The post services are a part of everyday life nowdays. There's a couple different reasons that point toward the necessity of the postal service. Number one will always be because of its ability to connect people. Number two the E-commerce, which nowdays is booming and many other reasons make the postal services so important.

But what happens when a mail comes? Most people have to walk to their mail box in order to check if there a packet, a bill or a letter from a family member in the inbox. This can be very exhausting if you live on the fourth floor in a building without elevator and the mail boxes are located on the ground floor.

So the postbot is going to make your life much easier by informing you that the post man has dropped something in your mail box. The principle is simple, two leds illuminate two photo resistors, if there is something between the detecting elements, a big amount og the light will not reach the photo resistors. This way detects the microcontroller the letter and sends to the second module in your appartment a notification with sound and text!

Supplies

2x Arduino Nano

2x 10k Resistor

2x 220 Resistor

2x 5mm LED

2x Photoresistor

1x 433 MHz Receiver/Transmitter Module

1x MAX7219 Dot LED Matrix Display

1x 297x420x4mm Plywood sheet

1x Active Beeper

4x 2x 1,5V Battery Holder or two 5V Power Banks

25x Dupont Jumper Wire Cables

1x 2x 8cm Prototyping Board

Material costs about $30.

Step 1: Making the Letter Holder

The letter hoolder is a wooden case which will be placed in the post box and will hold the sensors, the microcontroller, the battery pack and the transmitter. I used 4 mm plywood to construct the holder but you can also use other materials.

You can find and download the patterns of the pieces for this part at the pdf file LetterHolderPattern. This holder is designed to fit in a mail box with the following dimensions 310 x 210 x 80mm. You can modify the dimensions direct at the AutoCAD file if you want to keep the design but the dimensions of your post box don't pass to my design.

Cut the parts and sand the edges with a fine sandpaper them, then glue the parts together as seen on the pictures and let 24 hours the glue dry .

Step 2: Prepairing and Installing the Electronics in the Letterbox

Each 5mm led needs a 220 Ohm resistor, which has to be soldered on the positive side. Both leds use a common wire to the ground. The digital pins D8 and D9 are driving the leds and the analog pins A0, A1 are reading the input voltage from the LDRs. From the circuit diagramm you can find more information about the detecting element.

The transmitter 433 MHz needs a power supply and the third pin at the middle is connected to the pin 10 of the Nano microcontroller. The modules comes usually without antenna which reduces dramatically the communication range, to extend the range i soldered a 34.6 cm wire at each module.

For the battery pack i used two 2 x 1.5 V AA battery holders, which i glued together and connected in series by soldering the positive cable of the first one to the negative of the second one in order to have a voltage of 6 V for alkaline batteries and 4.8 V when four Ni-MH rechargable batteries are used. Another option is to use a power bank directly connected to the usb power supply of the arduino.

The power supply was placed at the left side, in the middle the microcontroller and at the right side the 433 MHz transmitter. The cables to connect the parts are common arduino project jumper Dupont wires. I have used a prototyping board to connect in a row all positive and another row for all negative cable wires, i finally placed this part on the middle next to the arduino nano.

Step 3: Making the Receiver Case

The receiver case should hold the led dot matrix, the microcontroller with the 433 MHz receiver and the power suppply. I have made a custom design matching the instructables robot and named it postbot. The design was first copied from paper to the plywood, then the frame for the display removed and finally using a pyrograph the pattern was created.

The matrix display has two pins for the power supply, the DataIn is connected to pin 12, LOAD(CS) is connected pin 11, and the CLK pin is connected to pin 10. The anode of the beeper is connected to the pin 13 and the arduino can be either powered by a power bank or a 5 Volt plug power supply.

Step 4: Programming Receiver and Transmitter

In order to communicate with the 433MHz modules Arduino needs the RCSwitch.h library and the LedControl.h library is used for controlling the dot matrix display. I also used the LowPower.h library for energy saving purposes at the transmitter module as it is powered with batteries.

The code at the transmitter is at first setting on the leds and then reads the input values of the photo resistors. The difference of the two readings is used to calibrate sensors. The next step is to read the value of the first led and determine if there is an obstacle between led and photo resistor, if there is nothing between them the second led is turned on and if there is also nothing detected then the value of the last reading is transmitted to the receiver.

Once the Receiver-arduino receives a signal, it has to be determined if the value corresponds an empty post box or not. If there is no mail a short beep informs that the box is empty and a X appears on the dot display, otherwise a mail symbol is shown an a long beep sound infroms you that You've got Mail!

Congratulations the you have made everything correct. If you like the instructable, have questions or need help please let me know.