@▷ Photosensitive devices, Photosensitive devices, LEDs, sample circuits | Diagram for Schematic

Photosensitive devices, Photosensitive devices, LEDs, sample circuits

We will look at Light-Sensitive devices in this article and find out how they can be used in various practical control circuits. Light-sensitive devices include photocells, photodiodes, and phototransistors. Visible and infrared light (or the absence of that light) can trigger many different kinds of circuit for the control of alarms, lights, motors, relays, and other actuators. Light-sensitive devices, sometimes called photoelectric transducers, alter their electrical characteristics in the presence of visible or infrared light.

Photocell Basics:
Photocells are also called by many other names including photoconductive cells, light-dependent resistors (LDR's), and photoresistors. They are variable resistors with an extremely wide range of resistance values (up to hundreds of orders of magnitude) that are dependent on the level of incident light. Resistance in photocells varies inversely with the strength of light that falls on them. In other words, resistance is very high in the dark, but low under bright light.

Figure 1 is a cutaway view of a typical photocell showing the pattern of photoconductive material deposited in the serpentine slot separating the two electrodes that have been formed on a ceramic insulating substrate. This pattern maximizes contact between the crystalline photoconductive material and the adjacent metal electrodes.

The photoconductive material is typically Cadmium Sulfide (CdS) or Cadmium Selenide (CdSe). The selection of the material and the thickness and width of its deposition determine the resistance value and power rating of the device. The two-terminal assembly is enclosed in a metal or opaque plastic case with a clear glass or plastic window over the photo-conductive material. Newer types just uses a very hard clear lacquer coating on a very thin wafer. Figure 2 is the schematic symbol for the photocell.

Curves Photocells are made with diameters from about one-eight inch (3mm) to over one inch (25mm); the most popular devices have diameters of about three-eight inch (10mm). The smaller units are suitable for applications where space is limited, such as in card-reading applications (which method is no longer used I think), but they have low-power dissipation ratings. Another application is a so-called 'sun-tracker' used to keep large solar panels in the sun. Some photocells are hermetically sealed to withstand the effects of demanding environments.

Figure 3 compares the response of photosensitivity devices characteristics with that of the human eye. Relative spectral response is plotted against wavelength from 300 to 1200 nanometers (nm). The bell-shaped human eye response curve shows that the eye is sensitive to a relatively narrow band of the electromagnetic spectrum, between 400 and 750 nm. The curve peaks in the green light region at about 550nm and extends down into the violet region (400 to 450nm) at one end, and up into the dark red light region (700 to 780nm) at the other end.

Resistance Also, Figure 3 shows why cadmium sulfide (CdS) photocells are so popular for light-controlled circuits; the CdS spectral response curve peaks near 600nm, and it closely matches that of the human eye. By contrast, the response curve for Cadmium Selenide (CdSe) peaks further out at about 720nm. However, CdSe is also sensitive to most of the visible-light region.

A typical CdS photocell characteristic curve is shown in Fig. 4. Its dark resistance is about five mega-ohms. This value falls to about 600 ohms at a light intensity of 100 lux, typical of a well illuminated room and to about 30 ohms at an intensity of 8000 lux, typical of bright sunlight. (The lux is the SI unit of illuminance produced by a luminous flux of 1 lumen uniformly distributed over a surface of 1 square meter).

Commercial photocells have good power and voltage ratings, similar to those of conventional resistors. Power dissipation ratings could be between 50 and 500 milliwatts, depending on detector material. Their only significant drawbacks are their slow response times. Cadmium Selenide photocells generally have shorter time constants than Cadmium Sulfide photocells (approximately 10 milliseconds versus 100 milliseconds). They also offer lower resistance values, higher sensitivities, and higher temperature coefficients of resistance.

Photocells are included in photographic exposure meters, light-and dark-activated lights, and intrusion alarms. Some light-activated alarms are Figure 5 triggered by breaking a light beam. There are even light-reflective smoke alarms based on photocells. Fig. 5 to 20 show practical photocell circuits; each will work with almost any photocell.

