue) &&
        value !== null
      ) {
        text.textContent = value;
        lastValue = value;
      }
    });

    // Get LCD Screen contrast
    getLastValue(VAR_ID_CONTRAST, TOKEN, function (res) {
      var value = null;
      try {
        value = res.results[0].value;
      } catch (e) {
        console.log("No data");
      }
      if (
        (lastContrast === null || value !== lastContrast) &&
        value !== null
      ) {
        bg.style.filter = "contrast(" + scale(value) + ")";
        lastContrast = value;
      }
    });
  }, 2000);


function makeHttpRequest(method, url, headers, payload) {
  return new Promise(function (resolve, reject) {
    var options = {
      method: method,
      headers: headers,
    };

    if (payload) {
      options.body = JSON.stringify(payload);
    }

    fetch(url, options)
      .then(function (response) {
        if (!response.ok) {
          throw new Error(response.statusText);
        }
        return response.json();
      })
      .then(function (data) {
        resolve(data);
      })
      .catch(function (error) {
        reject(error);
      });
  });
}

function getLastValue(variableId, token, cb) {
  var url =
    "https://industrial.api.ubidots.com/api/v1.6/variables/" +
    variableId +
    "/values";
  var headers = {
    "X-Auth-Token": token,
    "Content-Type": "application/json",
  };

  makeHttpRequest("GET", url + "?page_size=1", headers)
    .then(function (res) {
      if (typeof cb === "function") cb(res);
    })
    .catch(function (error) {
      console.error(error);
    });
}

```

Lastly, add the third-party library required for this implementation:

```
https://cdnjs.cloudflare.com/ajax/libs/d3/4.9.0/d3.min.js
```

Result:

<figure><img src="https://884329393-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-MhzNRg0B4ECiNXc093G%2Fuploads%2FXtF1JXwws92UFNvQhCLo%2Fimage.png?alt=media&amp;token=5300a7c7-6eea-4665-a409-f0b221c46bfc" alt=""><figcaption></figcaption></figure>


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation by asking a question.

Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters:

```
GET https://dev.ubidots.com/dashboards-and-widgets/html-canvas/examples/create-an-lcd-screen-with-the-html-canvas.md?ask=<question>&goal=<user_goal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.