The "At Last!" Radio TR Sequencer
By: Bertrand Zauhar, VE2ZAZ
Published in the March 2007 issue of ARRL's QST Magazine

Page last updated: 06/10/2015


This page complements article "The "At Last!" Radio TR Sequencer" published in the March 2007 issue of ARRL's QST magazine. This page also provides updates to the original article.

Please visit this page frequently, and right before assembling the project, as new information may get added regularly.

Once again, I would like to express my gratitude to Jocelyne, my wife.This project took 6+ months of spare time to develop, so she deserves it! Also, thanks to Jim-WB4GCS, Hugh-VA3TO and Dave-W6TE for beta-testing this system.

Bert, VE2ZAZ


For those who have not read the article yet, this is the best place to start learning about this project.

Reprinted with permission. Copyright ARRL, 2007 all rights reserved. This material originally appeared in QST: Official journal of the ARRL (


Note: As of October 2015, I no longer provide the blank PCB, as the demand cannot justify a quantity order of PCBs from my part. Sorry for the inconvenience.

Please contact me via email if you would like more information
on the following item described in the article:
  • A pre-programmed PIC 18F2220 micro-controller.
I will be happy to provide you with procurement info in my email reply.


(Added 29/08/2008)
Optoisolators Lumex OCP-PCT124 are no longer stocked by Digikey. You can substitute with Fairchild LTV-814 (Digikey 160-1344-5-ND) or Sharp PC814XJ0000F (Digikey 425-2439-5-ND).

(Added 31/07/2008)
A user reported back that the Firmware Version field under the Settings Transfer tab of the Windows Setup tool reports version "01" even though the firmware is supposed to be version "02". I traced back the problem to a field I did not update in the PIC program memory back when I compiled version 2. Please rest assured, all users received a version 2 chip. This is just a display issue. It does not affect functionality and it does not call for an increment in the version number. From here on though, I will ship version 2 chips that display "02".

(Added 02/04/2007)
I added a note in the Firmware Programming section to highlight that the 5V In-Circuit Serial Programming (ICSP) is not supported on the sequencer PIC firmware load.

(Added 01/04/2007)
The original QST article is now available for download in PDF form. See above.

(Added 09/03/2007)
Version 1 of  the Remote Monitoring tool is now available for download. See the software section below.

(Added 02/03/2007)
I am now shipping version 2 of the PIC firmware. This version fixes a small bug in the disarming process. It also adds remote monitoring capability. A Windows tool will allow to remotely monitor all 6 output LEDs, the PTT LED and will produce tones. This saves a lot of wiring for those who would like to remotely install the sequencer PCB (tower-mounted, for example).

I apreciate reading back from experimenters who build this project. This allows me to improve this page by providing additional clarification if necessary. Thanks!


Due to space constaints, the top-down view of the PCB layout was not kept in the QST article. Here it is for reference purpose.

Click on the figure to view it in larger size ->


This is the Windows program that provides a user-friendly interface to the T/R Sequencer system to configure it. The software provides the following features:

  • Microsoft Windows-based program with single window and tabs,
  • All parameters are set up and saved from within one program,
  • Serial port driven; supports COM1 to COM4,
  • Help provided via mouse cursor hints,
  • Only required at configuration time.
  • Non-invasive installation; does not modify the registry nor install DLLs. All files are installed within the same directory.

Version 2 of this software must be used with version 2 of the PIC firmware (currently shipping).

This software was developed and tested in a Windows 98 environment. It was also tested in Windows 2000 and Windows XP. Since these operating systems cover two main branches of Windows (9x and NT), the software is expected to run in all Windows environments from Windows 95 through Windows XP.


This optional Windows software provides the ability to remotely monitor all 6 output LEDs, the PTT LED and produces tones through the PC sound card. This saves lots of wiring for those who would like to remotely install the sequencer PCB (tower-mounted, for example). The RS-232 link is used to accomplish this.

  • Provides a visual indication of Output LEDs and PTT LED status.
  • Provides sequence completion tones, a switching alarm tone and a timeout tone.
  • Microsoft Windows-based program with a single small window.
  • Serial port driven; supports COM1 to COM4.
  • Non-invasive installation; does not modify the registry nor install DLLs. All files are installed within the same directory.


For those of you who would like to make their own "At Last!" Sequencer PCB, here are the top layer, bottom layer and top silkscreen layers saved in .PDF format. The document prints on Letter-size paper. When printing in full size (no scaling), the size and proportions should be accurate.


