There are typically two kinds of light sensors or light-sensing heads currently in use in the light control industry. One is a photodiode type sensor employing a photocell that outputs a proportional varying voltage depending on the amount of light impinging on the cell. The other is a photoconductive sensor employing a cell whose resistance changes as the light on the cell varies. Either type is usable in many of the same situations. However, a well-designed photodiode sensor can output an extremely accurate analog signal that is compatible for input to computer controls. A good example of this is the sensor disclosed in U.S. Pat. No. 4,758,767 which was designed by Fred Blake, one of the inventors of the invention disclosed herein. The photoconductive type, on the other hand, cannot match the accuracy of the photodiode type but nevertheless is adequate for use in many light control systems with good results. Although less accurate, the advantage of the photoconductive type over the photodiode type is that the photoconductive type is cheaper. The invention of the present case relates to photoconductive type sensors, although it includes an improved housing design which could be easily implemented with either a photodiode or photoconductive type indoor sensor.
The typical photoconductive cell sensor is a two-lead system where the cell is connected to control circuitry. Usually, the cell is remotely located from the control circuitry. In some situations, however, both the cell and circuitry reside in the same housing.
One of the cell's leads is grounded while the other is connected to a low-voltage power source (typically 12 VDC) through a load limiting resistor. Typically, the control circuitry processes the cell's output, which is a change in resistance varying with light input that is "read" as a voltage change. The control circuitry has a comparator circuit that monitors this and outputs an "on" or "off" signal depending on the cell's output.
Referring to FIG. 11, for example, a conventional control circuit for a photoconductive type sensor is illustrated there. This particular configuration utilizes a high-resistance photoconductive cell 1 having a resistance of approximately 1000 K ohms at 2 foot candles input, and 10 K ohms at 500 foot candles input. The control circuit 2 utilizes a current limiting resistor 3 between a low-voltage power source 4 (12- volts DC) and circuit junction 5. The control circuit 2 has conventional sensor delay circuitry, indicated generally at 6, and conventional buffer circuitry, indicated generally at 7. The total range of output from the buffer 7 with the circuitry as shown is approximately 6 to 7.5 volts DC for 2 foot candles and approximately 0.1 volts DC for 500 foot candles.
The photoconductive cell 1 illustrated in FIG. 11 is a standard wide range type and, as the skilled person would know, its output is nonlinear. As the skilled person would also know, when it is used in conjunction with the circuitry of FIG. 11, approximately 90% of its voltage output response is used up in the first 10% of the input to the cell, which is approximately 0 to 50 foot candles. The remaining 10% of the voltage output range is available for the last 90% of the light level range, i.e. 50 to 500 foot candles. This is illustrated in the two left-handed columns of FIG. 14.
Referring again to FIG. 11, the circuitry shown there also has an operational amplifier circuit 8 that is used as a voltage comparator, and outputs an "on" or "off" signal, depending on the relative setting of a potentiometer indicated schematically at 9. This signal triggers one or more relays which correspondingly turn on or shut off artificial lights.
The kind of potentiometer used in the above-described circuitry is a type whose voltage input is adjusted by turning the potentiometer. FIG. 14 shows how the voltage input varies with the degree of potentiometer adjustment. Although not shown on the table, the 0.degree. position corresponds to approximately 12 volts and the input approaches zero by the time the potentiometer is turned to approximately the 270.degree. position.
In the FIG. 11 circuit, since the upper limit of output from buffer 7 is approximately -6 to 7.5 volts DC, any potentiometer settings in the 12 to 7.5 volt range (0.degree. to 120.degree. of turning motion) are not usable and are therefore wasted. This is illustrated, for example, in FIG. 15. Even worse, and as FIGS. 14 and 15 show, most of the potentiometer's remaining settings are used up within a small range of 1 to 100 foot candles input to the photocell.
Although the potentiometer is relatively easy to set in this range, due to the action of the current-limiting resistor 5 in the configuration of FIG. 11, this usable portion is not linearly proportional because the voltage read by the potentiometer follows the nonlinear output of the photocell. This is illustrated in FIG. 15 by that portion of the curve identified by the words "Easy Setting Range." As the table of FIG. 14 shows, above 100 foot candles use of the potentiometer is made extremely difficult. The invention disclosed herein, among other things, provides a way to make nearly full use of the range of potentiometer motion, and makes the scale of the potentiometer more uniform or, in other words, more linear. It therefore makes for an easier, more accurate-to-adjust system. This will become better understood upon consideration of the description which follows, and the drawings.
Further, inside ceiling-type sensor housings of the type which house either photoconductive or photodiode cells have, in the past, been bulky and relatively problematic in their installation. Referring to FIGS. 12 and 13, for example, the sensor housing shown there is typical to the current field. In order to install this housing, the housing's body 11 first requires that a circular hole be cut in a ceiling tile (not shown) where the sensor is to be installed. Then, a clamping ring 12 is positioned behind the hole, and the body 11 is inserted therethrough. The sensor's face plate 13 is screwed directly to the clamping ring as shown at 14 and sandwiches the ceiling tile inbetween. A significant amount of labor is required to make this installation. As will become apparent from considering the following description when taken in conjunction with the drawings, the inside ceiling sensor housing disclosed herein is of a superior design. Not only is it much easier to install than previous designs, but its smaller size is less obtrusive to the eye.