week_3: analog sensors

During class we discussed the use of analog sensors as means to obtain data stream from the environment that can be used to design interactive experiences. We also discussed the use of Arduino as computer, writing code to modify the behavior of hardware.

Here I describe the use of a phototransistor coupled to a piezo element to generate sound in response to changes in light conditions. I used this circuit configuration to experiment with generation of different sounds depending on the light-absorbance properties of plant leaves exposed to different light sources. Furthermore, I encouraged my own daughter (5 years old) to interact with the circuit so I could observe the process closely.

I also built and experimented with a circuit configuration that used a temperature sensor as analog input to change the color of LED lights when touched. For this post, I only document and describe the first example suing photoresistor and piezo element.

Leaves from different plant species generate different sounds when illuminated with same light source

Plants use a region from the electromagnetic spectrum (between 400 nm to 700nm) to drive photosynthesis. Light is absorbed by pigments (chlorophyll a, chlorophyll b, and carotenoids), and their relative amount synthesized in the leaf and their distribution (upper part of leaf relative to lower part) contribute to difference in light absorbing properties between species. Plant leaves absorb light primarily at the red and blue ends of the spectrum, with distinctly less absorbance at the green spectrum (500-600nm). This property made for an interesting case in experimenting with sound generation when the photoresistor was illuminated with a single light source and covered with leaves from different plants.

Figure 1. (Left) A phototransistor connected to ground through a 10kohm resistor and to an analog pin A0. Notice the placement of the piezo element on the circuit board. Example taken from p.71-77 of Arduino Projects Book. The code that runs the Arduino was slightly modified to generate different tones of sound in response to light. Other than that, it remained essentially unchanged. (Right) Sources of light beams used and plants leaves from my garden for a quick experimentation/proof-of-concept.

I observed that different light sources would produce different tones of sound. When leaves were placed covering the phototransistor, different sounds would emerge that were different from the ones produced with the light source alone. Furthermore, placing the upper of lower part of the leaf altered slightly the sound being generated. This is because the distribution of pigments across the leaf varies.

Interactive experience with light-sound circuit configuration was fun

As I observed my daughter playing with the circuit, I could confirm that the sole concept of 'translating' light to sound was highly engaging. The combination of electronic components (that are unusual for children to interact with) and commonly known plant leaves from the garden (that she helped planting them on pots several months ago) was an interesting assemblage in helping her relate one concept to the other. Implicit in the interaction, there were:

1 plant biology & physics of light concepts_ leaves have pigments that absorb a certain part of the electromagnetic spectrum and allow other to pass through (this is the light hitting the sensor)

2 electronic concepts_ what is a sensor and how can control sound produced by a speaker

3 exploration and play_ she had to experiment with different leaves and light sources

4 body engagement_ she engaged and interacted using her eyes, hands and ears (seeing, touch, hearing)

Facebook link to video: https://www.facebook.com/profile.php?id=100009404870867

Taking it further

Several ideas can be derived from this implementation of the example presented on the Arduino Projects Book. One possible scenario would entail a classification system of the stress response of plants to drought based on sound. As plant experience drought stress, a series of biological processed take place that affect chlorophyll synthesis and photosynthesis, consequently altering the sound generated.


The Arduino Project Book (2015). Published by Arduino LLC