If you would like to look at the firmware load running inside the PIC micro-controller, well here it is! The ASM file is a text file of the source code I wrote. Beware! This is Assembly language...The code is well documented though. Have fun... ;-)

I also provide the latest HEX file required to upload the firmware load into the PIC 18F2220's program flash. This file is in 8-bit Intel HEX format, which is the industry standard for 8-bit micro-controllers. Note that 5V In-Circuit Serial Programming (ICSP) is not supported on this firmware load. All pins are assigned to sequencer functions, so the low voltage
ICSP programming pins could not be spared for that task. In order to accomplish the firmware upload, you need a PIC programmer that can handle the PIC 18F series chips and standard high-voltage programming.

Version 2 of the firmware supports the Remote Monitoring feature through a Windows program. It also
fixes a small bug in the disarming process. Updated 18/03/2011: I have updated the Version 2 files provided here. There was one small difference which I had overlooked. If you tried to program the PIC with the previous files, you may have had problems, with the PIC not starting up.


  • The 7805 voltage regulator should be mounted with its case "grounded". A small TO-220 heatsink should be inserted between the PCB and the regulator case. A good example of a suitable heatsink is Digikey's HS107-ND. A small bead of heat-conductive paste should be applied to both surfaces.
  • In-Shack Wiring. The figure to the right shows how the sequencer must be wired in order to configure and operate the sequencer from within the shack.
    • RS-232. The RS-232 connection is a simple 3-wire communication. No handshaking or strapping is required at the PC's end. As well, you do not need to set any COM port parameters within Windows; the software sets these when data exchange occurs. 
    • LED wiring is not clear since the PCB does not have any pin marking showing proper orientation during soldering. Here is how the LED's should be mounted (also see figure): All LED's (D3, D6, D9, D12, D14, D18, D21) should have their green diode anode soldered to the square pad. For Digikey's MV6461A-ND, this is the longest lead. This orientation will make the LED's turn on in green at first power up (i.e. configuration is still at default state).
  • Remote Location Wiring. The figure to the right shows what a typical remote installation of the "At Last!" TR Sequencer looks like. It basically uses (UTP) unshielded twisted pair CAT-5 network cable. The data rate used by the sequencer is 2400 bps. This slower rate allows to extend the cable run without any adverse effect on transmission quality. I have made test with 200 feet (60m) of CAT-5 cable and was amazed on how little signal quality is affected with that run. I have every reason to believe that we could double that length and still get good results. Since this installation is subject to strong RF exposure, clip-on ferrite cores can be used at both ends to reduce EMI pickup. I obvioulsy cannot provide any guarantee that the remote setup will work under any conditions. The user may have to experiment with wiring type, EMI filtering, etc.
In-Shack Wiring

Remote Location Wiring
Click on the figures above to view them in larger size.

  • Remote Monitoring Tool
    • In order for the sequence tones to be heard through the sound card and PC speakers, the sequence tones must be enabled on the TR Sequencer using the configuration software.
    • The timeout tone feature of the remote monitoring tool is managed by the tool itself and is totally independent from the timeout tone setting on the TR Sequencer.

There are a few more features that are not documented in the article, that are worth mentioning here:

  • Remote Monitoring capability. A Windows tool provides the ability to remotely monitor all 6 output LEDs, the PTT LED and will produce tones. This will save lots of wiring for those who would like to remotely install the sequencer PCB (tower-mounted, for example). The RS-232 link is used to accomplish this.
  • Configuration Checksum. In order to make the re-configuration process more robust against noisy RS-232 lines, an 8-bit checksum is calculated when the Windows software sends the configuration data to the sequencer. The PIC firmware calculates its own checksum based on the data received and compares it to the received checksum. If both checksums match, the new configuration overwrites the old one in the sequencer, otherwise the re-configuration does not occur and a failure message will result in Windows.
  • Auto-Disarm under a feedback alarm condition. When an output feedback alarm occurs, the sequencer freezes the outputs in their current state to allow the user to troubleshoot the setup and wiring. It will stay in that frozen state for a maximum of 3 minutes, after which the sequencer will auto-disarm to preserve any radio equipment that could be transmitting. Of course, the user can manuallly disarm the sequencer whenever appropriate within this 3 minute delay.
  • Auto-Disarm when transmission reaches 11 minutes.  When a transmission (Tx mode) is kept active for 11 minutes non-stop, the sequencer will auto-disarm to preserve any radio equipment that could be transmitting. This is provided in addition to the Timeout tone feature and is seen as a "last resort" measure.
  • Debouncing of the sequencer PTT. The sequencer PTT line is debounced in software. This provides a protection against fast transitions that could lead to radio system malfunction.
  • Watchdog. The micro-controller also has a hardware watchdog reset feature. In the unlikely event that the firmware should crash, the built-in watchdog will reset the micro-controller. This forces all outputs to go floating (open state). The maximum time for the watchdog to kick in is 4 milliseconds.