RoastLogger Hottop Controller (RL-HT-CTRL) board:

1. General
2. Perceived advantages/disadvantages of the RL-HT-CTRL board
3. Installation notes
4. Extent of testing so far
5. Disclaimer

Update on availability - February 2014:

I am delighted to say that JimG (MLG Properties) is back in production of limited quantities of the Hottop Roaster Interface and TC4/C boards and is taking orders via his website. 

It has always been made clear that the RL-HT-CTRL board was being made available in small numbers for people who could not wait for the production of the HTRI boards to start again.  As Jim is back in production of these boards I have no plans to build further batches of RL-HT-CTRL boards.  All those who have registered an interest in obtaining a RL-HT-CTRL board have been advised by email that they can now order an HTRI board.

The information on the RL-HT-CTRL board will be maintained as a reference for those who have obtained a board.  All the information on the RL-HT-CTRL boards will be made available to JimG for him to consider if he wishes to incorporate this in the future development of the HTRI boards.

1. General

Computer control of Hottop coffee roasters using RoastLogger, the Hottop Roaster Interface (HTRI) and TC4/C boards has proved popular.  Unfortunately the HTRI and TC4/C boards are not available at the moment although the supplier, MLG Properties, LLC., hopes to be back in production towards the end of this year.

I have been experimenting with a different version of these boards which provides the same computer control of Hottop roasters via the RoastLogger.  The new version (RL-HT-CTRL) board has been developed to test some perceived advantages of using a Teensy 2.0 instead of an Arduino.  See the "Perceived advantage" section below for details.   The new boards have now been sucessfully tested.  I have also modified the RoastLogger (version 1.9.2) to communicate with the new board via USB HID.  I have tested the boards on both the Hottop B and P and they should work without problems on any of Hottop models D, B, B2, B2-K, P and P2.  Please note that this will not currently work on the Hottop P2-K.  I am now using these boards for all my roasts.

Randy Glass (of espressomyespresso.com fame) has tested a RL-HT-CTRL board in a Hottop B2-K.  You can see his review here.

Board with heater LED cable:
Board with LED cable

The RL-HT-CTRL board provides control of a Hottop roaster using the RoastLogger offering the functionality of a TC4C/HTC or Arduino/TC4/HTShield.   The board is intended to be installed inside the roaster only.  Please note that this is NOT a general purpose TC4/C and has none of the inputs or outputs of a TC4/C other than those required for control of a Hottop roaster. 

As stated above, it is hoped that MLG Properties will be back in production towards the end of this year.  In the meantime, for anyone who can not wait,  I have a few PCB's left and could make up a few more boards on a first come first served basis.  The boards can be supplied fully built and tested as shown in the photos, including a Teensy 2.0, with all firmware installed ready to communicate with the RoastLogger via USB HID. 

If anyone is interested in obtaining one of these boards please email me at the email address provided in the feedback section of the RoastLogger website.  Please DO NOT post requests for boards on toomuchcoffee.com or any other forum.   Only email requests will be considered.

2. Perceived advantages/disadvantages of the RL-HT-CTRL board:


  • Provides the option of communicating with the RoastLogger via USB HID instead of RS232 serial.  This provides much more robust communication and makes it plug and play from a user perspective.  That is the user does not need to select serial ports, set baud rates etc. They just plug in the USB cable and click the start button on the RoastLoggers Arduino tab.  The RL-HT-CTRL is automatically recognised and communication begins automatically.  A separate sketch is available for RS232 Serial communication if the user prefers this for some reason.
  • When using the recommended USB HID communication no drivers are required for communication with the RoastLogger in any operating system.   If you wish to use serial communication then no driver is required in any operating system but an .inf file is required in Windows.  A suitable .inf file for Windows users can be downloaded using the "Windows Serial Installer" link on the  PJRC Teesy site.
  • Allows the user to load precompiled sketches without needing to download and install the Arduino IDE or know anything about programming or compiling sketches.  They would just download a very small Teensy Loader program that loads sketches onto the Teensy included on the RL-HT-CTRL board.   I will compile the necessary sketches and and make them available for download.   Users just plug in the board via USB, open the loader program, click file open and point to the compiled sketch that I provide and in about a second it is loaded ready to go.  Simple enough so anyone can do it.  
  • Has a smaller footprint than existing boards allowing for installation on the left hand side wall of the roaster as viewed from the rear fan cover.  This removes the need for a separate bracket to be made.  See the following installation instructions for details.


