Well, the device I mentioned at the beginning of this thread chewed its way through another pair of 9V batteries in just a few months, so rather than keep buying batteries I suggested we try one of the solar-powered ones that are common on eBay. We got one of these:
A quick mini-review:
Pros:- Very sensitive to movement through quite a wide arc of view.
- Sturdy spike that locates it securely in the ground.
Cons:- Sensor's arc of view is needless wide in the vertical axis for something monitoring near ground-level (so triggered by birds in nearby trees, etc.).
- Ni-MH batteries are not user-accessible, nor externally rechargeable.
- Not properly weatherproof, moisture easily gets inside.
- Makes a short, loud beeping noise every time it triggers (which wasn't mentioned in
any of the information on the eBay seller's page).
Now, about that last point: it's been installed in my mother's garden for a couple of weeks now, and because it's so sensitive and is triggered so frequently, she's just about fed up with the noise and wanted me to see if I could do anything to silence it. So I took it apart to see what could be done to modify it's behaviour.
This one is a lot more sensibly-designed (at least from an electronics point of view) than the previous one. It uses an integrated PIR sensor with a digital output, and the whole thing is controlled by a microcontroller (appears to be a PIC 12F510 clone - pin-out matches, and is marked '510') rather than some off-the-shelf dedicated function PIR controller chip. It was quite easy to reverse-engineer the circuit; schematic attached below.
An interesting point is that the circuit includes functionality to flash an external LED (or perhaps multiple LEDs) while it is triggered, but the manufacturer did not equip this particular device with that option. So perhaps this board is generic to more than one model. However, given the propensity for cost-cutting on cheapo electronic consumer devices like this, it's strange they still populated the board with the transistor and resistors for this feature.
Anyway, back to my mission: silence the damn thing!
I initially thought - given the external design of two 'speakers' on the front - that perhaps there was a separate buzzer being used for the audible beep, and a second for the ultrasonic, and I could simply disconnect the audible buzzer. But, teardown revealed that the second 'speaker' on the front is fake. So, the same transducer is obviously being used for the audible and ultrasonic output.
Analysing the output drive circuitry with a 'scope revealed the following signals: upon triggering it outputs a 5.1 kHz square wave for 2 secs, then 2 secs each of 26, 35, 45, 54, and 63 kHz square waves. The latter ultrasonic sequence repeats for as long as the device remains triggered.
So, given that the 5.1 kHz beep is integral to the device's output, how can I go about eliminating this without affecting the rest of the output?The only thing I can think of is a high-pass filter, but I'm not sure that something simple like a passive RC filter would work effectively. If I had, for example, a 20 kHz high-pass RC filter, I'm not convinced the shallow roll-off would attenuate the voltage level enough at 5.1 kHz to eliminate that part of the output. Also, I'm not certain to what level I need to attenuate the signal to nullify the drive transistor's switching. Would it just need to be below the typical base-emitter voltage of 0.6V?
Are there any better, simple solutions? I suspect there are more sophisticated high-pass filter setups (i.e. multiple-order and/or active) that would work great, but I don't want to start adding a dozen passives or op-amps, etc.
A crazy thought I had was to replace the existing microcontroller with something pin-compatible and write my own code for it - seems pretty simple, just an interrupt for the PIR trigger and a timer. Don't think I'm quite prepared to go to those lengths, though...