Light Meter Using I2C Level Converter, LCD, and TSL2561

I recently needed to measure how different materials affect light transmission for a gardening project. I decided this was the perfect opportunity to try out the new logic level converter to run both a 5v LCD and a 3.3v light sensor on a single i2c bus. I used the following parts in this project:

Optional (but definitely helps keep everything tidy):

Lady Ada’s Bento Box

First up is wiring power to the breadboard from the Arduino. I decided to keep the left side for 3.3v and the right side for 5v.

Breadboard right ground bus – black wire – Arduino ground pin
Breadboard right positive bus (5v bus) – red wire – Arduino 5v pin
Breadboard left ground bus – black wire – Breadboard right ground bus
Breadboard left positive bus (3.3v bus) – yellow wire – Arduino 3.3v pin

Connecting the breadboard to the Arduino

The next step is adding the logic level converter to the breadboard. It straddles the center notch like a DIP (but takes one additional column). I placed it toward the top of the breadboard to leave plenty of space for the light sensor to be added later.

Level converter LV pin – yellow wire – Breadboard 3.3v bus
Level converter HV pin – red wire – Breadboard 5v bus
Level converter left GND pin – black wire – Breadboard left ground bus
Level converter right GND pin – black wire – Breadboard right ground bus
Level converter B1 pin – green wire – Arduino analog pin 5 (i2c clock line)
Level converter B2 pin – blue wire – Arduino analog pin 4 (i2c data line)

Adding the level converter to the breadboard

The light sensor can now be added to the breadboard. I placed it on the left side of the breadboard with the pins in the rightmost column.

Light sensor GND pin – black wire – Breadboard left ground bus
Light sensor SCL pin – green wire – Level converter A1 pin
Light sensor SDA pin – blue wire – Level converter A2 pin
Light sensor VCC pin – yellow wire – Breadboard 3.3v bus

Adding the light sensor to the breadboard

Finally, the LCD is added to the circuit.

LCD DAT terminal – blue wire – Level converter B2 pin
LCD CLK terminal – green wire – Level converter B1 pin
LCD 5v terminal – red wire – Breadboard 5v bus
LCD GND terminal – black wire – Breadboard ground bus

With all components added, the block diagram of the circuit looks like this:

Arduino Uno image courtesy of Fritzing

And the resulting display:

For my project, I’m gathering data at reasonably high light levels; so, I went with the following settings:

  tsl.setGain(TSL2561_GAIN_0X);
  tsl.setTiming(TSL2561_INTEGRATIONTIME_13MS);

If you are measuring low light levels, you may want to adjust these two lines accordingly:

  tsl.setGain(TSL2561_GAIN_16X);
  tsl.setTiming(TSL2561_INTEGRATIONTIME_402MS);

An additional area to highlight is how the sketch is printing the light level values:

  snprintf_P(output_buffer, 6, PSTR("%5d"), (full_spectrum - ir_spectrum));

snprintf_P is a variant of sprintf that adds a couple of nice features. The ‘n’ indicates that you can specify a maximum number of bytes to write into the buffer; this helps protect against accidental buffer overruns. The ‘_P’ indicates that the format string is read from program memory; this helps conserve RAM. In the invocation above, I’m using the companion macro PSTR() to keep the format string parameter in program memory.

Full source is available on github.

How will you be using the logic level converter in your projects?