This morning I threw together a keyer program for
Linux PCs and Raspberry Pi

 http://abelian.org/ebnaut/ebkey.c

On the RPi, install ntpd with
(you might need to prefix these commands with sudo)

 apt-get install ntp

Check it is working using

 ntpq -p

which displays the time servers the daemon is using.  After a
minute or two, at least one should show a '*' in the left column.

Download ebnaut.c and ebkey.c and compile with

 gcc -std=gnu99 -O3 -o /usr/local/bin/ebnaut -Wall \
      ebnaut.c -lm -lpthread

 gcc -Wall -o /usr/local/bin/ebkey ebkey.c

Run in test mode to toggle GPIO output 7 (pin 14) with
one second period

 ebkey -t -S1 -m rp,gpio=7 -v

A voltmeter between pins 13 (ground) and 14 (GPIO7)
should show alternating zero and 3.3V.

Send a message with

 echo 'test message' |
    ebnaut -et -N12 -p8K19A |
    ebkey -S0.1 -m rp,gpio=7 -T +60 -r 300 -vv

The -T +60 tells it to start at the next whole UTC minute
and -r 300 makes it repeat on 5 minute slots.

On a PC it uses a serial port DTR and RTS, eg

 echo 'test message' |
    ebnaut -et -N12 -p8K19A |
    ebkey -S0.1 -m dtr+rts,dev=/dev/ttyS0 -T +60 -r 300 -vv

For summary of options

 ebkey -?

See

 https://www.raspberrypi.org/documentation/usage/gpio/

for GPIO pin connections.

--
Paul Nicholson
--