The most critical part of any PECU is the light sensor. Spectral sensitivity and long-term stability play an important role in providing reliable daylight detection.

Cadmium sulphide (CdS)

CdS sensors operate as light dependant resistors. They have conductivity approximately proportional to the level of light. The predominant choice 10 years ago, CdS sensors are still frequently used but are becoming less popular due to environmental considerations and are also subject to some long-term drift.

Photodiode sensors

It is possible to construct a semiconductor diode to produce a current proportional to the incident light level. The currents generated are small and need careful amplification by a circuit that compensates for thermal effects.

The fundamental physics of semiconductor junctions means that silicon photo-diodes have significant sensitivity to light outside the visible spectrum. Although semiconductor technology can limit this, other measures e.g. filters, are needed for matching sensitivity to that of the human eye. This is usually achieved by the addition of a filter that blocks light outside the visible spectrum.

Filters can take a number of forms, but are usually bulk-coloured glass slips added to the photodiode assembly during manufacture.

Zodion's DyeMatch™ is a novel approach whereby a special Infra-red blocking dye is added to a polymer and special lens caps moulded from this material are fixed over the photodiodes.

Table 3 : Comparison of filtered and unfiltered photodiode sensitivity with eye sensitivity

A variant of the above is to add a filter that blocks all visible light, so that the photodiode is only sensitive to Infra-red light. This can be incorporated into a PECU (Photocell) that is only sensitive to IR, and virtually insensitive to visible light.

This means that the PECU can be used where it is partially illuminated by visible light, often produced by the light that is being controlled by the PECU. Such a product is the Zodion ‘Lowlight'. It is typically used in bollards, where the PECU is incorporated into the base compartment and illuminated by visible light from the lamps within the base reflected from the inside of the cover.

The lowlight is insensitive to this light, and operates by sensing the IR component of daylight transmitted through the cover.

There are 3 cautionary notes:

•  Such PECUs can only be used with light sources that generate virtually no IR (incandescent lamps generate huge amounts of IR)

•  The PECU, although insensitive to visible light, will still be affected by very high levels of visible light. The arrangement of the PECU within the luminaire should minimise such light.

•  The proportion of IR to visible light in daylight varies with weather conditions so the PECU will not switch as accurately as a conventional PECU. An increase in annual burning hours of up to 25% can be expected.

Photodiode sensors can also be incorporated into integrated circuits although the functionality may be limited due to the competing requirements of semiconductor processing for optoelectronics and integrated circuits.

Also see

• What are PECUs (Photo Electric Control Units)? - Including part night PECUs (Photocells) and part night dimming PECU
• Energy consumption for photocells
• What are annual burning hours for PECUs (photocells)?
• Daylight and photocells- what you need to know
• Load switches used in PECUs
• Characteristics of a street lighting load
• Formats for photocells

To find out more about the Zodion Lighting Controls range of Photo Electric Control Units, please browse our catalogue pages.

Our PECUs range includes:

• Miniature Photocells
• One-Part Photocells
• Two-Part Photocells
• Sockets and Accessories for PECUs

If you have any questions, please contact us on 01422 317 317 or fill in our contact form we will be delighted to answer any questions you might have on PECUs.