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Copyright 2022 GrizzWorks, LLC - ALL RIGHTS RESERVED

Pico Keyer Electronics Notes

In order to build a suitable keyer "kit", let us take a look at what components will be necessary.

  1. Small enclosure. The enclosure would contain the RPi Pico device, a very small MAX232 UART to Serial converter module, a switching transistor such as the 2N2222A, a bias resistor for the transistor switch, and some wires to connect them all together.

  2. The UART to RS-232 converter board can be quite small, on the order of 3/8" x 5/8" in size. There are 6 wires which must be connected to it. RX, TX, Vcc and GND on one side and the RX and TX lines on the other side. A VERY GOOD OPTION is the USB to UART cables commonly used for the console connection to devices like the Raspberry Pi SBC's. DO NOT USE the "red" wire for V+ to try to power the keyer. USB 2.0 only supports 500mA of current which would be shared with the converter board in the cable, the Pico and the transistor switch. Best to have a separate power source.

  3. Raspberry Pi Pico microcontroller (or equivalent. There are defines in the source to use the Seeed XIAO 2040 board). Wiring may be soldered directly to the microcontroller via the header pin holes along both edges.

  4. Connectors: a. DB-9 Male (Plug) panel connector for the RS-232 interface.

b. 2.5mm jack, barrel, or RCA jack (or just a hole!) to supply the 3-5V power source for both the Pico and MAX232. This could be battery(ies) or a DC source from a wall wart or other DC power supply. 4 - 1.5V batteries with the V= fed through a single diode should provide approximately 5.3V max to the device, which is withing tolerance for the supply voltage. The Pico should function down to around +2V for a supply, but the MAX232 may need at least +3V. So as the battery(ies) discharge there may be intermittent function. If odd things happen, check the battery level.

Some may wish to simply lead the power wires out of a small hole in the box. Tying them in a knot inside the box helps to keep the wires from being pulled out and from straining on the Pico's and MAX232's soldered connections.

Lastly, if the USB port of the microcontroller is exposed then that may also be used to supply power to the devices. At this time, running stdio via the USB port is not suggested. In some cases, strange things happen when the Pico is plugged into a Windows machine. A second "device" is created along with the serial port and it causes some issues. For the sake of portability, use the UART version as written. Power can be taken from pin 39 for V+ and pin 38 for Ground if the USB powered option is chosen.

c. 3.5mm Stereo audio jack. Wire this to a cable to the radio or code practice oscillator for use as a straight key using the code provided here. If an iambic version of the code is ever offered, it would be wired for an iambic keyer which is part of the radio (built-in keyer). If the radio uses a 3.5mm key jack, a pre-made 3.5mm plug to 3.5mm plug cable may be used, which is rather convenient. If the radio uses a 1/4" plug, there are adapters available for those as well so a pre-made audio cable may be used.