Keyer circuit

Posted on Feb 7, 2014

A larger version of the circuit diagram by clicking here. The circuit was drawn using Eagle from CadSoft, a schematic and PCB layout package that is free to use for small non-commercial projects. The heart of the keyer is the PIC16F877A chip. This is a 20MHz microcontroller with a generous amount of I/O capability including up to 8 A

Keyer circuit
Click here to download the full size of the above Circuit.

DC`s on board with resolutions of 10 bits each. Power to the chip is obtained by 3 x AA cells in series and a power on/off switch is used. Even with the power left on, the sleep mode of the PIC chip reduces current draw to negligible amounts. C5 and C6 are used to decouple the VSS and VDD lines of the chip, one on either side close to the pins. X1, C3 and C4 form the oscillator circuit and in this case a 4MHz ceramic resonator is used which is well below the 20MHz maximum rating of the chip. The lower frequency was chosen to prevent any layout problems on the stripboard, and 4MHz still results in a throughput of one million instructions per second plenty for a morse keyer. R1 and R2 are the speed and weight controls respectively. These are commoned on the VDD line, although the tails are taken to the RB1 pin on the chip. RB1 is normally held at logic 1 so that no current flows through the pots, assisting with the low current drain for battery operated equipment. RB1 is pulsed low to read the pots and does so continuously while operating. Pins RB4 to RB7 are the four inputs for the dit and dah paddles, and the menu and memory pushbuttons. The significance of using these pins is that they can generate an interrupt and wake the device up from sleep mode. This is of critical importance as the PIC device is sent to sleep even between characters. Finally, Q1 drives the key input of the transmitter or transceiver limited to 33V...

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