PIR

From DIYWiki

Choosing a PIR security light


PIR detectors (Passive Infra-Red) detect movement of people (and pets), and switch lights on when movement is detected. They don't normally respond to movement of cold objects.

PIR light fittings are used as security, safety and courtesy lighting. They typically stay on for a few minutes when triggered, giving the benefit of lighting with little of the run cost.

There are plenty of variants to choose from. Hopefully this article will help with the choices.

There's no need to use an all-in-one PIR fitting. Standalone PIR detectors can be used with any type of light fitting that suits the situation, giving a very wide range of choices.


Power

Fittings with excessively bright lights aren't unusual, especially with halogen lamps. 118mm linear halogens can be had in a range of powers, but 80w is more than is ideal for a percentage of cases.

Power required depends on circumstances and desired light level. The following is just a rough starting point:

  • House very close to road 40w
  • Small terraced back garden 60w
  • Semi set back from road 100w
  • Large area 100w - 1kw, but going above 100w requires a fitting mounted higher than usual to get an increased light area.


Relay v triac

PIRs contain either a triac or a relay that switches the light. Relays make a quiet click, triacs are silent. Triacs are vulnerable to being killed by the various types of filament lamp, including halogen.

Sometimes one can identify which type they are by the max load rating on the label. Triacs typically provide for 100-200w, relays often 1kW or so.

Triacs can generally be replaced if you can solder, but most diyers don't do so.


Bulb type

Filament lamp

These occasionally arc over when they blow, killing triac type PIR units. They're low energy efficiency, but aren't on much time in total, so run cost is minimal.

Halogen

Halogen/xenon capsules inside GLS bulbs do the triac death trick much more frequently.

CFL

CFLs are unsuitable for PIRs unless specifically designed for them, and the unit is CFL compatible.

  • Ordinary CFLs have very low output in cold weather
  • with frequent switching they have short lives
  • since they're always in warming up mode the energy efficiency is not good in winter time.
  • CFLs and triac fittings are often incompatible (though you can use CFLs plus one small filament lamp)
  • Fittings that take only a CFL use one designed for the job; these are different to general purpose indoor CFLs.

PIRs that have a connection from the detector (not the lampholder) to neutral are CFL compatible. (This doesn't mean CFLs will work well in them of course.) Ones with no such connection, where the only neutral connection is from the bulbholder, are not CFL compatible.

LED

LEDs are a good choice where they provide enough light, though they have their pitfalls, mainly the poor colour of some white LEDs, and in some situations bulb theft risk. Some fittings come with non-replaceable LEDs; lifetime should be fine if it doesn't stay on too long.

  • 3 minutes 10x a day = 180 hours a year
    • 25,000 hour life would give 138 years life
  • 12 hours a night = 4380hrs/year
    • 25,000 hour life would give 5.7 years life

Linear halogen are very prone to excess brightness and severe glare. Tenants can be confused about which of the various similar replacement bulbs to get and don't know how to fit them, making them not always maintained in rental properties.

Some fittings only take one bulb type. Fittings designed for filament lamps can take a wide range, typically including filament, halogen capsule, CFL, LED, and even oddities like neon lamps.


Reflectors types

Bright metal reflectors send light in a specific direction, good for when you want a limited angle of illumination.

White reflectors send light every which way, and give a softer appearance. Good for short range illumination.

Grey reflectors have been spotted! Needless to say these are a daft choice. Painting them white improves light output per power used.

Many fittings send light skyward. That's energy you pay for wasted.

Many fittings have large areas of black interior, wasting light. This is true of a lot of Victorian coach style fittings. Painting white or lining with aluminium can improve light output per power in.

Coach style lights put a lot of light out behind them. Replacing or lining 1-3 rear glass panels with aluminium can reduce the bulb power needed. Fizzy drink cans provide free aluminium that's easy to work. Folding the ali over double conceals the painted side.


Glass v polycarbonate

Glass can be shattered by vandals, or with globes by tightening the fixing screws. Polycarbonate is tougher, but goes cloudy eventually and can be burnt by vandals.


Relamping

Fittings are a lot more likely to get relamped if its easy. Unless you need to for extra security, don't mount the fitting too high, and pick one that's easy to open and uses bulbs that are easy to find.

Steel screws in cast ali fittings are a recipe for corrosion, making relamping very difficult. Sometimes such screws can be replaced with a wire tie, or just lubricate them.


Number & position of lights

2 or more lights creates a much more pleasing appearance than just one. It also means there's still light when one bulb is dead.

Its normally better to place lights where they illuminate the visitor's face and don't illuminate the person indoors. Doing the opposite makes seeing people outside very difficult.


Controls

Some PIRs have no controls at all. IMLE its unwise to assume the presets are reasonable.


Range & position

The claimed detection range is normally for a person in full view walking across the field of vision. People walking toward the fitting aren't detected so easily.

Sun shining into a lamp's detector reduces detection ability. Some won't work usefully in this situation, some do.

Some fittings are sold specifically for short range detection. Useful where an entrance is close to the street.


Synchronising lamps

With a lot of fittings its easy to connect an external fitting that's also controlled by the internal PIR.

Its also possible to have 2 PIR fittings with the slave line commoned so that both light when either detects a visitor.


Extra modes

There's some tendency for PIRs to need adjusting years later, and they often have a walk test mode. Keep the instructions for when you need to do this.

Many also have a mode or 2 entered by operating the lightswitch repeatedly. Needless to say these modes are more often an annoyance than useful. Switching power off to the fitting for a couple of minutes resets them.


Dimming fittings

Some can cost thousands of pounds in wasted electricity. Running a fitting that keeps a filament or halogen lamp on all night in dimmed mode is an especially inefficient way to operate. There are much better ways to provide all night low level lighting if its what you want.

This issue doesn't apply to fittings using a separate 2nd low power bulb for dim light; in these neither bulb is dimmed.


Fitting syles

A few warrant specific functionality comments.

Coach lamp type fittings can benefit from replacing or lining some glass panels with ali, ditto the lid on black units. Panes can be filled to deliberately control where light goes, eg to avoid annoying a neighbour or bedroom.


Quality

Varies greatly. Some brands we like:

And some criticisms:

  • Argos: grey plastic reflector


Large areas

These are not effectively lit by a single light 7-8' up, no matter what its power. Options for large area lighting are:

  • Multiple fittings to spread light output
  • 1 or 2 fittings mounted very high. Sometimes one can be put in reach of a top floor window for relamping.


Nuisance

Outdoor lights and especially PIR lights should not shine directly onto road traffic. Local authorities can sometimes object where this occurs, mainly with high power fittings.

Light shining into a neighbouring house can be annoying.

Observatories are affected by fittings that light up the sky. Fittings that cut off all upward light are used in some areas.


Alternatives

The main alternatives to PIR fittings:

Photocell or timer with high energy efficieny light eg sodium

Beam break detectors are seldom used now. Not affected by movement outside of the beam.

Rundown timer switches can also be used, though they must be designed for outdoor use. Pressing the switch button gives a preset on time.


See also