Ambient light sensor FYI

Products Support DragonBoard410c
#1

Just had another quick look over the documentation for these boards, and found a new document called “Grove Digital Light I2C Sensor Integration”. Looking at it, I saw something really strange; the suggestion to power the sensor with 5 volts and add an i2c level converter to interface with the 1.8 volt Snapdragon.

The thing that is particularly strange about this, is that this particular sensor (TSL2563) actually communicates at 1.8 volts. In addition, it only likes input voltages in the range of 2.4 to 3.0 volt.

Turns out that they are suggesting not just the sensor itself, but on a specific breakout board that is specially adapted to arduino or rpi application, which operate on 3.3 or 5 volt I/O. This specific board translates voltages from the low voltage sensor, to the high voltage arduino/rpi. Then add in ANOTHER voltage level conversion from that high voltage back down to 1.8 volt… which just happens to be the same voltage level that the sensor actually works at.

Kind of redundant, isn’t it?

So the correct way to do this sensor is actually to pick up just the bare sensor with TMB package. Use a voltage divider (2 resistors) to cut the 5 volt power in half, and just hook straight up to it.

The bare chip costs $1-$3 depending on the supplier. TSL2563T.
The resistors cost about $0.01 combined.

The “wrong” one on the voltage level converter board costs $10-$15, PLUS the extra voltage level converter runs another $7.

#2

I agree that the two level shifters are redundant. The App note is explicitly titled “Groove Digital Light Senseo…”, and is intended for show how to interface a Groove sensor from Seeed studios, not a bare light sensor. The document is also a baseline example for interfacing other Groove devices.

Not everybody wants to connect to a bare device, they like their break-out board mounted devices because wiring is easy. The TMB version of the device is quite small the pins are on 0.9mm spacing which most people find quite difficult to solder to, the device doesn’t simply plug into a breadboard. The I/O voltage on the TSL2562 is 1.8V and the 410c provides 1.8V I2C on the Low speed connector, however the bare device also needs a 2.5V power supply. The 96Boards spec doesn’t provide any voltages between 1.8V and 5.0V so you need an external supply of some kind (likely a linear regulator from the 5.0V supply). Again many users are not comfortable with hand wiring a regulator and a 0.9mm device.

Hopefully the third parties like Seeed Studios will start building 1.8V versions of their devices soon. Removing the level shifters should improve the unit cost.

Full Disclosure: I am a employee of Qualcomm Canada. Any opinions I have expressed in this or any other post, may not reflect the opinions of my employer.

#3

If you can’t handle that, you can’t handle hooking it up to the expansion port either. Plus, there are plenty of generic breakout boards that could convert the thing to DIP without all those redundant parts.

#4

Hi Doitright: Can you point me to a generic breakout board that will convert 0.9mm pitch 6-pin parts to DIP please.

#5

This will work;

Spec on SOT23-6 package for comparison;
http://www.diodes.com/_files/zetex_files/pack/SOT23-6.pdf

Notice dimension “e” at 0.95 mm vs 0.90, which mean that it is close enough that you couldn’t tell the difference.

The other important dimension is “E”, up to 3.2 on the SOT23-6, 3.8 on the TMB-6, but there are two places where there is some leniency added; the breakout board probably extends a small amount past the specified width, the TMB-6 has indentations on both sides of 0.20 (i.e., total down to 3.4), and the TMB-6 pads extend in toward the center of the package by 0.9.

#6

Thanks, that should work.

#7

Hi there,
I have been trying to do this using the tutorial but when it comes to kernel modification step I am not find any support for AMBIENT light sensor , my I2C support column is empty.
Can someone help me with this.