In the comments section of my previous article I demonstrated an optical Theremin. This article is the build for that circuit. The circuit and circuits that are similar to it in function have also been called a “Light Sensitive Tone Generator,” “Photo Theremin,” and “Audible Light Meter.” These three circuits are usually based on the two-transistor Light Sensitive Tone Generator from The Forrest Mims Circuit Scrapbook, Volume 1 (Radio Shack, 1976). The circuit built in this article is based instead on the 555 timer IC. For an introduction to Snap Circuits and the 555 timer, see this article.

Initially, I didn't think there would be much practical use for an Audible Light Meter, but while I was doing research for this article I came across an article in an old newsletter from NASA’s George C. Marshall Space Flight Center (Huntsville, AL) titled, “AUDIBLE LIGHT METER HELPS THE BLIND TO 'SEE,'” so I suppose you can call it an audible light meter for the visually impaired:

An audible light meter which can serve as a portable light detection instrument for the blind, has been developed by the Southwest Research Institute using Marshall Center technology. The 5-ounce hand held device is actuated by a button switch, much like a flashlight, and is about one-half the size of a pack of cigarettes. Light striking the instrument’s photocell activates a transistor which energizes an audible buzzer signal. The tones of the signal vary with the amount of light reflected (as in colors of clothing) on the light detection system. By listening to these varying tones, a blind person can know when the electric lights are off, or when the indicator lights on automatic appliances are on. The device also gives a blind person greater mobility by signaling when he is near doorways, walls, hallways, windows or other barriers.

The instrument’s tiny variable resistor can be adjusted to make the device sensitive enough to detect a cooking light or coarse enough to distinguish between day and night.

The unit, which markets for less than $100 and is operated by a regular 9-volt transistor battery, also provides blind persons working in industry and business with a valuable aid for knowing, for example, when stock supplies are not correctly displayed. (Source)

Parts needed:

555 Timer IC (I used a KIA555p, but the NE555 will do just fine)

Snap Circuits Parts:

1 Base Grid (11” x 7.7”) # 6SC BG
1 Eight-Pin IC Socket # 6SC ?U8
1 0.02uF Capacitor # 6SC C1
1 Variable Resistor #6SC RV
1 Whistle Chip # 6SC WC
1 4.5 Volt Battery Holder # 6SC B3
1 Slide Switch # 6SC S1
1 Photosensitive Resistor # 6SC RP
1 Single Snap Conductor # 6SC 01
5 Conductor with 2-snaps # 6SC 02
3 Conductor with 3-snaps # 6SC 03
3 Conductor with 4-snaps # 6SC 04
1 Conductor with 5-snaps # 6SC 05
1 Conductor with 6-snaps # 6SC 06

Build the circuit shown:

The following is the video demonstrating the “Audible Light Meter”:

The change in pitch is caused by increasing or decreasing the resistance in the circuit. The Photosensitive Resistor (RP) is made with a photoconductive material such as Cadmium Sulfide (CdS). The amount of light that falls on the photoresistor changes the resistance—that is, as more light falls on the photoresistor, it decreases the resistance in the circuit. As less light falls on the photoresistor, resistance in the circuit increases.

You'll notice when more light that falls on the photoresistor, this makes the pitch higher. Less light makes the pitch lower.

--------------------

NOTE: Some sources also cite Optoelectronic Projects Vol 1 by Forrest Mims, III (Radio Shack, 1977) for the Photo-Theremin circuit design.