A Passive Infra-Red (PIR) sensor is an electronic device commonly used in security lighting, and burglar alarm systems. A PIR sensor is a motion detector which detects the heat (infrared) emitted naturally by humans and animals. When a person in the field of vision of the sensor moves, the sensor detects a sudden change in infrared energy and the sensor is triggered (activated).
Passive Infrared Detectors
Fresnel Lens on the front of a typical PIR sensor
At the front of a PIR sensor unit is a fresnel lens (pictured above). This special kind of lens is used to gather light from a very wide field of view and focus it directly onto a passive infrared sensor (as shown below).
PIR sensor
Some electronics inside the PIR sensor unit then decide whether the infrared light detected is from a warm moving body, and if so, triggers a switch.
Uses for PIR Sensors
PIR sensor with integrated spotlight
The commonest use for a PIR sensor is automatic security lighting. Whenever a suitably large (and therefore probably human) warm body moves in the field of view of the sensor, a floodlight is switched on automatically and left on for a fixed period of time - typically 30-90 seconds *. This can be used to deter burglars as well as providing lighting when you arrive home at night.
* The duration the light is left on for can usually be set by the user.
Such PIR security lighting systems are available from a wide range of sources from as little as £10.00 up in the UK.
PIR Sensors and Renewable Energy
The vast majority of commercial PIR sensor products are built to run on 240 VAC mains electricity. This makes them unsuitable for most renewable energy powered systems such as motion activated CCTV systems in remote locations. Although a power inverter can be connected to the RE system battery bank to obtain the necessary voltage, using it for 24 hours per day for such as small load would be a huge waste of energy.
12 Volt DC powered PIR sensor
Fortunately 12 Volt DC powered PIR sensor units are available. These (such as this 12V PIR sensor available in our REUK Shop and pictured above) can be connected directly to a 12V battery or battery bank, and used to trigger a Relay when motion is detected. The relay will switch on/off a light or any other 12VDC or even 240VAC device if required.
If a power inverter is necessary to power a 240 Volt device switched on by the PIR sensor, then the inverter can also be turned on as and when required rather than needing to leave it on 24 hours per day. This saves a considerable amount of energy.
This PIR sensor will run off 10-14 Volts DC electricity and draws a current of under 10mA.
PIR Sensor Timer Circuit
12 Volt PIR Sensors do not usually include a built-in timer. Therefore, it is necessary to put together a very simple circuit so that whatever device is connected to the PIR sensor will stay switched on for the desired length of time.
PIR sensor timer circuit - turn on a device and leave it on for a time determined by the values of C and R
The circuit shown above will turn on a floodlight or other device for a time in seconds approximated by the value of the capacitor C (meaured in microFarads) multiplied by the resistor R (measured in megaOhms) multiplied by 0.75.
For example, if R = 1M and C = 22uF, the floodlight would remain lit for around 16 seconds after the PIR sensor switched off.
Weaknesses of PIR sensors
In order for a PIR sensor to work well most of the time, they are designed with certain limitations. A PIR sensor cannot detect a stationary or very slowly moving body - if the sensor was set to the required sensitivity, it would be activated by the cooling of a nearby wall in the evening, or by very small animals. Similarly, if someone walks straight towards a PIR sensor, it will not detect them until they are very close by.
PIR sensors are temperature sensitive - they work optimally at ambient air temperatures of around 15-20 degrees Celcius. If the temperature is over 30 degrees, the field of view narrows and the sensor will be less sensitive. Alternatively, if the temperature is below 15 degrees, the field of view widens and smaller or more distant objects will activate the sensor.As we have previously discussed in our article 12 Volt Programmable Timer Switch, it is not possible near impossible to purchase a programmable timer which can be powered directly from a battery bank used to switch on and off connected devices.
Programmable Thermostat
Most domestic central heating systems come complete with a thermostat - a device used to switch on the heating when the temperature in a room is below a particular value. During the last 20 years, these devices have become more intelligent - enabling the user to program the times of the day the heating system is to be active. Domestic thermostats have also become less power hungry, and are now often powered by a couple of AA batteries.
Programmable thermostat can be used as a low voltage timer
It is therefore very simple to use such a programmable thermostat (pictured above) to switch items other than central heating on and off at different times of the day - for example, security lighting, irrigation pumps, and so on.
Using a Thermostat as a Programmable Timer
Within the thermostat is a relay which is only triggered when the ambient air temperature is below the target temperature programmed into it. Therefore, if the thermostat is programmed with a target temperature of say 30 degrees Celcius (here in the UK), the relay will be remain closed (i.e. ON) since we almost never get to 30 degrees here. We can then use the programmable timer to set the times through the day at which the device to be controlled should be on or off.
Note that many modern thermostats can also be switched to a cooling mode with the relay triggered only when the temperature is above a target value. Such thermostats can therefore also be used in refrigeration and air-conditioning systems to switch on cooling devices. A typical thermostat has a programmable range from 5 to 30 degrees Celcius, therefore if you live somewhere where temperatures of 30 degrees are likely, the cooling mode should instead be used with a target of 5 degrees Celcius. Therefore the relay will remain triggered as long as the temperature remains above 5 degrees.
Once that is set up, all that remains is to follow the instructions provided with the thermostat to set the on/off times. Some thermostats repeat the same programme every day, others can have each of the seven days of the week programmed differently giving the user more control.
Connecting to the Thermostat
At the back of most thermostats is a piece of terminal strip into which wires from the device to be turned on are connected.
