Alter Existing PIR Circuit

[OrbitalD]

Hi,

I have a board with an existing PIR circuit designed for close operation. This circuit was used as a handwave signal.

I wondering how I would alter this circuit to work for whole room motion sensing using the same PIR sensor

http://farm1.static.flickr.com/200/467631191_6e01d24adb_b.jpg

Any ideas are greatly appreciated.

David
OrbitalD

 

[ScottyUK]

Much of the sensing behaviour is determined by the lens design. For what these things cost you would almost certainly be better off using one intended for security systems which are designed from the outset to cover rooms. I can find a cheap one for £5 ($10 US) and a top quality one costs about £20 ($40 US). The cost of the expensive one equates to less than an hour of devlopment time at commercial rates.


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Sometimes I only open my mouth to swap feet...

 

[OrbitalD]

Hi Scotty and thanks for the advice.

I'm familiar with the off-the-shelf offerings and would agree that it would be simpler and cheaper for most applications.

However, I working on a non-profit interactive art project that will use between 500-1000 sensors and these boards are available relatively inexpensively, much cheaper than the $10 per sensor.

So I’m going to try if I can to modify them to work although I’m not exactly sure that they will.

And yes, lens design is important and we will need to consider that as well.

Thanks again,

David
OrbitalD

 

[ScottyUK]

At that volume it might be worth seeing if one of the smaller manufacturers would do a 'special' for you. As long as you (or they) can use a standard lens and pyrometer (optical sensor) and modify the processing firmware to make it behave the way you would like, that would seem reasonable. Once you move into the realm of custom lenses the cost of a new mould tool to produce them will make your hair curl.

You could also check out the OEM discounts available: my prices were single quantity retail - for 1000 modules I would expect to see that virtually halve.


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Sometimes I only open my mouth to swap feet...

 

[OrbitalD]

These boards are available at under $1 each and they come with a 1W audio amp as well, which we need. This is a super deal for us if we can make it work and would leave more money for other important aspects of the project.

We will start looking around at other options if we are sure these wont work but the price is right and the idea of re-using these reject parts goes with the environmental thrust of the art installation.

Thanks again,

David

 

[logbook]

The design of this board is very "unpleasant". The standard TLC272 has an input offset spec of ±10mV. The noise gain of the first stage is double what it needs to be because of unintentional poistive feedback caused by the bias circuit. Unless the opamp is selected the first stage will hit its output limit in a high proportion of circuits. selected versions of the opmp are available. The best quality "B grade" parts are needed to get 2mV offset. If the components were rearranged a little the performance would be improved at no additional cost per unit.

 

[logbook]

Interestingly I just found some of these boards advertised on ebay, item 270114454004. 10 for £9.99 starting price, no bids yet.

Nice photos and the same circuit as given by the original poster, plus some extra psu stuff. They are not using the selected grade of opamp.

Of course if they biassed the opamp to 2V rather than 2.5V nominally they would have got a bit more range since these opamps don't like going above about 3.5V on the output.

 

[OrbitalD]

Hi Logbook,

Original poster here:

When you say that that "they are not using the selected grade of opamp" exactly what do you mean?

I'm kind of new to Op-Amps.

Thanks,

David

 

[logbook]

The opamp comes in three “selection grades”. The opamp manufacturer tests them and the best ones are called “B”, the next best “A” and the rest are unmarked. On the Ebay pictures you can read the part number on the chip. It says TLC272C… . The better parts would say TLC272BC … for example.

Beware of the “A” and “B” grading in general. It used to be the “A” grades were better than “B” so you have to check for each opamp which is the best grade part.

Having seen the design details, I am not surprised that these types of products are so unreliable. I got a complete PIR sensor light and it only worked for a few hours before failing. It was of course ridiculously cheap.