Nuts & Volts - July 2004

Near Space

by L. Paul Verhage

Approaching the Final Frontier

Near Space

Making a Light Sensor for the HOBO Data Logger

By using a photocell (CdS) as one element in a voltage divider, you can construct a simple light sensor. Because it uses a photocell, the light sensor’s spectral sensitivity is very similar to that of the human eye. After you finish reading about how to build this light sensor for your HOBO data logger, I’ll explain a very interesting finding about designing voltage divider-based sensors.

Any two resistors wired in series with a voltage source (battery) form a voltage divider. In the voltage divider, the voltage dropped across one resistor element is proportional to its resistance in sum with the second resistor element. The voltage drop across the resistor of interest is given by the following formula:

condition. Now, by measuring the voltage drop across the variable resistor, you can measure the environmental variable of interest.

The cadmium sulfide (CdS) photocell is a light sensitive resistor. Its resistance decreases when exposed to bright light and increases when exposed to dim light. The photocell responds very quickly to changing light conditions, but not as fast as a phototransistor or photodiode.

You will need the following components to make a CdS light sensor for your HOBO data logger:

Note: The value of the fixed resistor depends on the resistance range of the CdS cell. For my light sensor, I used a 1.5K resistor. At the end of this article, I’ll explain how you can determine the best resistor value for your particular CdS cell.

• Cadmium sulfide cell (My particular cell has a resistance that ranges from 100 Ω in bright light to 20K in the dark.)

Vdrop = Vapplied X (Ri / Ri + Ro)

• Fixed resistor (A 1/4 watt resistor is sufficient.)

In this formula, Ri is the resistor of interest and Ro is the other resistor.

By itself, the voltage divider circuit is not very interesting. However, things do get interesting when one element becomes variable and changes its resistance due to changes in some environmental

• 3/32” stereo jack kit

• Thin heat shrink tubing

Each input to the HOBO is through a 3/32” stereo jack. The stereo jack has three contacts: tip, ring, and base. Voltage to operate the sensor comes from the tip. The signal to be digitized is connected to the ring of the stereo jack and the ground is connected to the base. The diagram in Figure 1 illustrates the connections.

I placed the fixed resistor close to the stereo jack and ran a long extension out to the CdS cell. I decided to keep the fixed resistor close to the HOBO to minimize the amount of wire my sensor needed. Note that the HOBO is digitizing the voltage drop across the CdS cell because the ground is connected to one end of the CdS cell and the signal is connected to the other end.