If the input is found to have changed from a low to high state then the timer starts. Even a slight perturbation to the beam is enough to trigger an output (blue).īased on a program for a stopwatch, the Arduino sketch works by monitoring pin 8 and comparing its state to the last time it was read. D3 is replaced with an AC source such that its simulated trace (red) represents the amount of the IR beam which is obscured. The circuit was modelled in MultiSim before prototyping on a breadboard. I used an online tool to easily determine the resistor values: Inverting Schmitt Trigger Calculator. The trigger was made using an op amp and R3,4,5 determine the thresholds upon which the op amp saturates and delivers a constant voltage. The trigger is inverting as the detector is still active low. This is an issue when dealing with slow moving objects, thus an inverting schmitt trigger processes the the output from the photodiode resulting in a true digital signal. As an analogue device, D3 will produce a variable output only reaching a high state once the beam is mostly obscured. While the IR beam is incident on D3 a current will flow thus the detector is active low. The reverse biased photodiode D3 is operating in photoconductive mode, R6 controls its sensitivity. It has hinges and 'folds' when not in use. The IR LED was taken from an old TV remote control. The frame is made from those panels covering the expansion bays on the back of a PC case which are brazed to another part of the same case. Power supply should be that from USB, either from a computer or cheap wall adaptor.The photogte should deliver a digital signal (ideally TTL compatible) to trigger some timing unit.The interface should be via a computer and not a proprietary receiving unit.I began to envision my ideal photogate timer. Large, expensive, with a cumbersome interface and a little single line display to read the data from. While I cant remember the manufacturer or model of the photogate equipment I used, its design featured elements found in modern equipment too such as proprietary computer software interfaces and obscure connectors. The inspiration came while teaching a Higher Physics evening class where I used the ubiquitous photogate in various demonstrations. The timer may operate in various modes, for example, measuring the duration of a single event or the time between two consecutive events.Ī while back I made a photogate intended to trigger an Arduino-based timer. This project uses Mathematica, Arduino and Java source code and is presented as a zipped folder via GitHub: PhotogateDriver.zipĪ staple of experimental physics an infrared beam forms a tripwire which when crossed registers as an event and triggers a timing unit.
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