Hack 02-02: Light Sensitive

Tags

speaker, pulser, oscillator, light, photoresistor, photocell, light-dependent

Difficulty Rating

2 on my scale, if you're starting from here. But if you already built the last project, then this hack is just a 1!

Purpose

A simple change to our previous circuit, Speeding Up the Pulse. This change will make the output frequency change according to light level.

Bill of Materials

We need the circuit from Project 02 and the last hack, Speeding Up the Pulse, assembled and working, plus this part:

Label	Description	Image
PR	Photoresistor

A photocell is a light-dependent resistor. Your photoresistor may be bigger or smaller. That's OK — it should work fine as long as it sort of looks like the one above.

Assembly

Click on the image to see a larger version.

We need to disconnect the orange wire (POTM lead #3), and add one part (PR).

Adjust POTM (the potentiometer) so you get wide change in frequency with changes in light level.

Success is when you hear a tone coming from the speaker, and its pitch changes as you change the amount of light falling on the photoresistor.

Schematic

Click on the image to see a larger version.

Images

Click on the image to see a larger version.

Video

[Under Construction]

How Does It Work

The Photoresistor

The photoresistor, also known as the light-dependent resistor and photoconductive cell, responds to light because its active material – either cadmium sulfide (CdS) or cadmium selenide (CdSe) – is a photosensitive semiconductor. You can see the material as the orange- or red-colored serpentine stripe on the surface. In the absence of light, this material is poor conductor of electricity — it has high resistance. But when struck by light, its conductivity goes up greatly — its resistance decreases significantly.

In this hack, the photocell is controlling the input voltage to the circuit. In bright light, the photocell has low resistance, and feeds more current into the potentiometer. This raises the voltage on that node, which raises the current flowing thru R1. And as we saw in the previous project, more current thru R1 leads to higher oscillation frequency.

Only two leads of the potentiometer are used in this circuit — this is called a variable-resistor or "rheostat" connection.

History

The photoelectric effect was discovered in 1873 in gray selenium. Selenium photoresistors were soon commercialized in the mid-1870's. Photoresistors based on cadmium sulfide and cadmium selenide came some time later. Over the years, scientists have experimented with many other metal-sulfide formulas.

Simple Mods

Reaction to Light — The circuit as-is produces a higher frequency output when more light strikes the photocell. But if you want the opposite effect, to get lower frequency output with increasing light, swap the circuit placement of the potentiometer and the photocell.

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