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A "photogate" is a common piece of equipment found in a physics lab. A photogate is typically used to indicate the presence of an object or measure the time an object is at a certain position. This is accomplished with a light source and a light detector. If you have walked into a store and heard a tone as you passed through the doorway, this was probably done with a similar piece of equipment. In this Instructable I will show you how I made one. Be advised this version of a photogate uses a custom BTA cable from Vernier (more on this later). I intend to have my high school students engineer their own photogates, and this version is a proof-of-concept. We will use these in our physics lab for actual experiments and offer our designs to other physics teachers.

Step 1: Electrical

There are a number of different circuits one can use to perform the function of a photogate. The circuit I chose is shown in the picture. The light source is an LTE5280 infrared (IR) LED and the detector is an SFH310fa IR phototransistor. Both of these devices are available from Mouser. The devices were chosen based on their emission and sensitivity spectra, viewing and detection angles, package size (T1-3/4 which has a 5mm diameter), and cost. Resistors are chosen to limit the LED current to ~20mA and phototransistor collector current to ~1mA. A low collector current ensures the transistor will saturate and provide a wide voltage swing when an object is detected. The power supply is from a PC's USB port via a Vernier Lab Quest Mini. The cost of all electrical components is about 50 cents (excluding the cable which I will specify later).

Step 2: BTA Cable & Wiring

The BTA cable makes this photogate somewhat less generic, since the Vernier Logger Pro software is also needed. But if you have the Lab Quest Mini, you certainly have the Logger Pro software as many schools do. The BTA connector pinout is shown (please forgive the pic taken from my lab notebook). Only pins 1 (output), 2 (ground), and 5 (+5V) are needed. The remaining 3 wires are twisted and wrapped around the central screw post for strain relief.

Step 3: Enclosure

There are really no tight constraints on the enclosure dimensions, so I chose the dimensions using the "about this much" and "about that much" approach. There is a top half and a bottom half, and the two are joined with 4-40 machine screws. Also I wanted to be able to mount the photogate using a 1/4"-20 threaded screw. A simple way to add a 1/4"-20 mount is with a 1/4"-20 hex nut, so I designed into the enclosure a place to insert a common 1/4" hex nut. The 5 screw posts are also used to aid wire routing and provide strain relief. Simple point-to-point soldering works just fine, and no special isolation is required.

I've attached the STL files for 3D printing. I made these in Autocad Fusion360. When I looked at them today, the dimensions seem off by a factor of 10. In other words something that is 8cm is really 8mm. This has happened to me before; I think Autocad defaulted to cm and I used mm. Be aware when printing that you may need to scale the object by a factor of 10.

Step 4: Testing & Using

I made a "picket fence" structure out of craft sticks and hot glue, dropped this through the photogate. As the rungs and spaces of the fence alternately blocked the light and allowed the light to pass, the data shown in the Logger Pro graph was produced. Data was collected at a sampling rate of 10kHz. When the output voltage is high (~5V), the LED light is blocked and the phototransistor is off. Similarly, when the output voltage is low (~1V), the LED light is striking the phototransistor, causing it to turn on.

That's it -- a working photogate for under $6!

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