April 6, 2012 AT 10:43 am

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):

 

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
Adding the LCD to the breadboard

 

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

Light meter circuit diagram

Arduino Uno image courtesy of Fritzing

And the resulting display:

Light meter 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?

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