Sensors2006,6, 514-525
nsors
ISSN 1424-8220
© 2006 by MDPI
/nsors Light Dependent Resistance as a Sensor in Spectroscopy Setups Using Puld Light and Compared with Electret Microphones Derci Felix da Silva 1 and Daniel Acosta-Avalos 2,*
1 Instituto de Pesquisa e Denvolvimento (IP&D), Universidade do Vale do Paraíba, Av. Shishima Hifumi 2911, CEP 12244-000, São José dos Campos, SP, Brasil
E-mail: br
古筝英文2 Centro Brasileiro de Pesquisas Físicas, R. Dr. Xavier Sigaud 150, CEP 22290-180, Rio de Janeiro, RJ, Brasil
云南旅游必去十大景点E-mail: dacosta@cbpf.br
* Author to whom correspondence should be addresd.
Received: 1 February 2006; in revid form: 27 April 2006 / Accepted: 9 May 2006 /
Published: 9 May 2006
Abstract: Light-dependent resistances (LDR) are cheap light nsors. A less known light
detector is the electret microphone, who electret membrane functions as a perfect
absorber, but only detects puld light. The aim of this study was to analyze the u of a
LDR and an electret microphone as a light nsor in an optical spectroscopy system using
puld light. A photoacoustic spectroscopy tup was ud, substituting the photoacoustic
chamber by the light nsor propod. The absorption spectra of two different liquids were
analyzed. The results obtained allow the recommendation of the LDR as the first choice in
幼儿园成长故事
the construction of cheap homemade puld light spectroscopy systems.
Keywords: LDR, electret microphone, photoacoustic, puld light.
1. Introduction
The light dependent resistor (LDR) is a nsor who resistance decreas when light impinges on it. This kind of nsor is commonly ud in light nsor circuits in open areas, to control street lamps for example. Another possible u is in spectroscopic apparatus [1]. In this kind of apparatus, continuous light or puld light can be ud. Continuous light is ud in common spectroscopic apparatus. The u of lock-in amplifiers made the u of puld light in spectroscopy easier, as is commonly ud in photoacoustic spectroscopy [2]. LDR’s are made of miconductors as light
nsitive materials, on an isolating ba. The most common miconductors ud in this system are cadmium sulphide, lead sulphide, germanium, silicon and gallium arnide [3]. A less known light nsor is the electret microphone. As the electret membrane functions as an absorbing black body, and as the electret microphone ca has an air chamber that can be ud as photoacoustic chamber, the electret microphone can be ud as a detector of puld light [4]. This type of microphone can be ud to obtain the transmission spectrum of any transparent material.
The aim of this communication is to study the respon of LDR to puld light and the analysis of the
spectral curves obtained with a LDR and an electret microphone as light nsors in an optical spectroscopy device.
2. Experimental Section
To study the respon of the LDR to luminous stimulus, it was ud a voltage divider circuit, compod by a 4.7 kΩ resistance, a LDR and a 9 V battery. The voltage was measured on the LDR using a multimeter or a lock-in amplifier.
First the respon of the LDR to continuous light was studied. This was done using a He-Ne lar as light source (UNIPHASE, mod. 1201-1) emitting at 633 nm with mean power output of 1.9 mW. To control the light power, two linear polarizers were ud, crossing their polarizing axis at a fixed angle that permits the light power to be changed following the Malus’ law. In this way, the light power was decread and measured with a power meter (MELLES GRIOT, mod. 13 PEM 001). The curve of the voltage as function of light power was constructed, and analyzed using the software Microcal Origin.
After the continuous light analysis, a puld light analysis was done. In this ca, the same light source was ud. The lar power was constant (1.9 mW) and a mechanical chopper (STANFORD
世界上最不值钱的货币
RESEARCH SYSTEMS Mod. SRS540) was ud to pul the light beam. A two pha lock-in amplifier (Stanford Rearch Systems Mod. SR530) was ud to measure the amplitude and pha of the LDR voltage.
Absorption spectra were obtained using a home-built photoacoustic spectrometer tup. A light beam supplied by a 1000 W Xenon lamp (model 66071, Oriel) was modulated at 17 Hz by a mechanical chopper (model 197, EG&G) and pasd through a monochromator (model 77250, Oriel). Then the monochromatic beam was focud into the LDR or a commercial electret microphone using mirrors and lens.
To record simultaneously the amplitude and pha of the signal, the voltage signal was fed to a two-pha lock-in amplifier (SR850, Stanford Rearch Systems). The lock-in amplifier was interfaced with a microcomputer to record the signal.
