Wireless Doorbell Morse Code Transmitter and Printer

In this instructable, I will show you how to modify a wireless door bell into a Morse code transmitter and receiver. Also, I will show you how to make a real time Morse code printer using simple parts.

Background information:

Morse code was developed since late 1830s. It encodes letters and numbers into sequences of short and long signals called “dots” and dashes”. Morse code is used in electrical telegraph system at the beginning, and later it was adapted to radio communication.  Today Morse code is still using in military, ham radio communication, dire emergencies, etc. Morse code is an old-school way for long distance communication, and it is very easy to learn.  You can send encoded messages to your friends in a fun way.

The door bell I have is from ebay (~$5, 100-meter range). There are some advantages using wireless door bell as sending/receiving unit:
A. Low cost and easy to find.
B. It has pretty good transmitting range.
C. The frequency of transmitter and receiver are preset, no adjustment is needed.
D. Low interference.

Just for fun, I make a telegraph key using a wooden spring type clothes clip, two M3 screw nuts, one M3X8mm screw and one M3x12mm screw, two M3 washers, a couple pieces of hook up wire, some double-sided tapes, and a piece of plastic board. (picture 1)

Pull the clothes clip apart, then place two wooden pegs together and dill a hole.  The hole should be just a little bigger than the screw you use. Use a smaller drill bit to drill a hole near the corner of the plastic board, and use M3 screw tap to tap the hole. (picture 2 and picture 3)

Put a small piece of double-sided tape on one wooden peg. Bond the peg on the plastic board. Overlap the hole on the peg and hole on the board. Use the screw to clamp down a piece of hook up wire. (picture 4 , 5, and 6)

Similarly, clamp down another piece of hook up wire on the other peg with screw and nut. (picture 7)

Put the clothes clip back together. Now the telegraph key is finished. It is acting like a momentary push switch. (picture 8)

Open the back cover of the transmitter. Take out the circuit board. Drill a small hole on the side of the enclosure for the telegraph key and buzzer wires. (picture 9 &10 )
 

The transmitter is using 12VDC. I use RadioShack (073-259) 76dB Piezo Buzzer to add sound effect. Locate the power switch pins on the transmitter PCB, solder the telegraph key wires parallel to the power switch pins. Also, solder the buzzer wires parallel to the transmitter circuit after the power switch. (picture 11 &12)

Use double-sided tape to bond the transmitter and the buzzer on the plastic board. Now the modified transmitter is finished. Don’t forget to install the battery. (picture 13 &14)

The wireless doorbell circuit contains the RF receiving part, on/off logic part, and the ringtone part. Wireless doorbell is just a simple on/off remote switch. When the power button of the remote control is pressed, the receiver receives the signal and triggers the ringtone IC to play music. Our goal is to replace the ringtone IC with a Piezo buzzer (RadioShack 273-054) to make beeping sound.
Open the receiver enclosure. Locate the ringtone IC and remove it from the PCB. (picture 15)

Now we need to find the logic signal output pin. Connect the buzzer negative pin to the receiver ground and touch the circuit pins on the PCB that for the ringtone IC with the positive pin while pressing the transmitter switch. You should find out which pin’s output is controlled by the transmitter signal. The control pin’s voltage will be high when the transmitter is on, and its voltage will be zero when the transmitter is off. (picture 16)

If you think the beeper sound effect from the buzzer is good enough for a Morse code transmitter, you can just connect the positive pin of the buzzer to the control pin and connect the negative pin to ground. The Morse code transmitter is now finished. If you want to have more fun, you can follow the steps below to make a Morse code printer.

  Here are the main parts for the printer. (picture 17 &18)  

Cut a ¼” thick, 1”X 0.5” plastic board and a ¼” thick, 4”X0.5” plastic block. (picture 19)

Drill a small hole near the edge of the long block. Use screw and washer to clamp down a paper clip. (picture 20)

Use super glue to bond the small block and the long block together (I called it “pen block”). (picture 21)

Find a 12VDC computer fan. Insert 2 screws on the mounting screw holes of the fan. Tight a small rubber band on one screw. Add super glue on the rubber band and screw to fix their position. (picture 22)

Use super glue to bond the plastic blocks (pen block) on the fan. The block should be about 2mm away from the screw. Use super glue to bond the rubber band on edge of the pen block. Insert a marker pen in the paper clip. When we apply voltage to the fan, it will turn counter-clockwise and move the marker pen forward until the plastic block hits the screw. When the fan is off, the rubber band will pull the plastic block back to its original position.(picture 23)

Use super glue to glue the fan on a piece of plastic board (base board).  Put a piece of plastic block (paper block) near the tip of the pen and mark down the position of the tip.(picture 24)

Drill four small holes on paper block near the pen tip position. Insert four push pin into the holes. Leave a small gap between the plastic part of the pin and the paper block. The push pins will be the paper feeding guide. (picture 25 &26)

Use super glue to glue the paper block on the base board.  Adjust the position of the paper block. When the pen block hits the limit screw, the pen tip should touches the surface of the paper block. You can also adjust the position of the pen in the paper clip. (picture 27 &28)

Connect a spacer to a small motor. Mount the motor on a small piece of plastic block (motor block). Adjust the height of the motor so that the pen tip and the mid-point of the spacer on the motor are in the same level. Use super glue to glue the motor block next to the paper block on base board. (picture 29 &30)

Drill a hole on the base board on the other side of the paper block. Insert a nail from bottom. Use super glue to fix the position of the nail. (picture 31)

Put two spacers on the nail. Cut a small piece of cardboard paper. Punch a hole in the middle. Place the paper on the nail between two spacers.(picture 32 &33)

Cut an A4 paper into 3/8”X11” strips. Use scotch tape to connect the strips together into a single long strip. Use scotch tape to tape the paper strip on the top spacer on the nail and make a paper roll. (picture 34)

Put the paper roll on the nail. Carefully insert the paper strip though the push pins. Use scotch tape to tape the strip on the motor spacer. Now you finish all the mechanical parts.(picture 35)

Here I use 9V DC power supply for the whole system. U2 and C1 convert 9V to 5V. When the control pin is high (transmitter is on), Q1 turns on the buzzer to make beeping sound. At the same time, Q3 turns on S1 motor (the 12VDC motor) to push the marker pen to draw dots and lines on paper strip.

D1, C2, R2, Q2, and S2 (5v relay) together is a time-delay switch. When the control pin is high (transmitter is on), C2 will be charged, and Q2 will turn on S2. When the control pin is low (transmitter is off), C2 discharges through R2. Q2 will be on until the voltage on C2 is below Q2’s threshold voltage. S2 will be on for several seconds although the control pin is low. S2 is the power switch of the 555 PWM speed control circuit. The frequency is controlled by R4 and C4. Frequency = 1.44 / (R4 * C4). The speed of the motor S3 is determined by R4. When the S2 is on, S3 will be on and pull the paper strip. At the same time, the pen will draw dots and lines on the paper strip basing on the control pin signal. For a better speed control, you can replace S3 with a Continuous Rotation Servo (Radioshack 273-457). You need to change R4 and C4 basing on the servo frequency requirement. For more information, check this website: http://www.dprg.org/tutorials/2005-11a/index.html

 

Put all the electronic parts in a small box. Use double-sided tape to fix their positions and connect the motors. A nice Morse code printer is ready to go. Have fun