Electronic – Noisy amplification and low light: photodiode and op-amps question

Assuming that the laser tag variant will be played in relative darkness, not in daylight, a thin film photovoltaic cell (TFPV) or thin film solar cell of requisite thickness and flexibility could serve as a wide-area photodetector.

TFPV cells are available that use very thin, yet tear-resistant polyimide substrate, which can be cut and sewn into, for instance, the top surface of a jacket or cap.

The challenge will be distinguishing a relatively small area exposed to the laser, as opposed to the overall photovoltaic surface being exposed to ambient light. Clearly, if the intensity of the laser is not significantly higher than the combined ambient light that may be incident on the entire surface of the clothing, this won't work. Unfortunately increasing the laser intensity leads to laser eye safety concerns.

Permissible eye-safe laser power is pretty low, typically single-digit milliwatts for visible lasers, and fractional milliwatts for IR or UV lasers - keep in mind that eye safety concerns apply even to brief accidental exposure to the laser from very close, such as if a player accidentally triggers the laser tag gun while looking into it.

The solution is to use a TFPV sheet that consists of a multitude of separate cells, rather than one large photosensing area, for instance:

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The detection mechanism would need to sense individual PV cells within the mesh. The cell which produces an anomalously high voltage would be the one with a laser beam incident upon it - all the other cells would have lower baseline voltages representative of the ambient light upon each.

Color differentiation would not work for source identification, if using the TFPV approach: Consider a tiny red laser spot, a tiny green laser spot, and a larger surface area illuminated by the laser tag arena lighting: The PV cells would not be able to distinguish between these.

Another challenge with using TFPV material is that signal rise times are pretty slow - and slower for larger cell sizes. Using multiple smaller cells mitigates this somewhat.

Therefore, traditionally used signal modulation for distinguishing between different light sources, e.g. 38 KHz pulse modulation as used in infrared remotes, will not work. A much slower modulation frequency, and identification of distinct code sequences from different emitters, would be the way to go.

Just because the battery voltage is 3V doesn't mean there isn't some sort of switching regulator to boost the voltage. However, even if there isn't it is possible to achieve using just a single 3V rail, 2 op-amps, a couple of resistors, and a photodiode.

You just need an op-amp which can operate on low voltages. You will either need to bias the non-inverting reference point, or put an capacitor switch pump to generate a negative rail.

Here's an example with a dual op-amp package, one which is used to provide a virtual ground reference.

You'll want an op-amp which has very low noise, low input bias current, and low input offset current. Something like this TL032.

^{simulate this circuit – Schematic created using CircuitLab}

The output is \$V=I_D R_G\$. Make \$R_G\$ small enough to avoid saturating the op-amp, but large enough to give you the full resolution. This is designed to operate with a differential voltage sensor, you can in theory measure Vout+ single-endedly, but it won't necessarily work as well because you'll miss out on fluctuations on the 1.5V reference.

Oh, and it probably goes without saying that this circuit doesn't include any of the logic/adc stuff. That is all extra, but simple microcontroller with a differential ADC should be sufficient to get a reading (these can operate in the 3V range as well).