New Sensor Update – Ambient Light Sensor

We recently evaluated an ambient light sensor (TEPT5700) which can be used outside an enclosure. Our requirement is to keep the sensor outside  while the rest of the electronics  stays inside a  weather  proof  enclosure. This allows us to use the light sensor in the outdoors. The other option is to use a PCB mounted light sensor inside an enclosure with a transparent lid.

We chose a through hole photo-transistor which is sensitive to visible light (just like the human eye).  The  current through a photo-transitor  is  directly proportional to the intensity of  light falling on it.

tept5700_ckt

We used used  an op-amp in negative feedback amplifier configuration to amplify the voltage across a current limiting resistor (between the photo-transistor’s  emitter and ground). To increase the supported range (of light intensity) we used an analog switch to change the  gain of the op-amp between two  values  (low gain (2) and high gain (102)). The amplified output (of the  op-amp) was hooked up to a 10 bit ADC channel on the WSN1101L wireless mesh node. The software driver (for this sensor) gets  the ADC output for  both gain values but selects one for  calculating the photo-transistor current depending  on the two values. If the photo-transistor current is low (low light conditions), the ADC output (measured) in the high gain configuration gets  used.  If the photo-transistor current is high (bright light) causing the op-amp output to get clamped at the positive rail in the high gain configuration, the ADC output (measured) in the low gain configuration gets  used.

Here  is a pic of the circuit built on a WiSense  proto board (PB-200).

th_als_eval

 

References:

 

For more information, please visit http://www.wisense.in

Posted on August 29, 2016, in Uncategorized. Bookmark the permalink. Leave a comment.

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