In trying to reduce the power consumption of the Cat Feeder Unreminder, I am going to explore using some really low power comparators to build the AC drive voltage I need to run the TN LCD segments.
The MAX9019 is a dual package comparator fits the bill in the power requirements.
In the switching frequencies I am using (50-100Hz) it should only need a supply current of ~1uA.
We setup the first comparator to generate a square wave and then the second comparator in the package as an inverter.
The design breadboarded up. It drives the screen!
This is well under the resolution of my Siglent SDM3045X. Will have to wait to get the right equipment to measure the actual current the circuit is drawing.
Here are the two output drive signals on the scope.
Next step to work on is the power retention system. The largest draw on the system right now is the leakage on the super capacitors! I found some super capacitors made by Eaton, HSL0814-3R8106-R, that specialize in having low leakage. Slightly higher ESR then some super capacitors but that isn’t that important for this project.
For the Cat Feeder Unreminder, I am going to pivot from using LEDs to indicate the “feeding” status and use a TN-Effect Display instead. These displays are much lower power then illuminated LEDs but they require slightly more circuitry to drive.
TN LCDs run off low AC voltage from around 3VAC to 6VAC depending on the screen. The one I picked, Varitronix‘s VI-422-DP-RC-S, operates over this range. They are not particular picky about the quality of AC voltage, just that it has zero DC offset. Driving the displays with square waves seems common. Anything north of 50Hz should do as well.
I used a 0.1uF capacitor and a 100K ohm resistor. Should get the oscillator to jiggle around 70Hz. Then I fed the output of the oscillator into another inverter on the CD4049. This gives me two square waves that are out of phase which will give us the AC voltage we need!
This works great! However the downside is this oscillator uses ~120uA at 3.3V without driving the Display. The display takes sub 5uA to drive so this is a big part of the power budget!
For the Cat Feeder Unreminder, I originally wanted to run LED indicators but I think it would be cool to run a reflective LCD display like on solar powered calculators.
For a display I was looking at Varitronix‘s VI-422-DP-RC-S. I was thinking I can hardwire the display to say FEED. What is weird with these displays is they run on alternating current (AC).
The solar power subsystem provides 3.3VDC which won’t work for activating the segments. Google searching around shows that these displays run on AC square waves. Initial thoughts are to make a push/pull transistor circuit that can drive and source high and the low side can sink giving us a 3.3V AC drive source.
But after chatting with some folks I think using a 4049 Hex Inverter like this will work great.
Received the 3AAEM10941CPCX10 evaluation kit for the AEM10941 solar harvesting chip today.
Its possible it won’t light up right away. It takes sometime for the super capacitor to charge up. You can charge up the super capacitor with a power supply set to 3.3V and current limited to around 10-20mA. Make sure to not reverse bias the charging!
The AEM10941 is a solar harvesting IC. It handles a small solar panel and charges either a lithium battery or a super capacitor. I ordered a evaluation board (part number: 3AAEM10941CPCX10) to test it out for the Cat Feeder Unreminder but I went ahead and made a footprint for it in Eagle. The evaluation board comes with a couple solar panels and some super capacitors to mess around with.
Its a small electronic device that makes sure you don’t overfeed your cat by feeding more often then needed. Low power consumption with no need of changing batteries or external power sources. Solar power? Basically a resettable egg timer that doesn’t need batteries cause if the batteries die then you won’t be reminded to feed the cat!
User interface should be simple. One button to reset the timer, then a LED that lights up when you are ok to feed the beasties and another LED that lets you know the system is working correctly. Maybe one led that is turned on when its not time to feed and then turns off when time to feed. Pressing the button resets and turns the LED back on? Verifying the circuit is still powered? Prevents hungry cats at least.
Simple BOM so far:
This is a Solar Energy Harvesting IC
Solar Panel AM-1816CA
84µA at peak power
This is specced at 200lx
Low power, configurable timer
Napkin math on power requirements. The LTC2956 draws 0.8μA. LED at 40uA. Total draw when Cat Feed Indicator is on is 40.8μA.
5F x (4.5V-3.6V) / 0.0000608 = 74,013 seconds -> 20 and a half hours!
So we have enough power from the fully charged Super Cap to run everything for almost a day. That is good. Should bump up the capacitance just to get a full day.