ELECTRONICS - [Morse Code Typewriter] - [page 2/3]

16F628A Morse Code Typewriter

The morse circuit

The image below contains the complete schematic diagram for the morse coder device. As you can see the hardware part is very easy to understand.

An AC/DC voltage is applied to the connector 'POWER'. This voltage is rectified and flattened by the diodes and the capacitor C1. The voltage regulator LM7805 will turn the voltage into a nice stable 5 VDC voltage.

The circuit is build around the PIC 16F628A. This PIC is running on a 20 MHz crystal. We need this high speed because our controller needs to be fast enough to decode the keyboard signals.

The PS/2 keyboard is connected to the PIC's ports RB0 (CLOCK) and RB1 (DATA). Our keyboard also needs some power: the PS/2 connector is also connected to the GND and +5V lines.

The LCD module requires a little bit more wires: We will run it in 4-bit mode which means we only need the data lines D4 to D7. We also need to control the E-signal (Enable, pin #6) and the RS-signal (pin #4).

The PIC also controls a LED on output RA0 and a speaker on output RB3. This port will deliver a PWM (Pulse Width Modulation) controlled signal. I used a small BC547 amplifying transistor because we can't connect our speaker directly to the PIC's output (it can't deliver enough power).

Information

I have built a very basic amplifier for the morse code speaker. It is no real audio amplifier. You should never apply a flat DC signal at the input of the amplifier. This wil literally cook the loudspeaker!

I used the PIC's build-in PWM generator to generate the audio signal. The PWM output frequence is approximately 1 kHz with a duty-cycle of 50%. This frequence is about the minimal PWM frequence the PIC can output.

The PS/2 connector

The image below shows you the connections of a PS/2 connector. Only 4 wires are actually in use: +5V, GND, CLOCK and DATA. It is important that you don't switch the polarity of the power supply! This can kill your keyboard as some keyboards aren't protected against a reversed polarity.