Light to Sound Convertor
This short Instructable describes a device that converts the changes of light into sound.
Recently I made a version of the Light-Probe-MK-I made by Nqtronix, although not as sophisticated as his as I do not have such a fast photo-diode. I used a "large" photo-diode (2.65 x 2.65 mm) which also meant that it has a much higher capacity and can't be used at high frequencies.
While I build this light-probe I remembered that I had read the other Instructable from AndresR145 about his LightSound device. So I combined them and this is the result.
Supplies
SFH2440 (or any other photo diode)
MCP6002 dual opamp
5k potentiometer
3.5 mm audiojack (chassis mount)
CR2032 battery + holder
perfboard
small plastic box
The next three Instructables
https://www.instructables.com/LightSound
The Photo Diode
As said the photo-diode is a "large-area" photo diode that is used in photoconductive mode. This means that it is reverse biased. So it does not conduct current, at least, it doesn't until light shines on it. You can think of it as though light makes the diode leak more current. The advantage of this mode is that it is much faster than photovoltaic mode (which is used in solar cells). For more information read the Instructable by Nqtronix or on Wikipedia :
Schematic
The current of the photo diode is quite small, in the region of several micro amps. The SFH2440 has a maximum of 750 uA when it is in bright sunlight, under normal circumstances the current is less than 10 uA and in total darkness it is just a few nA.
This current is then fed to a trans-impedance amplifier (opamp U1B) that converts this current into a voltage with R1, with a value of 2k7 it converts 750 uA into about 2 V. A capacitor (C2) blocks the DC component of the signal and the AC part is amplified by a second opamp (U1A).
The dual-opamp is a low power MCP6002 from Microchip, its bandwidth-gain product is 1 MHz, more than good enough for audio. An advantage of this dual-opamp is that it works from 1.8 V to 6 V and uses 100 uA per amplifier. The total current consumption of the whole device is just over 200 uA so it can easily be powered with a CR2032 battery. Finally a potentiometer that was scavenged from a broken RC-servo, so the sound level can be adjusted.
Build on Perfboard
As usual I build it on a piece of perfboard so I designed no PCB. The case is a small plastic box (80x50x20 mm) that I spray painted black. The front panel is designed with Front Panel Designer, a program that I came across in yet another Instructable that was written by Killawhat.
Results
To my surprise the LED lighting above my workbench produces a 100 Hz hum, (mains is 50 Hz where I live). So the mains adapter doesn't feed it with a smooth DC current.
Seven segment led displays with a TM1637 driver make nice sounds because the digits are scanned one after the other. A clock made with four MAX7219s and DOT-Matrix displays even makes a sound that one might call melodious :) A seconds surprise was my bike light that uses PWM even when it is on the brightest level, I had expected to see a PWM signal only when it was at half intensity. Video on youtube: