Mike's Universal Exposure Timer and Event Counter

Overview:

• Software configurable to time exposure or interruption in exposure to light source,  or dual event counters
• Inexpensive: cost to build is well under $50.00
• Easy to construct, layout is a single layer PC board
• All components are off the shelf and easily obtainable
  
• A pair of photo transistors  of virtually any configuration can be plugged in using simple modular connector
• Fast response - as low as 20 microseconds with a optimally designed sensor configuration
• Smallest event measurable is well under 1/2000 second (timing error should be under 5%)
• Longest event measurable is over 4 hours
  
• Rearm interval between events is under 30 milliseconds, the time needed to transmit capture data to the serial port
• Includes 2 LEDs that indicate state of sensor, also providing verification that the CPU is operating
  

   

• Camera shutter tester
• Pinewood derby car timer
• Pinewood derby car speedometer
  
• Slot car speedometer
• Model railroad speedometer
• Slot car 2 lane lap counter
  

Hardware:

Bill Of Materials

Circuit Board

This single layer circuit board can  be easily made at home using instructions provided at the site (http://www.fullnet.com/u/tomg/gooteepc.htm) and the artwork above.   PCB size is 4.042 by 2.222 inches, so this art should be printed at exactly that size.

A couple of additional suggestions regarding fabrication follow.

1. Use 320 grit paper to prepare the copper surface prior to ironing on the resist.  A scotch brite pad doesn't provide enough bite for the ink to stick.   Same is true for the silkscreen layer.
2. To get all of the paper residue off off the silk screen side, soak the board overnight in water.  Then scrub with a toothbrush.
   
3. Use of a drill press will vastly simplify and speed drilling all the holes required.   For the most part a size 64 drill bit will work for most of the holes, though a 68 would be preferable for the transistors (3904) and the voltage regulator (7805) parts.  I found a pin vice can be used to hold these small bits in a normal sized drill press.  Small drill bits can be found at hobby shops catering to model railroaders.
   

This artwork can be applied to the top side via making a laser printer decal or though ironing it on from laser printed photo paper.  Also note that this silk-screen artwork must be printed reversed so that when it is applied to the top side of the PCB, it comes out correctly.

To assist interested enthusiasts in building their own Universal Exposure Timer and Event Counter:

• I will sell interested experimenters preprogrammed AVR 2313 parts and laser printed artwork on photo paper for $15.00.  Contact me via email for more information.
• I am considering having a number of PC Boards built and made available for sale, but this depends on level of interest.
  

End Panels

Sensor Designs

 

• 1 ordinary telephone headset connector.  This is 4 position, 4 connector RJ style  headset connector, not the 6 position, 4 connector RJ-11 used to connect the phone to the wall.   I have found that cheap 6/4 crimping tools I had works OK for crimping these smaller 4/4 connectors.

• A couple of feet of telephone wire.  Certain wires contain conductors braided with some sort of non-metalic material, and are hard to solder.  Find wire that is not braided in this fashion.  One end is attached to the connector and the other end is stripped for soldering to the photo-transistors.  Keep consistant in how the wires are positioned in the connector from device to device.   I keep the green wire on the left when looking down on the connector.  I connect the emitters of the photo transistors (usually the longer leads) to the red and yellow wires.  The black and green wires will be connected to the collectors, as seen below.  The emitters connect  to the ground  of the RJ connector (pins 1 or 3) and the collector to the input connectors (pins 2 and 4).

• 2 photo transistors.  I've  settled on a specific photo transistor for my timer/counter applications.  This is a T1 sized device made by Panasonic and sold by Digikey as part number PNA1801L-ND.  Actually, almost any photo transistor will do for many applications.  However for high speed shutter testers, the size of the photo transistor can affect sensor design somewhat and this T1 Panasonic device is the smallest that I could find without an visible light filter.   Note how I mounted the photo transistors in position parallel to the slots, in the photo below.  I haven't experimented  with orientation, but be aware that other orientations may affect performance.

• 1 piece of copper to be used as the faceplate for the sensor.   At a local craft store, I found a product called ArtEmboss that is real copper of the perfect thickness.   This product is sold by American Art Clay Co, Inc.  Using the techniques used to make homemade PC boards, I etched out a piece of copper that was 36mm x 24mm and contains 2 slots 32 mm apart that were .33 mm wide.  For a vertical focal plane shutter tester, rearrange the slots to a verticle orientation spaced around 22 mm apart.  You can experiment with smaller slots, but be careful that you don't limit the light reaching the phototransistors, too much.  You might use electrical tape to create a slot in a test block to see how small you can go.  Smaller will generally provide a more accurate shutter tester.  See the links in the PCB fabrication section of this page for instructions on etching copper foil.  Before etching,  remember to paint the backside of the copper to prevent etching of the entire piece from the backside.  Once made, the copper plate is glued to the from of the block of wood with the slots aligned in front of the photo-transistors.

• 1 small block of wood, cut to 35mm width and about 60mm long.  Thickness can be standard 3/4" stock.  I cut, drilled and machined this block to form the chassis for my sensor.   First I cut it in 2 pieces, leaving a thin section  as the back and the rest as the chassis.  I continued to  hollow out the chassis using various sized regular and forsner drill bits.  I drilled holes for the phototransistors using the faceplace as a guide to ensure accuracy.  The phototransistors are carefully glued into holes prior to soldering.  Make sure you set up some kind of strain relief for the cable or you may find yourself spending a lot of time fixing broken connections.  In this case, the cable fits very tightly in the entry slot and I don't expect it to move.

