EDNE February 2013 - (Page 45)

Photo meter assesses ambient light Raju Baddi, Tata Institute of Fundemental Research, Pune, India ↘ Most PN-junction diodes can be used as photodiodes. While not optimized for this application, they do work. When the diode is reverse biased, it will produce a small photovoltaic output as the light level is increased. LEDs are particularly suited for this task because their housings are transparent. You can construct a simple circuit that will assess the condition of ambient lighting and, because many LEDs’ packages are tinted to enhance their emitted color, may even yield a reasonable evaluation of the detected color. The results are not as effective as those obtained using a high-quality optical filter, which typically has narrow bandpass characteristics, but they can be quite acceptable. Though the design described here does not produce the accuracy of designs with laboratory-grade photodetectors and transimpedance ampli- 5V + (R) (G) (Y) R4 1 TO 100M BC559 R1 10k Q2 Q1 BC549 1N4148 D1A D1B R2 2.7k R3 2.7k R5 + − M 1 TO 100 μA D2A D2B Figure 1 If the simplest version is required, use an inexpensive volume-unit meter to replace the 0- to 100-μA meter. fiers, it can be quickly assembled and will produce usable results at a low cost. Three LEDs are used; experimentation will indicate which device has the best sensitivity to which color (Figure 1). The ambient light falling on the LEDs causes some current flow—typically in the range of 10 to 100 nA—through each LED, depending on the applied illumination level. This current flows through the base of a transistor, Q1, and is amplified. Q1’s collector current then splits between potentiometer R4, which acts as a firststage gain calibration, and the base of Q2. Q2 provides further amplification and drives the left side of a bridge circuit (D1A and D1B). Note that R2/D1 and R3/D2 form a balanced bridge. Q2’s collector current provides a slight imbalance to the bridge. The meter, M, measures this imbalance. R5 adjusts the sensitivity of the meter. Set R4 and R5 such that the meter has an appropriate deflection. R4 is useful for selecting the quiescent point; R5 is useful for adjusting the sensitivity. Before building the circuit, check whether the LEDs can be used as photo sensors. To determine whether a given LED is a good photodiode, check the voltage across the LED using a common digital multimeter set to its most sensitive range—typically 200 mV. Typical output voltage should be approximately 0.3 to 1 mV with typical office illumination. EDN DI5242 Figure 1 e the Cub Tales from Pg 75 how to design source VOICE OF THE ENGINEER ER NOVEMB e 11/2012 Issu om -europe.c www.edn s Design Idea Pg 67 NE TH E EN GI VO IC E OF g for extendin Strategies reliability product ER to 32g the 8n Managin sor migratio bit proces Page 29 nces of The nua requency variable-f drives Page 57 Page 27 Page 40 www.edn-europe.com/new www.edn-europe.com ment EDN.Com 09 Pg : Baker’s Best g a antialiasin Delta-sigm with a modefilter circuit rejection Pg 36 february 2013 | EDN europe 45 tery-stack Volt’s bat aids manager drive Chevy’s Page 63 http://www.edn-europe.com/new http://www.edn-europe.com http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE February 2013

RS Components
Agilent Technologies
Maxim Integrated
Signal Integrity
Mesago SMT 2013
Test & Measurement World
Rohde & Schwarz
Mesago PCIM 2013
Mechatronics in Design
Hot technologies: trends to watch in 2013
Hot 100 products of 2012
Add LED intelligence to improve light quality, efficiency, and cost
Design Ideas
Product roundup
Tales from the Cube

EDNE February 2013