"Sound bulbs": audible light bulb replacements for introductory circuit labs

In introductory circuit labs, students collect data based on visual observations of the brightness of incandescent light bulbs in various configurations, which presents a challenge for blind and visually impaired students. We aimed to develop the design and evaluate the efficacy of audible drop-in replacements for incandescent bulbs. We have found that a simple circuit based around a 555 timer can be used to create an audible sound whose pitch changes with voltage. The "sound bulbs" performance showed ohmic behavior in both positive and negative polarity with fitted curves indicating equivalent resistances of approximately 323 Ω, only about 1% different from the nominal value that would be expected from having a 100 Ω resistor (R3) in series with a 220 Ω resistor (R4). The frequency-vs-voltage curves for the sound bulbs show that around zero volts the audible pitch changes by a half step approximately every 0.3 V, and near the extremes of +/- 6 V the sensitivity increases, with pitch changing by a half step approximately every 0.2 V. When combined in series and/or parallel, the measured resistance behaves in the same way as resistors. Calculated values of equivalent resistances were within 1% of the expected values for series, parallel, and combination circuits. The frequency response is reasonably uniform across a typical voltage range used in introductory physics labs, showing a promising functionality. Sensitivity is such that changes of fractions of a volt create distinguishable sounds across the full input voltage range. While the motivation for this work is to improve accessibility for blind and visually impaired students, providing "sound bulbs" as an option is a step toward universal design that has the potential to benefit many more students.