Digital Lux Meter

Posted on Sep 19, 2012

Photo diode outputs light current that is well proportional to input light power when it is used in short mode. In this lux meter, the output current is converted to voltage with an I-V converter, it is captured by a micrcontroller and displayed it in term of lux. The ppamp U1 in the Circuit Diagram acts as the I-V conversion circuit and its conversion rate becomes 50mV/µA according to a feedback register R5. The capacitor C5 is to gain correction, it cancels Ct of the photo diode (approx. 200 pF). In this case, larger capacitance may be good rather than 220 pF because high frequency signal is not needed. The lux meter is powered by a 9V battery. To minimize battery space, it is made contact with two piece of springs made of nickel plated copper phosphorus alloy instead of a battery snap. However it is not anti-reverse insertion structure, any protection circuit is needed.

Digital Lux Meter
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Q1 is the reverse insertion protector which has not voltage loss like series diode. Q2 is a main swich constructs power hold circuit with Q3 and Q4. Main function of the firmware is only captureing sensor output with A-D converter and display it to the LED display. The built-in A-D converter of AVR has 10 bit resolution but it is slightly insufficient for industrial measurement system. The A-D converter unit has a gain amplifier which can be inserted between multiplexer and A-D converter. This enables to improve resolution by changing the input gain automatically. To implement this funciton, capture input in gain of 20, if overflow occured, capture it again in gain of 1. As a result, can improve resolution at small input resion that lower relative resolution. Maximum light input is approx. 30000 lux due to A-D input raige of 0 to 2.56 V. Any reference light source will not be available in most case. We have to depend on only the data sheet to calibrate lux meter. The short current is Isc = 0.16µA/100lux, according to S1087 data sheet. It becomes 2µA at 1250 lux. When -100 mV is applied to test pin (TP1), it is equivalent to 1250 lux at photo diode, and I-V converter will output 100 mV. To perform calibration, tie ISP pins via a several k? resister, turn on power, and will be calibrated to the reference input. Low range is calibrated when SCK-GND and -100 mV on TP1, High range is calibrated when MOSI-GND and -1 V on TP1....

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