Photocell Light Switches:
Fig. 5 to 10 illustrate practical light-activated switch circuits with relay contact outputs that are based on the photocell. The simple circuit shown in Fig. 5 is designed to react when light enters a normally dark space such as the inside of a cabinet or closet. The photocell R3 and resistor R2 form a voltage divider that sets the base bias of Q1. Under dark conditions, the photocell has a high resistance, so zero bias is applied to the base of Q1; in this state, Q1 and the relay RY1are off. when a sufficient amount of light falls on the photocell, its resistance drops to a low value, and bias is applied to the base of Q1. That bias activates RY1, and its contacts can control external circuitry.

The simple Fig. 5 circuit has low sensitivity and no provision for sensitivity adjustment. Fig. 6 illustrates how these drawbacks can be overcome with Darling-coupled transistors Q1 and Q2 replacing Q1, and the use of a potentiometer R2 for sensitivity control, replacing fixed resistor R2. The diagram also shows how the circuit can be made self-latching with the second set of relay contacts. Normally-closed (NC) pushbutton switch S1 permits the circuit to be reset (unlatched) when required.

Figure 6  Figure 7

Figure 7 shows how a photocell can form a simple dark-activated relay that turns on when the light level falls below a value preset by potentiometer R1. Resistor R2 and the photocell R3 form a voltage divider. The voltage at the R2-R3 junction increases with falling light. That voltage, buffered by emitter-follower Q1, controls relay RY1 with common-emitter amplifier Q2 and current-limiting resistor R4.

The light trigger or threshold levels of the circuits shown in Fig. 6 and 7 are susceptible to variations in supply voltage and ambient temperature. Figure 8 shows a very sensitive precision light-activated circuit that is not influenced by those variables. In this circuit the photocell R5, potentiometer R6, and resistors R1 and R2 are connected to form a Wheatstone Bridge, and op-amp IC1 and the combination of transistor Q1 and RY1 act as a highly sensitive balance-detecting switch. The bridge balance point is independent of variations in supply voltage and temperature, and is influenced only by variations in the relative values of the bridge components.

Figure 8  Figure 9


In Fig. 8, the photocell R5 and potentiometer R6 form one arm of the bridge, and R1 and R2 form the other arm. Those arms can be considered as voltage dividers. The R1-R2 arm applies a fixed half-supply voltage to the non-inverting input of the op-amp, while the photocell-potentiometer divider applies a light-dependent variable voltage to the inverting pin of the op-amp.

To use this circuit, potentiometer R6 is adjusted so that the voltage across the photocell and the potentiometer rises fractionally above that across R1 and R2 as the light intensity rises to the desired trigger level. Under that condition, the op-amp output switches to negative saturation, which turns on Q1 and thus RY1. When the light intensity falls below that level, the op-amp output switches to positive saturation, and Q1 and the relay are turned off.

Figure 10

The circuit in In Fig. 8 is so sensitive that it is able to respond to changes in light-level that are too small to be detected by the human eye. The circuit can be modified to act as a precision dark-activated switch by either transposing the inverting and non-inverting input pins of the op-amp, or by transposing the photocell and the adjacent potentiometer.

The circuit in In Fig. 9 also shows how a small amount of hysteresis can be added to the circuit with the feedback resistor R5 so hat relay RY1 is actuated when the light level falls to a preset value. However, the relay is not de-actuated again until the light intensity increases substantially about that value. The hysteresis magnitude is inversely proportional to the value of R5, but is zero when R5 is open circuited.
Figure 10 shows how a precision light/dark switch can be made by combining op-amp light and dark switches. The switch activates relay RY1 if the light intensity rises above one preset value or falls below another preset value. Potentiometer R1 controls the dark level, potentiometer R2 controls the supply voltage, and potentiometer R3 controls the light level.

