Sharp IR Sensor Problems

Hi All,

I'm having some trouble with a Sharp 2Y0A21 IR sensor and I hope someone can shed some light on it for me. I'm sure this is just a newbee problem, but it's making me crazy!

My entire circuit is more complex, but I tracked my problem down to something very simple:

When I connect (4) AAA Alkaline cells (5.71V) to the (+) and (-) of the sensor and there is no obstruction, the sensor lead reads around 100mV to ground. This is perfect, basically zero volts. When I wave a hand in front of the sensor, the voltage reads between 1V and 4V depending on the distance. Again, perfect!

However, when I connect (2) CR2032 coin cells (5.79V) I get around 1.4V on the sensor lead even with no obstruction.

Can anyone tell me why this would happen? Is there something about the coin cells that causes this result?

TIA, Mike

 

Just a guess - maybe they cannot supply enough current? Is that voltage you cite the voltage while running? If not, try measuring that.

 

My loaded voltage from the AAA's is 5.56V @ 28mA. From the coin cells, I'm getting 4.98V @ 25mA. According to the specs, the sensor "likes" 4.5 to 5.5V so I'm still in the right range.

 

CR2032 are supposed to drive loads of 15k ohms and more at 3V (less than 0.2 mAmp). According to the datasheet for this sensor, it needs 30 mA (approximately 200 ohm load on 6 volts).

This is way too much for a coin cell. Your sensor is in some type of latch-up. Try a different battery that is intended for higher current applications.

 

Thanks, GopherT! Looks like that solves the mystery. Strange symptom, though...

 

Not really strange... The Sharp datasheet mentions clearly that the IR LED draws current in high current pulses, so even though it might take 30mA average the PSU must be able to deliver higher currents like 100 or 200mA, depending on sensor type.

The datasheet also recommends adding a large cap right at the power terminals of the sensor, something like 100uF or 220uF, which may help to fix your problem.