Optical Probe – a tool for reverse engineering optical protocols.

 

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From TV remote controls to high speed networking via laser fiber optics, optical communications is done every day. Because most of this information is carried on short pulses of either visible or invisible light, it is impossible to reverse engineer these signals without proper test equipment.

But, before you can reverse engineer anything, you have to know it exists.  A blinking light is obvious, but it could also be several pulses close together as several bursts of data. A dim light may be a dim light, or it could be a high speed pulse train that is fooling your eye into thinking it’s dim. Older telephone modems used to connect their “RX” LED to the data stream, and, because it faithfully reproduced the data optically, you could eavesdrop on the communication from across the street.
Once you know you have a signal of interest, test equipment allows you to sample it ways that exceed your senses. Humans can’t see infrared yet. Sure, there are experiments to see if the eye can become sensitive to infrared by substituting one type of vitamin A for another, but that’s a bit extreme just to see if your remote is working, isn’t it?  Equipment, from night vision scopes to cell phone cameras can all detect invisible light, but they are not fast enough to analyze the data contained in their pulse trains. Amplitude, frequency, and other characteristics need a more advanced probe — this probe.
I designed this probe when working with a power meter reader. It output a dim IR light (that was a pulse train) to a meter and the meter would respond back with … something. I didn’t know what, it just looked like a blink of light.
optical_probe_schematic
The circuit for viewing those light pulses is easy. An oscilloscope can only sample voltage over time, so we need to give it the ability to sample light. A phototransistor (Q1) will modulate electrical current flow based on how much light strikes it, but an oscilloscope measures voltage, not current.  We add a power source (B1) and resistor (R1) into the circuit: according to Ohm’s law, the voltage across the resistor will be proportional to the current through R1, which is the same current that flow through Q1. Varying the resistor’s value adjusts the gain of the circuit. Because the phototransistor is sensitive to infrared light that we can’t see, it would be helpful to add a visible LED (D1) and a transistor to drive it (Q2) to give us a bit of visual feedback. This enables us to use the optical probe to check if there are signals present without the need of the oscilloscope. Of course it won’t see the pulses you’d see on an oscilloscope, but it will work well as a remote checker.
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