Keyer circuit

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Keyer circuit, last updated 2018-11-13

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Keyer circuit

You can see 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.

Circuit description

The circuit diagram is displayed on the link above a description is given here.

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 ADC’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.

R3 brings the MCLR# line up to perform a device reset when power is applied.

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.

Capacitors C7 and C8 decouple the inputs from the paddle to avoid stray RF latching the inputs to the keyer IC.

Finally, Q1 drives the key input of the transmitter or transceiver limited to 33V and 100mA. It is assumed that the key input of the TX is some positive voltage which is shorted to ground by the “key”. If negative voltages are used, or a different protocol, then changes are necessary to this area.

D1 a 33V Zener diode protects Q1 from both high voltages and reverse voltages. C9 is another decoupler to prevent RF getting back into the keyer.

Menu system

Pressing the menu button invokes the menu mode. It will stay in the menu mode for up to 5 seconds, then timeout with dit meaning E for Exit. Pressing the menu button again will take it out of the menu.

The reason for the timeout is that the power saving sleep mode is not activated while in the menu!

The menu commands follow, bold type indicates responses from the keyer itself.

E – Exit

Exits the menu. You can also accomplish this by waiting 5 seconds, or pressing the menu button again. Keyer responds with E.

I – Information

Key in dit-dit and the keyer will respond with M1, M2 or M3 depending on which memory is in use.

M – Memory select

Key in dah-dah and the keyer will respond with N to indicate a number is needed. Valid entries are dit-dah-dah-dah-dah, dit-dit-dah-dah-dah or dit-dit-dit-dah-dah. Shortcuts are dit, dit-dit, and dit-dit-dit respectively.

R – Record

Records over the current memory. Key in dit-dah-dit and the keyer will respond with R to signify record mode. Record the message on the paddles and stop, the keyer will automatically recognise the end of the message and respond with K. Up to 64 characters can be recorded.

S – Speed enquiry

Key in dit-dit-dit for a speed enquiry. The keyer will respond with the current WPM in quarter WPM steps, e.g. 18.25.

T – Tone on/off

Key in dah and the sidetone will be changed from its current setting. The keyer will respond with Y or N depending on whether the sidetone is on or not. The sidetone is always on during menu operations, even if switched off with T.

X – eXchange paddles

Swaps left and right paddles over (the change is instant). Handy for those who are left-handed or people like me who can't make their mind up...