Relay connections for a programmable thermostat
The image above shows the rear of a typical programmable thermostat with three connections: NO (normally open), COM (common), and NC (normally closed). One wire of the external circuit is always connected to the common terminal. The second wire must either be connected to the normally closed or the normally open terminal. Normally open is used if the external circuit is to be ON unless the thermostat relay is triggered. Normally closed is used if the external circuit is ON only if the thermostat relay is triggered. Usually the normally closed terminal is used so that when the thermostat 'turns on' the relay, it turns on the external device.
Buying a Suitable Programmable Thermostat
When choosing a programmable thermostat is must be battery powered (so that it does not need to be connected to a mains supply). If it uses 2 AA batteries as is common, it is possible to use a couple of rechargeable AA batteries or use a voltage regulator to connect it directly to a renewable system battery bank.
One key thing to look out for is the maximum current for the relay. Values from 3 to 10 Amps are typical in cheap domestic thermostats, which means that 36 to 120 Watt 12 Volt devices can be switched on/off using them.
On ebay.co.uk there are many suitable new and used thermostats going for as little as £7 plus £3 for delivery. It is usually better to go for a second-hand good quality unit, rather than a cheap Chinese import as you will get more for your money (and it's always good to recycle and reuse when possible).When Ray Driver asked someone to write this article several months ago, I immediately volunteered. As always, I gave myself plenty of time, which, of course, means I started yesterday. Now, with the deadline fast approaching, I had to come up with something to fill this space, 500 words to be exact.
What could I write?
Oh, I thought, the JBA’s YLS section is sponsoring a happy hour Sept. 15, at River City Brewing Co. from 5:30 to 7:30 p.m. The results for the July Bar Exam are available that afternoon, so the idea is for all members of the JBA to celebrate with those that are fortunate (or unfortunate) enough to pass (and for those of you that don’t pass, you’re still welcome to show up but you’ll have to pay full price for your drinks). There will be drink specials and hors d’oeuvres. See, that was about 120 words right there. A great start, only 340 words left.
Seriously though, I had to first decide on a tone for the article: serious, sarcastic, irreverent, paternalistic? In honor of Hank Coxe, I chose serious irreverence with a hint of sarcastic paternalism.
OK. I’d chosen a tone, now I needed a topic. Some of my ideas really weren’t that bad. Most would read an article about What They Don’t Teach You in Law School, others should be ordered to read an article about What They Tried to Teach You in Law School. Still others may simply want some answers to those age-old questions: Why are beagles so cute and why judges aren’t? Why is it so hot? Why would the FDOT ever raise the Main Street Bridge during morning, lunch, or afternoon rush hour? Do you offer something IN evidence, INTO evidence, or does it even matter? Who changes the payout amounts on the lottery billboards? Learning to merge: the causes of traffic on JTB? Yes, those would have been great articles.
Suddenly, I had an even better idea.
Billing, as we all know, is the cornerstone of an associate’s life (2,000 hours and the partners learn your name; 2,200 and your spouse forgets it; more than that and your just plain lying).
My idea: write this article about something for which I could also bill — the proverbial two birds with one stone.
Alright, I thought, what case could I write this article about and bill for at the same time. The divorce case where the wife ran away with her husband’s best friend (after the husband billed a record 2,500 hours) or the case about the abusive tax shelter? I chose the latter because tax law is simply more interesting (oh yes, and my motion to compel in that case was due the next day). Here are some of my notes on that motion:
Winston (W125-2526)
Atty notes (KBC) 08/26/03
If the government is relying on financial statements prepared in conjunction with allegedly privileged third-party documents, does that waive the accountant/client privilege for those documents? Can the government, as a party, waive a non-party’s privilege?
Under the Patriot Act, only John Ashcroft can answer this.
Note to file: write opinion letter to attorney general Ashcroft.
[Time Entry: Find time sheet (0.1); consider issues for motion to compel (0.7); discuss this article, I mean case, with senior partner (0.4); write this time entry (0.1); return time sheet to place where I’ll again have to search for it (0.1)]
While working on this tax case (and this article), I went to get my afternoon Diet Dr. Pepper only to discover that Ashcroft had taken the last one. No, seriously, I was told that associates are required to “bill” 100 non-billable hours each year. Holy smokes, that was great news.
My first thought: would golf qualify? My second thought: I could actually spend time on this article, and not try to simultaneously bill and non-bill. This realization eliminated my unspoken, but very real, apprehension that my plan violated The Florida Bar’s double billing rule: writing my motion to compel as a pleading (which is billable) and writing my motion to compel as an article (which is non-billable). If one were billable and the other non-billable but required, is that “double billing?”
I was back to thinking about what to write. Remember, I only had 500 words and really want something that was worth reading. Hey, I thought, what about the Internet? I could go online and find an idea for this article.
(One non-billable hour later) Nope, but the Red Sox won last night. The online satirical newspaper, The Onion, was funny: George Bush Diagnosed with Attention-to-Deficit Disorder. That’s good. And, Jennifer Lopez is dating someone. My Google search: “last minute articles, ideas or stories about the Jacksonville Bar Association, Young Lawyers Section” produced only an Expedia.com pop-up ad.
In the end, I still hadn’t found something to write about, and I was out $550 for two round-trip tickets to Chicago. The article was starting to cost me money. I had to end it.
In conclusion, and with only 97
non-billable hours remaining and
my magic number of 500 in the rearview mirror, it’s safe to say that we all could learn something from this experience . . . and, if any of you know what that is, please let everyone else know. I guess what I am really trying to say is that my 500 words are finally up.
http://www.jaxdailyrecord.com/showstory.php?Story_id=39412
