Circuit symbol of a photodiode

What is a photodiode? It is a semiconductor device, usually made from the element silicon. Photodiodes are used as light sensors in many different forms of technology, especially fiber optic communications. They have a lot in common with solar cells (photovoltaics) for capturing solar power. In fact, a solar cell is basically a giant photodiode. But when we’re interested making an efficient sensor, the best results come from small packages.

The photodiodes used in our Lite2Sound PX DIY kit are the QSD2030 type, with an active area of 1 square millimeter. This small slice of silicon is encapsulated in clear epoxy resin, and it looks just like an LED:

When light hits a photodiode in a circuit, photons are absorbed and transformed to electricity. The anode terminal becomes positively charged, and the cathode becomes negative. The amount of current generated is proportional to the intensity of the light, and also depends on its color.

Photodiodes are sensitive to light of all visible colors, from deep blue (400 nm) to far red (700 nm), and beyond. In fact, they are most sensitive to invisible light in the near infrared region; the peak sensitivity of a PIN photodiode sensor is at about 950 nm. It is no coincidence that most infrared remote controls use this wavelength to communicate, since efficient 950nm LED emitters are available to match it.

Photodiodes that are made for use in IR remote controls have a dark colored lens, looking like a black LED. The black lens blocks visible light but infrared light passes through. You can see below the spectral sensitivity curves of two versions of the QSD2030 photodiode.

Spectral sensitivity of unfiltered and IR-filtered photodiodes

The plain QSD2030, with a clear lens as pictured above, responds to light from 400 nm to 1100 nm. The dark-lensed QSD2030F (F- for “filtered”) is blind to visible light, and only responds to near-IR radiation in the 750- to 1100 nm region

There are many models of inexpensive silicon PIN photodiodes on the market, but most have similar wavelength-dependent sensitivity curves. If you are interested in sensing infrared wavelengths beyond 1200 nm, then silicon photodiodes won’t work. On the other end of the color spectrum, some inexpensive PIN photodiodes are sensitive in the ultraviolet region 200-400 nm; therefore its conceivable you could discover optical sound sources hidden in the invisible ultraviolet range.

Getting the signals

Simple circuit illustrating current flow

As we discussed above, when light hits a photodiode in a circuit, an electric current is generated, as it would be in a solar cell (photovoltaic). This is illustrated in the simple circuit above, which is just a photodiode with a resistor connected between its anode and cathode. The amount of current generated is much smaller than a solar cell; for example, with a 100-ohm resistor across its terminals, the QSD2030 photodiode produces about 0.1 mA when a desk lamp is shining on it.

This circuit is good for nothing, however, since all it does is convert light into an electric current, then immediately wastes it as a tiny bit of heat inside the resistor. To produce an electrical signal that can be used for audio, one needs a way to convert current into voltage.

Transimpedance amplifiers

Transimpedance amplifiers convert small currents from sensors into useful voltage. In our case the voltage will be an analog audio signal that we can listen to. This is possible by adding a key element to the circuit above: an operational amplifier, or op-amp.

Transimpedance amplifier with a photodiode at its input

The op-amp’s job is to generate an output voltage Vout. It continously adjusts Vout so there is zero volts difference between its (-) and (+) inputs. The resistor Rf in the circuit creates a negative feedback loop that makes this possible. While the photodiode is delivering a current I1 to the op amp’s (-) input, the op amp sends I2, which is an inverted “copy” of I1, back through the feedback resistor. When I1 and I2 add together, they cancel each other out, maintaining zero volts between the op-amp’s input terminals. As the photodiode’s current changes, the op-amp varies Vout with it to preserve this balance through the principle of negative feedback.

The audio signal that we want, Vout, can be made louder or quieter by adjusting the feedback resistor Rf. You can understand by applying Ohm’s law, V = I * R, that the output voltage is the product of the photodiode’s current and the feedback resistor’s value in ohms. Therefore, the ampilfication factor, or gain, of our photodiode amplifier is directly controlled by the value of Rf… more ohms gives you more volts, and generally speaking, this makes it a more sensitive receiver.

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Thats because the smaller the photodiode is, the less capacitance it has. The internal capacitance of a photodiode interacts with the amplifier it is hooked up to, creating a low pass filter effect. So, a solar cell has a lot of capacitance, and therefore it will naturally have a “stronger” low pass filter effect, in other words, it is big and slow…