Coding

Ever since I advanced beyond the “hello worlds” of Java, I had never stopped coding. This section is for documenting code usages that I had employed while solving the technological problems that I had encountered.

How to prevent “Uncaught ReferenceError: jQuery / $ is not defined”

When you want to build a microsite, jQuery can help you simplify client side interactivity. If you build your website with WordPress, then you will be able to use jQuery in the custom theme that you are building.

Since you will base your site interactivity on jQuery, you may write JavaScript codes that throw the following error:

Uncaught ReferenceError: jQuery / $ is not defined.

In some cases, the error may appear randomly, making it hard to detect.

So how can we prevent our JavaScript codes from throwing a “Uncaught ReferenceError: jQuery / $ is not defined”?

How to use pycrypto, python-qrcode and Flask-RESTPlus to create QR codes that can send encrypted data to an endpoint

If you want to direct someone to a web page without saying a word, then you can use QR codes to do so.

For example, your QR code reader will direct your phone’s browser to visit our home page when you scan the following QR Code:

Techcoil home page QRCode

Given that, you can use QR codes to send HTTP requests to an endpoint of your HTTP server. In addition, you can embed data that you wish to send to the endpoint in the QR codes.

With this in mind, let’s look at how we can use pycrypto, python-qrcode and Flask-RESTPlus to create QR codes that can send encrypted data to an endpoint.

How to use threading.Condition to wait for several Flask-APScheduler one-off jobs to complete execution in your Python 3 application

Previously, I discussed how to use Flask-APScheduler in your Python 3 Flask application to run multiple tasks in parallel, from a single HTTP request.

When we run jobs as discussed in that post, jobs are ran once by the underlying ApScheduler instance. In addition, our Flask endpoint return the HTTP response back to the HTTP client as soon as the jobs are scheduled.

If we do not want the HTTP client to know the outcome of the jobs within that HTTP call, then we are good. But what if we want to include any errors that the jobs encounter in the same HTTP response?

In such a situation, we will need a mechanism to wait for the one-off jobs to complete execution before returning that response.

Given that in mind, this post shows how we can use threading.Condition to wait for several Flask-APScheduler one-off jobs to complete execution.

How to create an interval task that runs periodically within your Python 3 Flask application with Flask-APScheduler

Previously, I talked about how to use Flask-APScheduler in your Python 3 Flask application to run multiple tasks in parallel, from a single HTTP request.

If you wish to run long running tasks triggered by an HTTP request, then that post will help you do so.

However, what if you want to run jobs periodically without blocking your Flask HTTP server from serving HTTP requests?

In this case, you will want to run an interval task with Flask-APScheduler.

Given that, let’s look at how we can use Flask-APScheduler to create an interval task within your Python 3 Flask application.

How to post JSON data to a HTTP server endpoint from your ESP32 development board with ArduinoJson

When sending data from your ESP32 development board, you may choose to send the data in JSON.

In such a situation, you can consider using ArduinoJson to help you construct the JSON data.

Given that, let us look at how we can post JSON data to a HTTP server endpoint from a ESP32 development board with ArduinoJson.

How to read RFID tags from SparkFun RFID USB Reader with Python 3

When you have an RFID starter kit from Sparkfun, you will be able to read RFID tags through serial.

Previously, we saw how to use an ESP32 board to read RFID tags from a SparkFun RFID USB Reader.

Given that, I was able to build a ESP32 prototype to scan tag ids from RFID cards.

In order for that ESP32 prototype to recognise what each of my tag card represents, I need to label the ids. Whenever my ESP32 prototype gets a tag id, it will query a tag catalogue to see what that tag id represent.

Since it is easier to label the tag cards from a computer, I built a Python 3 application to read the RFID tags from the SparkFun RFID reader.

So how we can read RFID tags from SparkFun RFID USB Reader with Python 3?

If you are looking for a way to read RFID tags from SparkFun RFID USB Reader with Python 3, then this post is for you.

How to control a servo motor with an ESP32 development board

When I try to reuse the Ardunio Sketch to control a servo motor with a ESP8266, I encountered the following error during compile time:

WARNING: library Servo claims to run on (avr, sam, samd, nrf52, stm32f4) architecture(s) and may be incompatible with your current board which runs on (esp32) architecture(s).
In file included from /Users/i309663/Documents/learning/Arduino-sketches/sketch_arduino_servo/sketch_arduino_servo.ino:1:0:
/Users/i309663/Arduino.1.8.7/Arduino.app/Contents/Java/libraries/Servo/src/Servo.h:73:2: error: #error “This library only supports boards with an AVR, SAM, SAMD, NRF52 or STM32F4 processor.”
#error “This library only supports boards with an AVR, SAM, SAMD, NRF52 or STM32F4 processor.”
^
exit status 1
Error compiling for board ESP32 Dev Module.

Therefore, I had to find another way to control my SG90 servo motor with an ESP32 development board.

Given that, this post shows how to control a servo motor with an ESP32 development board.

ESP32 devkit-c v4 connected to sg90 servo motor