  • I do not have the expertise of Jim Gallt (MLG Properties) in PCB design, or his skills or facilities for building boards.  Whilst the design is functional and tested it is probably sub optimal in some regards.
  • The board was developed as a testbed for USB HID communication.  I am willing to build a few extra boards for people who can not wait for Jim to get back into production of the HTRI boards but only a few.  The small numbers do not justify thorough documentation so users will have to make do with the limited documentation available.
  • The RL-HT-CTRL does not include a general purpose TC4/C and has none of the inputs or outputs of a TC4/C other than those required for RoastLogger control of a Hottop roaster.  In particular it does not include an interface for an external LCD.

3. Installation notes:

Most steps of an installation are very similar to that required for the HTRI boards, see the excellent manual by Randy Glass for details.  The following notes cover the few differences and some key issues associated with the installation:

Connections to board:
Connection to board

1. The RL-HT-CTRL board is designed to be fitted to the left hand side frame of the roaster, looking from the rear fan cover.  This removes the need for a separate bracket.  It is suggested that it is fitted with it's outside edge in line with the outside edge of the metal frame and about 2 to 3 cm above the bottom of the side frame:

  • With the USB connection facing up and to the inside where it can easily be accessed to plug in and out when removing/replacing the rear cover.
  • The thermocouple connectors at the top near the outside for easy connection with good screwdriver access above the power board.
  • The Hottop control cable headers at the bottom near the front where access is good (this is in front of the power board).

Board installed:
Board installed

2. The board should be fitted using 6 to 10 mm long M3 insulating nylon standoffs.  The standoffs can be all nylon or with metal inserts as long as they are insulated such as:

M3 standoff

3. Holes can be drilled in the side frame and metal M3 bolts, 5 to 6 mm long, used to fix the standoffs to the frame.   Randy Glass has provided a PDF template which can be used to mark the holes for drilling.  If using this template please print it at full size with no scaling.  It is suggested that you check the printed template against the board to ensure it has printed without scaling before drilling any holes.

4. Nylon nuts can be used to fix the board to the standoffs.  I would recommend, however, that you use short female threaded nylon standoffs instead of nuts as these can be tightened easily by hand without the need for a small spanner which may damage the PCB.  Fixing the board to three of the standoffs in this way is recommended.  The fourth standoff is under the Teensy and more difficult to access, the standoff in this location will still support the board without a nut in place.

5. Note that the location of the board with it's outside edge in line with the outside edge of the metal frame is to give good access to the connections.  The rear cover has significant clearance from this location and the cooling airflow caused by the rear fan will keep this location cool.

6. The Teensy has a mini USB type B connector.  It is suggested you use a panel mounted USB connection on the rear cover such as this.

USB mini panel mount
To ensure a reliable USB connection you should minimise sideways stress on the mini USB connection.  This can be achieved by careful positioning of the panel mounted end on the rear cover in a location that minimises stresses with the cover in place.

7. An additional control cable is required to connect the new board.  These are available from Hottop for about $10.  Two types of cable are available, a seven conductor cable for the Hottop model D and an 8 conductor cable for all other models.

8.  This solution is intended for installation inside the roaster connected to BT and ET thermocouples fitted into the rear wall of the roast chamber.  It is highly recommended that you use insulated thermocouples (ungrounded) or, if you use grounded thermocouples you must ensure that they are well insulated from the roasters frame.  Please review the guidance notes on installation of thermocouples in a Hottop.

9. The board includes a two pin connector for an optional LED to indicate when the heater is on.  A 3 mm red led fitted into a 3 mm hole in the front plastic cover of the roaster to the right of the control panel is suggested.

10.  Set the dip switch on the board to suit your Hottop model.  See the RoastLogger manual Appendix G for details but in summary:

Switch 1
Switch function
Hottop D OFF ON
Hottop B, B2, B2-K ON
Hottop P, P2 OFF

11. Program the control panel with the correct program to suit the above settings and your Hottop model.  See the RoastLogger manual Appendix G for details.  You need to select this program each time you wish to control your roast with RoastLogger.