To organize the circuit shown in Fig. 10, first preset potentiometer R2 so that about half the supply voltage appears at the junction between R6 and potentiometer R2 when the photocell is illuminated at its normal intensity level. Potentiometer R1 can then be preset so that RY1 is actuated when the light intensity falls to the desired dark level and potentiometer R3 can be adjusted so that RY1 is actuated at the desired brightness level.

In the circuits shown in Fig. 8 to 10, the resistance values of the series potentiometers should equal the photocell's resistance values at the normal light level of each circuit.

Figure 11  Figure 12

Bell-Output Photocell Alarms:
The light-activated photocell circuits in Figs. 5 to 10 all have relay outputs that can control many different kinds of external circuits. In many light-activated circuit applications, however, the circuits must trigger audible alarms. This response can also be obtained without relays as shown in Figs. 11 to 17.

Fig. 11 shows a simple light-activated alarm circuit with a direct output to an alarm buzzer or bell. The bell or buzzer must be self-interrupting and have an operating current rating less than 2 amperes. The supply voltage should be 1.5 to 2 volts greater than the nominal operating value of the bell or buzzer. Photocell R3 and resistor R2 form a voltage divider. Under dark conditions, the photocell resistance is high. so the voltage at the junction R3 and R2 is too small to activate the gate of the silicon-controlled rectifier SCR1. Under bright light conditions with the photocell resistance low, gate bias is applied to the SCR which turns on and activates the alarm.

In the circuit of Fig. 11, keep in mind that although the SCR is self-latching, the fact that the alarm is self-interrupting ensures that the SCR repeatedly unlatches automatically as the alarm sounds. (The SCR anode current falls to zero in each self-interrupting phase.) Consequently, the alarm automatically turns off again when the light level falls below the circuit's threshold level.

The circuit of Fig. 11 has fairly low sensitivity and no sensitivity adjustment. Figure 12 shows how that drawback can be overcome: potentiometer R6 replaces a fixed resistor and Q1 is inserted as a buffer between photocell R5 and the SCR1 gate. The diagram also shows how to make the circuit self-latching by wiring R4 in parallel with the alarm so the SCR anode current remains above zero as the alarm self-interrupts. Switch S1 permits the circuit to be reset (un-latched) when required.

Figure 13 Figure 13 shows how to make a precision light-alarm with an SCR-actuated output based on a Wheatstone Bridge formed by the photocell R6, potentiometer R5, and op-amp IC1. The op-amp balance detector provides precision control. That circuit can be converted into a dark-activated alarm by simply transposing the photocell and potentiometer. Hysteresis can also be added, if required.

Speaker-output Alarms:
Figures 14 to 17 show different ways of using CMOS 4001B quad 2-input NOR-gate IC's to make light-activated alarms that generate audible outputs with loud speakers. The 4001B is available at the CD4001B from Harris and Motorola, National Semiconductor, Signetics, and others under various designations that include 4001B.

The circuit of Fig. 14 is a dark-activated alarm circuit that generates a low-power 800Hz pulsed-tone signal at the speaker. NOR gates IC1-c and IC1-d are wired as an 800Hz astable multivibrator that can feed tone signals into the speaker from Q1. It is gated on only when the output of IC1-b is low. NOR gates IC1-a and IC1-b are wired as a 6Hz astable circuit that is gated on only when its gate pin 1 is pulled low. (Pin 1 is coupled to the voltage divider formed by photocell R4 and potentiometer R5.)

Figure 14 The action of the circuit is as follows: under bright light conditions, the voltage at the junction of the photocell R4 and potentiometer R5 voltage is high, so both astable circuits are disabled and no output is generated at the speaker. Under dark conditions, the photocell-potentiometer junction voltage is low, so the 6Hz astable circuit is activated, gating the 800Hz astable on and off at a 6Hz rate. As a result, a signal from Q1 produces a pulsed-tone in the speaker.
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