The electret microphone permits to obtain transmission spectra becau it functions as a photoacoustic chamber. In this ca the chamber is the frontal air gap of a cylindrical electret microphone, and the sample is always mounted directly on top of it. The front sound-inlet of the electret microphone is a 3 mm diameter hole, the front air chamber adjacent to the metallized face of
the electret diaphragm has a diameter of 7 mm and is roughly 1 mm high. To obtain the transmission spectra, the air chamber of the electret microphone is tightened with a glass slide of thickness 150 µm, and the sample is put on it without good thermal contact between them. In the conditions the
photoacoustic signal is due only to the light absorbed by the microphone membrane after passing through the sample, and is proportional to the amount of light reaching it.
To eliminate from our spectra the contribution of the spectral respon of the optical apparatus, the measured signal was divided by the one measured at the same photon energy on a completely absorbing material. In both cas (LDR and microphone) the reference spectrum was obtained without the sample on the optical pathway.
As the spectrum obtained corresponds with the sample transmittance, absorption must be proportional to the negative of the natural logarithm, as stated by the Lambert-Beer law. So, the absorption spectra analyzed are in fact the graph of the negative natural logarithm of the normalized signal.
The samples ud were two different cachaças that are distilled Brazilian beverages. The first one,identified as sample 1 was from the “Morro Velho” brand and has a yellowish color. The cond
one,identified as sample 2, is know as Aguardente de Cambuci. This beverage is made with Cambuci that is a Brazilian fruit, and the liquid has a brown-reddish color.
3. Results and Discussion
Figure 1 shows the LDR voltage as function of the continuous light power at log-log scale. It can be obrved that after 1 mW the voltage has a linear behaviour with linear coefficient of -0.547 ± 0.008,that can be understood as a functional dependence as:
P V P V 0)(= (1)
10
100
V o l t a g e (m V )
Power (mW)
Figure 1. LDR voltage as function of the continuous light power. For values higher than 1 mW the curve is linear. The linear fit inclination calculated is –0.55 ±0.01. The fit was done with the Origin software.
For voltages lower than 1 mW the functional dependence is more complicated, becau in the log-log graph it follows a cond order polynomial curve.
Figure 2 shows the LDR voltage as function of the light pul frequency. As the signal must ari through non-radiative process, the signal must follow approximately the function [5]:
20
)2(1)(f V f V πτ+= (2)
口若悬河是贬义词吗
危房怎么鉴定
where f is the light pul frequency and τ is the non-radiative time decay, that can be related with the time respon of the LDR.
0.05
0.10
0.150.20
V o l t a g e (m V ) f (Hz)
Figure 2. LDR voltage in function of the frequency of light puls. The continuous line is the result of the fit of the experimental points to the equation (2). The fit was done with the Origin software.The fit to the experimental values of Fig. 2 using Eq. (2) produce the following values: V 0 =0.268±0.002 and (2πτ)2 = 0.0024±0.0001. This produces a value for τ of 0.008 s or 8 ms. This means that a LDR must be innsible to frequencies higher than 125 Hz. Figure 3 shows the amplitude of the electret microphone signal as function of the light pul frequencies. The experimental points were also adjusted to the expression in equation (2). The fit produced the values V 0 = 0.0078 ± 0.00004 and (2πτ)2 = 0.0023 ± 0.00004, accounting for a τ value of about 0.008 s or 8 ms. Both detectors have similar values for their time respons. It can be obrved that the electret microphone has voltage values lower than that of the LDR. To obtain the curve in Fig. 3 a different light source was ud,becau the power of the He-Ne lar was insufficient to produce a signal in the microphone. In this ca, a black painted aluminum foil was closing the microphone aperture and a halogen tungsten lamp was ud as light source. So, if similar light sources are ud with both light detector, the LDR must produce higher voltages than the electret microphone.
立体五角星的折法
0.000
0.002
0.0040.006
V o l t a g e (m V ) f (Hz)
Figure 3. Microphone voltage in function of the frequency of light puls. The continuous line is the r
esult of the fit of the experimental points to the equation (2). The fit was made using the Origin software.
Figure 4 shows the LDR voltage as function of the light power with frequency of 71 Hz. The graph is in the log-log scale. As can be en, after 1 mW it has a linear behavior, with linear coefficient of -0.345± 0.005 that it can be interpreted as a functional dependence as follows:一目十行是什么意思
30
)(P V P V = (3)
For voltages lower than 1 mW the functional dependence is more complicated, becau in the log-log graph it follows a cond order polynomial curve, similarly as in the ca of continuous light.
Figure 5 and 6 show the amplitude and pha, respectively, of the microphone signal as function of the wavelength. As the electret microphone functions as light detector through the photoacoustic effect, it only detects chopped light and not continuous light. As can be en at Fig. 5, the signal amplitude reproduces fairly well the spectral emission of the light source ud [6]. The pha must be a constant when the microphone air chamber is appropriately aled with a glass window, giving
a stable signal. Fig. 6 shows that the pha is approximately stable with small oscillations on an average value of –176°.
