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:

• An LED has been added that lights when the laser hits the detector and is off when the detector does not register a signal. This is very useful in testing if the gate is ready for data collection.
• A more general graphing program is used to graph the data. After recording the times, the data is saved automatically in a file called data.txt. The student then runs graphdata in the browser to upload and graph the data. The new graphing program adds more options than the older version.

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.

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 for a 32 bit processor is: lasergate3.iso . For a 64 bit processor, the image file is lasergate64_1.iso . These images 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.

• conacc.desktop is a link to the program conacc that will run in terminal mode. This program can be used if you suspect that the motion has a constant acceleration. In our lab, we expect the acceleration to be constant for the Atwood Machine, ladder drop, and rotational dynamics experiments. The c program that is run is conaccusbf.c described below. After the data is collected, it is automatically saved in the file data.txt, which can be uploaded into graphdata.html described below.
• graphdata.html is a linear fitting program that gives the slope and intercept as well as the uncertainty in these two parameters for the data in the table. To graph the data from the lasergate, browse to find data.txt, which is located in the root directory. Then click upload file to upload the data, and click display data to graph and fit the data. Click (or unclick) the boxes to include (or exclude) the data points you want in the fit.
• tandv.desktop is a link to the program tandv that will run in terminal mode. The program measures the time and velocity of the motion when the gate is blocked. To calibrate the speed, you need to enter the effective blocking distance. The c program that is run is tandvf.c, which is described below. This program can be used in our Conserved Quantites experiment.
• linefitjs.html is a general linear fitting program that gives the slope and intercept as well as the uncertainty in these two parameters. This program is not set to upload any data, but the user can enter data of their choice.
• conaccusb.c and conaccfly.c (fast chip): These programs can be used if one expects constant acceleration, such as the ladder drop, atwood machine, rotational dynamics. Type "z" for calibration. After typing "z" you will be asked for the number of blockings for calibration and the distance between the first and the last calibration blocking. Check to see if the gate is working by blocking the detector with your hand and observing the LED. If the gate works, type "d", enter, to collect data. The source code is conaccusb.c (4 Mhz chip) and conaccfly.c (20 Mhz chip). After the data is collected it is automatically saved in the file data.txt in the same directory as the executable of conaccusb.c. To compile conaccusb.c or conaccfly.c type "gcc conaccfly.c -lm" in terminal mode. To run the executable you might need to be in super user mode to read the usb: sudo ./a.out.
• tandv.c and tandvfly.c (fast chip): This program records the time and speed of the object as it blocks the lasergate. You can enter the effective blocking distance and the number of data points you want. There is also the option to save the data to a file called tvout.txt, which will be located in the root directory.