Software:

Microprocessor

The processor is a AVR 8 bit micro controller produced by Atmel Corporation.  Full specifications are at the Atmel web site.    I do not intend to distribute source code for this processor, but an offering preprogrammed processors for those who wish to develop their own timer/counters.



Serial Port Protocol

Configuration and control of this counter/timer is through a RS232 port.  Cable configuration for a usual PC compatible would consist of a null modem cable, with a female connector on the timer/counter end of the cable. The correct cable for a PC or usb to serial adapter with a DB-9 interface is listed in the bill of materials.  The counter/timer reads the port to get configuration information and returns results of sensor activity back to a host computer through the RS232 port.  The port is configured to operate at 9600 baud, 8 bits data, 1 parity bit and 1 stop bit.



Configuring Counter/Timer

Bytes send via the RS-232 interface to the timer/counter control it's behavior.  The device samples the input port prior to entering a sampling state machine.  The configured behavior is changed every time a byte is read from the serial port.  Once a cycle starts, the RS-232 input port is not sampled until completion of that cycle.   The default mode is 0x00, which is normally off, timing mode.  You will probably have to cycle the timer/counters state machine after writing the config byte to the serial port to get it into the corrent mode.  This can be done by exposing the sensor to light and then removing the light a couple of times.

Bit 0    Off - sets the device to a normally off state (useful for testing camera shutters)
            On - sets the device to a normally on state (useful for checking speed of pine car derby cars, etc)
Bit 1    Off - sets the device to timing mode (produces an elapsed time)
            On - set the device to a counter mode (counts number of events occurring)


Receiving Data Back From Counter/Timer

Timing Mode

Depending upon the state of bit 1, the timer/counter will return a string of bytes every time a cycle is completed.

If config bit 1 is off (Timing mode):
        0xAA    start flag
        0X01   version
        0x??    control- echoes last config byte received from serial port - default is 0x0
        0x??    error byte-


err_pg0_fail = 1        first sensor opened second time, second sensor never  closed
err_pg1_fail = 2        second sensor opened second time, first sensor never closed
err_pg2_fail = 3        one sensor opened second time, other sensor never opened
err_timeout = 5          General timeout
err_timeout12 = 6      Sensor 1 didn't close
err_timeout1 = 7        Both sensors opened, but neither completed cycle
err_timeout2 = 8        Sensor 2 didn't close


                    4 bytes for time 1st sensor detects change out of normal condition - always 0 - base start time for this event
                    4 bytes for time 1st sensor detects return to normal condition
                    4 bytes for time 2nd sensor detects change out of normal condition
                    4 bytes for time 2nd sensor detects return to normal condition
        0x55    end flag
        0x??      check sum which is XOR of all previous bytes - checksum is initialized to zero prior to XOR sequence

Time is a count of samples which is clock rate of processor 3.6864 MHZ/13 (which is number of cpu clocks per cycle).   There is a small error factor depending on particular sequence of events that can be accounted for in host software.  See example shutter tester JAVA program for details on calculating this error factor.



Counter Mode

If config bit 1 is on (Counter mode):
        Bytes are sent every time an on transition is detected for either sensor.  The byte is a 7 bit value that contains the current lap count.  The most significant bit is cleared if the count is the number of transitions for sensor 1 or is set if the value is the number of transitions for sensor 2. Bytes are sent as transitions occur.  For example, multiple bytes can be sent for sensor 1, if no tranistions occur on sensor 2 in betwen the events for sensor 1.  If the transmitter is busy  sending the previous byte when a new transition occurs, the counter will  continue to count tranisitions and the updated count will be sent when the transmitter is done transmitting the previous byte.

•     Summary
•          The count value ranges from 0 to 127, wrapping automatically at 127 back to 0.  Software in the host must track wrap conditions.
•         The count for the first sensor always has the MSB (bit 7) cleared to 0.
•         The count for the second sensor always has the MSB (bit 7) set 1.
Performance of this counter is really limited by the serial port.  A little less than b1000 characters per second can be sent out of a 9600  baud serial port.   Updates must occur 3 or 4 times per 128 count prior to a wrap or the host will have trouble accounting for the wrap conditions.  Figure that 4 bytes must be sent prior to a wrap condition which gives 32 events between sending bytes per sensor.  With 2 sensors we must send 8 characters prior to a wrap. So max frequency is about 1000*(128/8) = 16000 cycles per second. Thus, this counter should be able to easily count transitions occurring at this frequency.  This frequency doubles if only one sensor is used.  


Applications

Shutter Tester

• display shutter times in milliseonds or...
• shutter times in 1/x seconds or...
• percentage error from ideal value or...
• worst case variation over several trials or...
• variation between saved trials
• also has screen useful for calibrating this device 

• Click here to download Application for Macintosh
• Click here to get the RXTX serial port driver for Macintosh

• Click here to Download JAR Executable File for Windows

• Click here to go to Sun's CommApi Webpage
• Click here to Download Platform Independant JAVA source

Speedometer

• Pinewood derby test track (in photo)
• Slot car speedometer
• Model railroad, locomotive tester/analyzer
  

Lap Counter

TTL to RS232 Level Converter

Other Ideas

Previous Shutter Tester Designs that I have Experimented With