New Laser Gate


In the Cal Poly Pomona Physics department we have designed a low cost accurate laser timing gate for use in physics experiments. We use the PIC16F690 chip to perform the timing and to transfer the data via USB to a PC. We first explain how to construct the gate, then how to run the software. The total cost for the system is under 30 dollars, with most of the cost being the USB-TTL cable (20 dollars).

The new laser gate is an improvement over the older version in the following ways:

The laser gate is very similar to the previous version with the addition of an LED on pin 13:

Hardware

One can construct the gate
by placing the components
onto a solderless breadboard,
or by soldering the components
onto a perf-board. The connections
(with the 20 MHz resonator)
are shown in the figure to the right:

Click here to see a parts list . A picture of the breadboard with connections is shown below:

The PIC chip is loaded with the assemply code timer3led.asm (4 MHz clock) or time3fastled.asm (20 MHz clock). With the 4 MHz clock on the PIC16F690, the shortest measured blocking time possible is 2 msec. For blocking times greater than 2 msec, the accuracy for the blocking time is 2 micro-seconds. For blocking times shorter than 2 msec, you will need to use our faster version with a 20 MHz resonator, time3fastled.asm . With the 20 MHz resonator, the blocking time can be as short as 0.4 msec, with an accuracy of 0.4 micro-seconds. For instructions on how to program the PIC16F690 see pprogs.html.

If the chip is running properly, the LED on pin 13 will light when the IR detector on pin 11 goes "low" (i.e. detects the laser light). The resistance of the IR detector changes when IR light strikes it. With IR light, the resistance is around 500 Ohms. When no IR light is detected the resistance of the detector is large.

Software

We run a C code in linux to read the usb port, collect the data and save it to disk. Then a javascript code is run in the browser to graph the data. We have also produced an image file that you can burn onto a CD or flash drive to boot up into. The image file is: lasergate3.iso . This image will start your computer in a nice distribution of linux, "Puppy Linux". After booting, hit "OK" in the setup screen, and there will be 4 programs on the desktop that you can run by clicking once on the desktop icon: conacc.desktop, tandv.desktop, graphdata.html, and linefitjs.html. A short page of instructions can be found at lgateinst.

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