Photogate Box

The photogate Box is an interface box intended to act as an interface between a photogate and a computer. The Photogate box includes a microcontroller with built-in hardware timers.

For more information on what a photogate is link.

This branch is for the PIC18F2620 MCU (used in Richmond photogate boxes).

There is also a branch for the PIC18F4525 MCU (used in Surrey photogate boxes).

Source code in C

The source code for this project is in C and is licensed under the GNU GPL v3. See the file PhotogateLV.c

Modified in 2014 for XC8 compiler and to use an external clock.

Added existing HEX file to project Files

Since this code has been working fine and was compiled years ago with an old compiler version and legacy peripheral libraries that we stooped using a number of years ago I have use a PICkit3 to read the memory of the existing unit and saved it as a hex file. If a new photogate timer box is to be constructed and programmed with this hex file.

• hex/timer_box_PIC18F2620.hex

There is a six pin programming header on the PIC board in the photogate timer box available for programming. In 2023 the MPlab IPE still works with a PICkit3.

Communication between Timer Box and Host

In general the timer box firmware sits in the main while loop waiting for a command from the host. A command always starts with a question mark followed by a single numeric character. The rest of the command is command dependent. Onces times have been aquired by the timer box they are sent to the host as four byte binary values.

Testing Timerbox with Coolterm

Coolterm can be used to test if a tiemrbox is working for troubleshooting.

The COM port used will vary from computer to computer. The baud rate and other serial port settings as below:

To check the status of the two photogates I just type ascii --> ?0

And it immediately responds with:

0 if both gates unblocked 1 if gate one blocked and gate two unblocked 2 if gate one unblocked and gate two blocked 3 if both gates blocked.

Notes

• CoolTerm is showing only characters received not what was sent)
• for that test I use CoolTerm in ASCII mode.

To get the times of two edges copy and paste from Notepad into the CoolTerm window

ascii --> ?10021    
      that is '?' -- next character is command
                   '1' -- falling edges
                   "002" two edges
                   '1' gate one

then I passed a finger through the photogate (gate 1). About ten seconds later I passed my finger through the photogate (gate 1) again.

Put CoolTerm in Binary mode

So that is hex 017D12DA - 00DCBC11 which works out to 10,597,977 microSeconds.

Prototype project from 2006.

Some changes have been made that were not included in the notes at the link given below. Details will be added to this README.md file to reflect the photogate timer box as is currently used.

• Photogate Box Notes from 2006-2007

PIC Wiring

There are traces on the board for the 5Vdc rail and ground rail running the length of the protoboard directly under the PIC IC (between the pins). The rest of the layout is like a solderless breadboard with 5 hole tie points running from near the power rails to the edges of the board.

The PIC inputs and outputs as defined in the source code

// Configure 2-Way Status LED

  TRISCbits.TRISC3 = 0;     // set as output 
  TRISCbits.TRISC4 = 0;     // and as output

  // Configure USART module

  TRISCbits.TRISC6 = 0;     // set TX (RC6) as output 
  TRISCbits.TRISC7 = 1;     // and RX (RC7) as input

  // Configure RC2/CCP1 and RB3/CCP2 as inputs
  // Photogate 1 is on RC2/CCP1/Pin 13 and 
  // Photogate 2 is on RB3/CCP2/Pin 24 
  TRISCbits.TRISC2 = 1;     // set RC2(CCP1) as input
  TRISBbits.TRISB3 = 1;     // set RB3(CCP2) as input 

The status LED was dropped from the design as comments were no one was looking at it and dropping it saved time building the units.

Parts List

• Link to Parts List

Microchip Documents

• Links to Microchip Documents and Install files

Enclosure

Photo of PCB's mounted on bracket plate which is mounted on the box lid. More detail about the mounting plate at https://github.com/danpeirce/pic-box-bracket