12. Your board will be set up for USB HID communication with the RoastLogger.  You must use RoastLogger version 1.9.2 or later.  Version 1.9.2 is the first version capable of communicating with the RL-HT-CTRL via USB HID.  Please ensure that the "Use USB HID" check box on the Arduino tab is checked.  No drivers are required for USB HID communication in any operating system.  If you are running on Linux then you need to grant permission for communication with the Teensy on the RL-HT-CTRL board.  This can be done with a udev rule.  Download the udev rule follow the link to "Linux udev rules" and the instructions in "Install Step 2".

13.  Conduct a final check of all connections:

  • Check Control panel connected to CTR header and the power board to the PWR header. 
  • Check thermocouples are connected the correct way around.  See this Wikipedia article for the colour coding of the various thermocouple types.  It is suggested you perform the following check after connecting the thermocouples and before fitting the rear cover to the Hottop:  Ensure the Hottop is unplugged from the mains, remove the roast chamber end cover and the drum.  Connect your computer via USB cable and run the RoastLogger.  On the Arduino tab check the "Use USB HID" check box and click start.  You should see the thermocouple temperatures appear on the display.  Warm each thermocouple with your fingers and ensure that the temperature increases as you warm it.  If the temperature decreases the thermocouple wires are connected the wrong way around.  Reverse the connections and try again.

14.  Please read the RoastLogger Manual, written by Randy Glass, for details of using RoastLogger to control your roasts.  In particular section 5. This includes a section on "Your first roast" and tips on "Fine tuning your profile".  Also please review the notes on adjusting the default action table settings to compensate for thermocouple installations that read low.

4. Extent of testing so far:

The USB HID communication has been tested on Windows XP, Windows Vista, Windows 7, Windows 8, OS X Snow Leopard, OS X Lion, OS X Mountain Lion, OSX Mavericks and Ubuntu 12.10.  In the unlikely event of USB HID not working on your hardware a compiled sketch to communicate via RS232 Serial is available for download from the RoastLogger downloads page.

I have tested the boards on both the Hottop B and P and Randy Glass has tested a board on a Hottop B2-K (see review here).  It should work without problems on any of Hottop models D, B, B2, B2-K, P and P2.  Please note that this will not currently work on the Hottop P2-K.

5. Disclaimer:

The RoastLogger Hottop Controller (RL-HT-CTRL)  device is an experimental electronics kit. It is intended to be used by experienced electronics hobbyists and other persons similarly knowledgeable with electronic devices.

Installation of one of these devices into your roaster may disable important safety features built into the roaster. It may also void the warranty on your roaster.

RoastLogger Hottop Controller devices have not been reviewed, tested, or certified by UL Laboratories, CSA, or any other recognised testing agencies. Any insurance you have that might otherwise cover damages resulting from a roaster fire may become voided by the use of non-approved devices such as the RoastLogger Hottop Controller.

Neither T Coxon nor any other persons or companies that may have been involved in the development, testing or documentation of the RoastLogger Hottop Controller device may be held liable for any damages resulting in part or whole from the installation or use of the RoastLogger Hottop Controller. This limitation shall apply even in the event of damages resulting from flaws in the devices that become known to T Coxon or other persons or companies that may have been involved in the testing or documentation of the RoastLogger Hottop Controller.

Whilst the RoastLogger Hottop Controller operates on 5 volts the Hottop roasters contain mains voltage connections.  Installation should only be carried out by competent persons who must ensure that the RoastLogger Hottop Controller and all associated cables and fixings are fixed in place and insulated where necessary to ensure that there is no possibility whatsoever of them coming into contact with any high voltage connections within the roaster.  Failure to adhere to this may result in high (mains) voltage being conducted outside the roaster where it may cause damage to equipment or death. If in doubt please seek expert help

All coffee roasters, whether fitted with a RoastLogger Hottop Controller or not, have to operate close to the autoignition temperature of chaff and coffee beans.  Constant careful operation is absolutely essential to avoid fires.  You should never leave a coffee roaster unattended when in operation.   You should roast in a location where a fire can be managed and suitable fire fighting equipment should always be on hand.

By purchasing, installing or using the RoastLogger Hottop Controller you are accepting any and all risks associated with its installation and use.

If you do not fully understand or fully accept all aspect of this disclaimer then you should not purchase or install the RoastLogger Hottop Controller.

While both Hottop roasters and Teensy micro-controller boards are mentioned in the documentation of the RoastLogger Hottop Controller, neither has endorsed nor is affiliated in any way with the RoastLogger Hottop Controller. Hottop and Teensy are trademarked or copyrighted names owned by their respective companies.