Kegboard

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The Kegboard is a special-purpose circuit built to control and monitor a kegerator. It consists of two relays and a PIC microcontroller, a few inputs for sensors, and a computer interface. It is designed to:

  • Control two AC outlets (one connected to a freezer and one connected to a solenoid valve, in the typical case)
  • Monitor a temperature sensor
  • Monitor a flow sensor

This page describes some of the details of the current kegboard's design, and ideas for revisions.

Contents

Current Design Details

Microcontroller
Kegboard 1.0 uses a Microchip PIC16f628 microcontroller. Main features for this chip are: onboard USART, plenty of GPIOs, cheap, easy to program, onboard oscillator, plenty of ways to flash. Interface with the host computer simply involves connecting a MAX232 to the TX/RX pins. Alternately, a module containing a FTDI USB-Serial chip can be plugged in for 'free' USB support.
Relays
Currently using two normally closed, non-latching relays rated for 10A @ 240VAC. Most fridge compressors should fall quite comfortably within this spec.
Flowmeter
A hall effect sensor flow meter is connected to an interrupt pin on the microcontroller. The microcontroller maintains a 2 byte wide counter of pulses.

Schematic

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Kegboard 1.01 Schematic

At right is an image of the most current Kegboard schematic. Version history below:

  • v1.01 - 08/03/2005 - part n/a
    • Fix(schemo): FTDI LED orientation
    • Fix(layout): Relay coil trace width
    • Fix(layout): Relay coil trace placement
  • v1.00 - 07/26/2005 - part kb-002
    • First PCB design
    • Single relay type for both switches (JS1a)
  • v0.10 - 10/01/2003 - part n/a
    • Breadboard version
    • Not widely produced :)

Parts List and Cost

Note: the following table is incomplete as of this revision. Discrete components (resistors, etc) are omitted for the time being. Please also see the Revision Plans section following this.

Kegboard 1.01 Parts List
Name Part Description PN (Digikey unless noted) Approx. Cost
IC1 PIC16f648A Microcontroller PIC16F648A-I/P-ND $3.63
IC2 MAX232 RS232 Level Convertor 296-1402-5-ND $0.90
RELAY1 JS1a-5v AC/DC Power Relay 255-1106-ND $1.15
RELAY2 JS1a-5v AC/DC Power Relay 255-1106-ND $1.15
U1 FTDI232BM USB-Serial Adapter (optional) 626-DLP-USB232M (Mouser PN) $25.00

Controller Interface Spec

The microcontroller presents a serial interface to the world. The Kegboard plugs in to a serial port and speaks at 115200bps. An old version of the firmware, written in JAL, is available here: Kegboard.jal.txt

Commands

You can say the following things the the kegboard, each by writing the specified character:

CMD_STATUS = 0x81
Tell the microcontroller to send a status packed (described below)
CMD_VALVEON = 0x83
Enable the solenoid valve (RELAY1)
CMD_VALVEOFF = 0x84
Disable the solenoid valve (RELAY1)
CMD_FRIDGEON = 0x90
Enable the freezer relay (RELAY2)
CMD_FRIDGEOFF = 0x91
Disable the freezer relay (RELAY2)

Reading the thermostat is not included in the current Kegboard.jal.txt but is mostly there. Matt S. also has a pure PIC ASM firmware that handles 1-wire reads of a ds1820.

Status Packet

The kegboard defines a single message, the status packet, which is returned under either of the following circumstances:

  1. When it is requested with CMD_STATUS
  2. Following any other command

(Yes, this means that CMD_STATUS is a NOOP!)

The following is the format of the status packet:

BYTE BYTE BYTE BYTE BYTE BYTE BYTE BYTE
'M' ':' fridge status valve status flow high flow low '\r' '\n'

Revision Plans

  • Kegboard 1.1
    • Replace the 16f6XX series PIC with an 18 series PIC that includes an onboard USB USART. (The DLP module should no longer be necessary; it was a kludge to support the older RS232-only PICs. Works nice, but way too expensive, and I'm not surface mounting the 232BM..)
    • Remove legacy MAX232 altogether (in conjunction with previous)
    • Modify power relays by adding two more pads so that the relays don't necessarily have to share the same source.
    • Possibly unite FLOW and THERMO connectors in to one 4-pin header, eliminating the redundant VCC/GND lines (but possibly reducing flexibility?)
  • Future
    • Investigate an alternate uC (eg AVR)
    • Use or support the Digi Connect ME as the controller itself or as an ethernet-to-serial interface
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