Passive (Pyroelectric) Infrared Motion Sensor Design? Answered
I am trying to build a pyroelectric infrared sensor using a KC778B from scratch. I have gotten my system to work using PIR modules, but because I am using 100 of them in my project, it ends up costing almost $900 more to do than building it from scratch (even including paying a machine to assemble the extra components).
However, the prototype I built doesn't work. It worked when I did it using a PIR module, so the LED charging circuit is fine -- but the sensor block is not. I went through the datasheet carefully and didn't see anything particularly odd, and scoping the pins also showed no noise to speak of. I found an example online of how to use the chip, and their system was very similar to mine, except with extra features that I had eliminated. Basically, the behavior is that it does not output a voltage indicating detected motion for a random appearing length of time, and then outputs only detection events. The attached LED light is off for most of the time, and then turns on, never to turn off again. Perhaps the sensitivity is too low, perhaps the controller and PIR sensor element aren't compatible, I just have no idea.
The PIR sensor is the PIR_D203S. It says it requires 3-15V to run and the drain reference on the PIR controller chip is only 2.5V, so I thought that could be the problem. However, connecting D directly to 5VDC did not fix anything. I just ordered two PIR sensors that say they go to 2VDC, but I'm not optimistic that this is the problem. I also did not include the 1k resistor on the output to the LED control transistor; could that cause the system to stick on or off? I did also not bypass the power supply, but I'm seeing it freeze, not flicker, so again I am doubtful. I really cannot think of any other places where this could fail -- perhaps the offset voltage on the PIR sensor, maybe the frequency of the turn off circuit -- but everywhere I look the datasheets imply that my circuit should be fine as is.
I'm kind of doubtful that instructables can help, but I have no where else to turn. These things are apparently too cheap for anyone to bother building them themselves, and there are only even one or two controller chips in existence.