Data Sheet

Project 11

Light Monitor

This project opens a window on your computer to

display measured data in real time. Build the

circuit shown. Load sketch Light_Monitor into

Snapino

using the programming instructions in

project 3. When the upload is finished, click on the

Tools menu, then pick Serial Monitor from the

list. A window opens on your computer screen,

displaying the measured light value. Vary the

amount of light shining on the phototransistor (Q4)

and see how the number displayed changes.

Programming Note:

The Serial commands open the window, then display some text

with the light measurement.

Changing the amount of light shining on the phototransistor

changes its resistance, and so changes the voltage measured at

the A0 input. The analogRead command has 10-bit accuracy, so

the measured number will be from 0 (darkest) to 1023 (brightest).

The 10KW resistor (R4) allows the voltage at the A0 input to fall

when it is dark and rise when there is light on the phototransistor.

The voltage measured depends on the ratio of the

phototransistor resistance to the 10KW resistor (R4). The

measured value will be about 512 when the phototransistor

resistance equals R4. Replacing R4 with another resistor would

shift the measured light value.

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Serial monitor window:

SCSNAPINO_Manual_112816.qxp 12/30/16 2:00 PM Page 23