There's a lot of photo electric devices, such as photo-cell relays, photoelectric switch units, silicon photo-electric cells and silicon blue cell.

Photo-cell relays. Basic component of photo-cell relay is an integral light activated switch. It combines a silicon planar photo diode with integrated circuit on a single substrate to provide a highly sensitive photo electric device. Operation is such that when light of a selected intensity falls upon it, the device switches on and supplies current to an external load. When the light intensity falls below the critical level the load current is turned off. This critical level can be adjusted within wide limits.

The equipment comprises a projector containing a light emitting diode and optical system projecting a beam of light either directly or by reflection onto a photo-cell mounted in a receiver unit. The relay coil is energized when the light beam is made and de-energized when it is broken. Thus the relay contacts can provide a change over operation which can be used to perform some external control function. The control unit can contain additional circuitry such as time delays or LED failure circuits to meet a wide variety of application requirement. System are available for operating over distances from 10-15 mm up to 50 m or more.

Applications include conveyor control, paper breakage alarm, carton sorting and counting, automatic spraying, machinery guarding, door opening, level controls, burglar alarms, edge alignment control and punched card reading.

Photoelectric switch units. Light sensitive switches are used for the economical control of lighting. They consist of a photo-cell which monitors the intensity of the light and automatically switches the lighting on or off. Construction of s typical unit is shown in figure 4.10 and this will switch a resistive load of 3 A at 250 VAC. The unit is based on a cadmium-sulphide cell and it incorporates a 2 minute time delay to prevent 'hunting'. Larger units are available with resistive switching capacities up to 10 A.

Silicon photo-electric cells. These cells are designed to provide large output current even under low illumination intensities. Currents of several milliamperes are obtainable. Structure of a photo-electric cell will be seen to consist of a thin p type layer on n type silicon. Due to its linear photo-voltaic effect there is no need for a bias power source. A linear output can be obtained by selecting a suitable load resistance for a wide range of illuminance. Like the silicon blue cell, described below, it has no directivity of receiving light, so there is no need to adjust the optical axis as is the case with photo-transistors.

Silicon blue cell. The sharps silicon blue cell manufactured by photain controls is claimed to be the world's first photo-electric diode possessing high sensitivity over the entire visible light spectrum. It is more reliable than the selenium or cadmium-sulphide photo-cells and has superior time response. No bias power is required, it has a lower noise level than the other two type and it is non-directional.

Application include illumination meters, exposure meters, optical readouts of film sound tracks, colorimetry, flame spectrometry, photo spectrometry and color or pattern recognition equipment.

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