Sensor Circuits

Broken Lawn Light

The solar panel charged lawn lights can be unreliable. I had one where maybe the factory forgot to seal the solar panel, which got damaged by moisture, perhaps damaging other components. I took the enclosure apart and found a 5252F IC, which is a "Solar LED Driver" ASIC. It needs an inductor to step up the voltage from a 1.2V Ni-MH. The switching frequency determined by inductor size also seems to limit current to the LEDs because those have no resistors that I could see.

I desoldered it and moved it to a new board, following the schematic in the QX5252 datasheet. From what I can tell, the battery won't charge over 1V... a fault or maybe safety margin. The solar panel is used as a light sensor, so the LEDs (3 white) are off during daylight. It turns on in dim light, perhaps compensating for light from the LEDs. Oddly enough it also shuts off in complete darkness... maybe it's to remain off while still in packaging... or my chip was damaged.

A little breakout board sketch + datasheet example circuit. Supplied inductor was 120uH if I'm reading the code right (brown red brown... it's hard to tell with coloured backing).

Because the lawn light had these fancy glass domes I didn't just want to throw it out, so I made a circuit to power it from 2xAA (2x1.5V) batteries. An LDR light sensor switches on a BC558 PNP transistor. Judging by the power draw, the (2500 mAh) batteries might last a few months.

Radio Stuff

I built this HEX3653 kit, but it's pretty boring. Awful clicky buttons makes for such a sterile user experience. *Click Clack* Was it the right button? Did the volume go up? Did it scan? Do I need to hold it? Which direction does it go? How far? A turning knob is very self-evident in comparison. Tempted to make a crystal radio from scratch instead, and maybe even attempt to make a diode for it. Though I might use an amplifier for speaker output. I suppose I could attempt to make a diode using different materials (needle(whisker) + oxidized things) then just measure with my component tester or multimeter to see if there's any diode'ing going on...

Random tip: Trouble getting AA batteries out of tight battery holders? Put a piece of flat present string around (behind) each battery when putting them in.

Daylight Heat Alarm

I might need an alarm for e.g. greenhouse or enclosed glass veranda which can easily reach 40-50°C during the summer if accidentally left shut and unattended. My circuit idea wasn't particularly original it seems. A comparator will turn on (drive) a buzzer/beeper if heat sensor value is over pot value. I ran into trouble using an LM311P so I switched to the LM358P (simpler, with two comparators).

«WIP» I wanted to use as few resistor values as possible for no particular reason, and choose 10K and 260 which I got baggies of. Looking at the NTC 10K curves it seems 40°C is at 6KΩ, 32 at 8KΩ. The warning should be at 37+ so I might need to adjust the resistors a bit. 5.5K + 2K pot would've been more suitable I guess.

Initial schematic. The pot and sensor are part of voltage dividers. Switching the whole circuit on with a light sensor could to save a lot of power as it's not needed during the night (I suppose the extra circuit does add a small cost when it's all ON). If there's a malfunction/short I guess there's less risk of waking up the neighbours in the middle of the night.

I've added a 100K from the output (green) back to input (black)... what I think it does is... when output goes high, it adds bias to the input making it harder to fluctuate on threshold noise (a Schmitt Trigger?). I don't know if 100K is ideal here. I've also added an jumper bypassing my small offset so I can test using e.g. body heat. Perhaps I didn't need to snake that red line around like that.

Total power draw @ 5V
 0.01 mA in darkness.
 0.9~ mA in daylight.
10.~~ mA alarmanizing.

I found these Christmas Ornament Hooks in a dollar store and bought them on a whim. Turns out they're reasonably conductive, sturdy but bendable, and take solder. Good thickness for breadboards and freeform PCBs. Brass? I dunno. Same gold tone where cut, so solid.

Random tip: Cable mess in a box? Maybe a peg board will work better? Maybe. Jury is out. I'm hoping it'll reduce mixing and tangling.

My old Hanimex 35hs camera makes a sudden appearance. Its aesthetics reminds me of going into electronics stores in the '80s, seeing freestyles (what be called portable cassette players here)... imagining all the features. So many buttons and dials. All gone now, replaced by dark pattern user interfaces, encased in black monoliths. *shakes cane at the sky*

Art by Arne Niklas Jansson