In order to help his friend prepare for a talk at DEFCON this weekend, [Craig] built an IR photodiode amplifier circuit. The circuit extended the detection range of the hack from a few inches to a few feet. We’re suckers for some well-designed analog circuitry, and if you are too, be sure to check out the video embedded below.
The talk concerns fingerprinting cellphones by the IR emissions that their proximity sensors emit. These sensors are there to tell the phone whether the phone is being held up to your ear, for instance. Of course, if the IR emitter in the proximity sensor were running all the time, it would be a battery drain, so the manufacturers turn them on only intermittently. If different manufacturers use different patterns, you can fingerprint the phones — if you can detect the IR from a long enough distance to be useful.
And that brings us to IR photodetector amplifiers. The circuit is “almost” a simple op-amp current-to-voltage (transimpedance) amplifier. But there are complications. To get very high gain, the circuit becomes prone to oscillating due to the inherent capacitance of the photodiode, so there’s a damping capacitor in the feedback loop. To avoid slamming rail to rail, [Craig] biases the positive input and adds some diodes in the feedback loop to reduce the output range. Since the output is going into a microcontroller, it’s run through a comparator to make it nice and digital. Finally, [Craig] used a nice big photodiode with good sensitivity.
We wonder why [Craig] spent so much effort keeping the first-stage op-amp out of saturation when he’s following it with a comparator anyway. Anyone?
Anyway, being able to detect IR pulses from far away is cool. And did you know that you can use photodiodes to detect (beta and gamma) radiation? The same caveats about oscillation and signal conditioning apply!