It is a "simple" "pick and place" machine, the (usually spherical and between 1 and 3 mm in size) seeds are picked up 5 at a time from the seed tray and dropped into the compost. When switching on the voltage, it is checked whether the button is pressed, if so, 2 seeds will be placed per cell. After this, the sowing beam, trolley and portal are moved to their home position. After a press on the button, the process will start; the compressor starts and the two electric valves suck and blow according to programming, so that the seeds are lifted out of the tray and dropped in the compost. During suction, when a seed is sucked to the needle, the needle lifts itself up due to the negative pressure. This makes the operation a little more visible to check. The beam also moves slowly when the needles towards the seeds in the seed tray, to prevent seed damage and the seeds from jumping out of the seed tray.
After the seed drop, the portal moves to the next column. and the process is repeated until the end of the columns (23) then the trolley moves to above the next 5 rows and the seeding starts again. Again, after 23 columns the next 5 rows and after finishing this 3rd one all 345 cells are seeded. Two small vibration motors ensure that the seed remains neatly distributed in the tray. Pause and stop with the same push button, pause short, stop > 5 sec. After a break, start a short press of the button.

Power supply for the compressor and pneumatic valves is 24V, the internal electronic supply is 5V generated on the PCB.
External structure: HW: 18mm plywood casing, linear plain bearings from IGUS for the gantry and the trolley, all the white plastic is 3-D printed, designed in Blender, printed on a Prusa MK4. PCB: design in Eagle, processor PIC16F767 I/SP programmed in assembler, PICKIT-3 (showing my age..), 5 stepper motor drivers, DRV8825, 2 for the gantry, one for the trolley and two for the beam. Electronic relay outputs for the two valves and the compressor (Air filter turned out to be essential, never knew how much very fine sand there is in compost). The speed of the compressor is regulated by a small PWM module; the different size of seeds require a different amount of suction to ensure a 100% pick-up. A 5V current-limited output for the vibration motors. Two outputs for the red/green indication LED, green indicates status ok.; orange flashing indicates pause, red flashing: error detected, red steady: stop command received. 6 digital interference suppressed inputs for the limit switches and the push button. The suction needles are two-part, a miniature cable sleeve is stuck in the seed tree, in which a hollow needle slides (dry). Silicone rings, cut from a piece of 1mm diameter silicone rig tubing ensure that the suction tip is soft and the needle cannot drop out.