(Re-)Installing and modifying a PIR module

A while back I ordered a cheap 230V PIR of AliExpress to control the light in our washing room because my housemates always forget to turn it back off. I came across a nice model that didn’t use the normal bulky (outdoor-ish) case but rather used a small external eye you could integrate in a ceiling. And because the room has a suspended system ceiling that was perfect.

When I got the device I opened it up, looked at it and thought “Yeahhhh, good enough”.👍 Then I quickly connected it and instead of drilling a whole I just sneaked the wire to the eye/PIR on the side of a panel and just suspended it there. And it worked fine! It turned on the light when you walked in and now most of the time the light was off. Win!😎

That was like a year ago. And although it did the job fine there where two issues.

  1. The eye wasn’t nicely integrated in the ceiling
  2. Although the advert said it has a duration (or “Time extension” like they call it) of 30 second it was more like 10 to 15 seconds…

And especially the second point was a bit of a pain. If you walked out for a sec of stood still (filling a machine) the light would turn off again. Pretty annoying and because it was a CFL (PL-S fitting) that’s not to great for the life of the bulb either.

So I took out my drill, marked a spot in the center of the panel next to the fixture and drilled some holes. Because obvious I didn’t had the right drill / a drill big enough. After which I filed it to size which was easy because it’s just drywall. After which I disconnected the cable, plugged the eye in the hole, ripped the cable from the connector because of crappy Chinese production, cursed because I didn’t had spare JST-SM connectors, broke the lip to hold the pin in because I couldn’t get it out, soldered the wire to the pin, glued the wire/pin in place and connected everything. Okay, that was easy.😂

Next, the timing issue. I expected this to be easy as well because I suspected it used the BIS0001 like 99,99% of all (China) PIR products do. Then it would be as simple as replacing R10 according to the formula and be done in 1 minute. So I disconnected the unit and opened it on my bench. Damn! They even scrapped of the numbering of the IC! Alright, with the datasheet of the BISS0001 in front I did a quick comparison. 16-pin chip, check! Power and ground on pins 7 and 8, uhmmm, nope. So it wasn’t a BISS0001 after all. So I started to reverse-engineer the schematic. It used a standard capacitive dropper connected to a LM78L05 to power the chip. Ground goes to pin 5 and 5V to pin 13. The relay is, as expected, driven by a SMD transistor and connected to pin 11.

Alright, enough info for now. I opened up Google and started looking for different IC’s for PIR sensors. Did a search for “PIR detector”, “PIR IC”, “PIR controller” and what not and came up with a small list of different IC’s. I opened the datasheets and started looking. Most used a different number of pins or power pins. But then I opened the datasheet of the LP8072 and win! It has 18-pins, uses pin 13 and 5 for power and ground and a relay output on pin 11. But the chip and datasheet are uhmm, Chinese… From it I figured a RC-circuit to TCI (pin 8) was responsible for the timing. To be sure I traced it out on the PCB and indeed, a 22k and a 4,7nF cap where connected to that pin.

The datasheet only has a table with  which R and C values correspond to what time. No formula… I assumed it most be a simple RC-circuit so the timing is linked to the RC-product. I entered the R and C values into Excel and made a plot. And yep, it looked pretty linear, especially up to the 300 seconds mark. So I told Excel to draw a linear trendline and to display the formula. And tadaaaaa, it came with Latex formula. I just wanted to swap the resistor and use the same capacitor. I re-wrote the formula Latex formula. I wanted around 2 minutes so that gives a resistor of 138k so I just grabbed a 120k. I de-soldered the 22k and replaced it with the 120k.

After that I closed the box and connected everything up again. And yes, it still turned on! So I started a stopwatch and timed it (though the glass of the door) and after 1 minute and 44 seconds it turned off. And if I recalculate it for the 120k I used: Latex formula. I would say that’s close enough!😁

And a picture of the modified PCB. Forgot to take one when it was on my desk so that explains the bad picture laying on the drywall. You can see the relays, the numberless IC and the big red dropper capacitor. The green cap in the front is the timing capacitor. The only (blue) metal-film resistor is the timing resistor I swapped.

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