Light to Sound with an iPad

Back in the early days of digital computing it was possible to design reasonably simple interfaces to convert analogue signals into digital inputs to a computer, like the Apple II or Radio Shack Tandy Model 100.  An investment of a few dollars could bring an evening or more of entertainment to the digitally amused.

In 1986 I designed a simple A2D converter for the purpose of capturing a varying light source as a digital input to a computer.  The heart of the converter was a '555' chip which was essentially an opto-isolator in a simple, small, and cheap package.  With the addition of a power supply, a capacitor, and one fixed and a variable resistor the A2D converter was created.  The variable resistor was a photocell for measuring the light source but could be any other variable resistor, such as a thermo-resistor.

The 555 generates a pulse with a rate dependent on the value of the input resistance.  As the resistance drops, the frequency of the pulse increases.  When the photocell is aimed at a light source, the resistance drops and the 555 generates a pulse stream that increases in frequency as the intensity of the light source increases.   The output from the A2D circuit is entered into the iPad via an iMic and Camera Connector. This combination is what I used to accommodate my original design using RCA jacks: the iMic has an RCA jack input end and a USB output end.  The USB output of the iMic is connected to the Camera Connector on the iPad.

Once everything is hooked up, the sound of the pulses can be captured using an app such as 'FiRe' for recording the output.  An example is in the link below.  The sounds are from moving my hand between a light source and a photocell connected to the A2D converter.

https://www.dropbox.com/sh/7ogsz2zna6lglbd/Ksbqw8C9FR/sound%20of%20light%203.aif

The schematic below shows the essential components: a 555 chip, input voltage (9V), variable resistance (R1), fixed resistance (R2), and a capacitor (C1).

 Figure 1: circuit diagram schematic

Input ('In Resistance'), output (Out RS232), and power (to 9V Source) connections used RCA jacks available inexpensively at electronic supply houses, shown in Figure 2.  The output was originally designed for an RS232 hookup, but here was connected to the iPad via an iMic interface, seen in Figure 3.  Figure 4 shows details of the hookup of the iMic to the iPad using the iPad Camera Connector.

Figure 2: A2D converter

Figure 3: Connection to iMic

Figure 4: iMic to iPad