The Speed Trap version 1.00b

When designing this circuit the goal was to use as few parts as possible so everyone can afford and manage to build it. The prototype was built using a PIC 16C84, but the 16F84 and 16F84A should work just as well.

Configure the display
Hold down Button 3 and turn on the power. Hold the button until the display shows [123]. The chip is now configured for the correct display type and will flash the version number [100] and show the ready sign [rdy].

Select a unit of measurement
It is up to the user to decide on the unit of measurement. The speed is reported in the same unit as the light gate measurements were done in. If you don't have a very good reason for using something else I suggest you use meter as the base unit. If you prefer the triggers to be spaced less than one unit apart, then use Button 4 to select a suitable fraction.

Triggers
The trigger pulse needs to last 10µS or more to be sure that it will always be registered. Pulses down to 1µS can be used if 100% success rate is not required.
RA3 needs to be triggered first and will start the timer, after the pulse is registered RA3 it is free to change logic level. RA4 requires the same pulse width and stops the timer when triggered. The triggers can be photodiodes, switches or any detector capable of emitting TTL compatible voltage levels. Button 5 chooses between  trigger on logic 1 and trigger on logic 0. RA3 and RA4 are CMOS inputs with TTL compatible voltage levels.

Power
The circuit will work at voltages from 2-6V, remember that the voltages on the trigger inputs should not exceed the supply voltage. C3 should be placed as close as possible to the chip, if omitted it may cause the chip to crash or behave erratically. The displays will be very dim when run on low voltages. Average supply current is 10-60mA depending on the type of displays and the supply voltage.

Buttons
All buttons except Button 1 are optional, you only need to add buttons for the functions that you want to use. Debouncing is done in software and most Normally Open buttons should work fine.

Oscillator (4 MHz)
A crystal is the most accurate and expensive option. A ceramic resonator is cheaper but can give errors of about 0.5%. The capacitors C1 and C2 are optional and will give slightly higher stability when present. The triggers will likely be the largest source of error even when a ceramic resonator is used.

Accuracy
The accuracy in testing was better than 1 m/s double checked by a digital oscilloscope. To test the circuit and your settings, place the trigger switches vertically and time an object in free fall. The average acceleration of gravity on earth is 9.81m/s^2. The time for an object to fall from a height h to the ground is t = sqrt( 2 * h / 9.81 ). So if you place the triggers one meter apart vertically it will take exactly t = sqrt( 2 / 9.81) = 0.452 seconds for the object to fall from one trigger to the next. If you use an object that is relatively heavy for it's size and start it just above the first trigger you should get a time that is very close to theory. With the prototype it was possible to get within two ms even with a handheld first trigger.

 

Operation of the Speed Trap

Word Definition
Unit The unit (meter, foot) you measured the light gate spacing in
µS Time in Microseconds, 1/1 000 000
mS Time in Miliseconds, 1/1000
Unit/s Speed in Unit per second
Kilo Unit/h Speed in (Unit*1000) per hour

 

Normal Operation
Button Name Action Display
1 Arm Enter timer mode [= =], [Err] (Check inputs)
2 Previous Cycle the 16 latest recorded values [000], [-E-] (Overflow)
3 Type Select µs, ms, Unit/s or Kilo Unit/h [ uS], [nnS], [U/S], [U/H]
4 Length Select 10, 20 ... 100% of one Unit [L01] ... [L10]
5 Trigger Select trigger on logic level 0 or 1 [--|_], [__|-]
6 Debug Display the RA3 and RA4 Trigger inputs [0-0]

 

During Startup
Button Name Action Display
1 Arm Set default state [CLr]
2 Previous Clear recorded values [C16]
3 Type Select display Type [123]

 

Default settings

Name

 Value
Previous 000
Type U/S
Length L03

Trigger

 [__|-]

 

 

Speed Trap Hardware

Shematics (display 3 is the most significant digit):
The Speed Trap - Schematics

PIC16C84/16F84/16F84A Pinout:
16c84.gif (6180 bytes)

 

PORTA - Pin direction and assignment
Name Pin Direction Function
RA0 17 Out Select display 1
RA1 18 Out Select display 2
RA2 1 Out Select display 3
RA3 2 In Trigger input 1
RA4 3 In Trigger input 2

 

PORTB - Pin direction and assignment
Name Pin Direction Function
RB0 6 Out Display segment a
RB1 7 Out Display segment b
RB2 8 Out Display segment c
RB3 9 Out Display segment d
RB4 10 Out Display segment e
RB5 11 Out Display segment f
RB6 12 Out Display segment g
RB7 13 In Button multiplexer (RB0-RB6)

 

Segments
a a a a
f b
f b
g g g g
e c
e c
d d d d


Programming the PIC

COM84 style PIC programmer:
picprogt.gif (3537 bytes)picprogpsut.gif (2936 bytes)
Some serialports have problems supplying enough current for stable operation, it may help to increase the value of C1. Another solution is to use an external regulated 5V PSU.

 

Configuration bits

Name

 16C84 16F84
_CP_OFF H'3FFF' H'3FFF'
_WDT_OFF H'3FFB' H'3FFB'
_XT_OSC H'3FFD' H'3FFD'

_PWRTE_ON

 H'3FFF' H'3FF7'

 

Serialport Pin Assignment
Name DB-9 DB-25
TXD Pin 3 Pin 2
DTR Pin 4 Pin 20
GND Pin 5 Pin 7
RTS Pin 7 Pin 4
CTS Pin 8 Pin 5


Reccommended software is WinPic: http://www.qsl.net/dl4yhf/winpicpr.html


Notes: