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.

This application note from NXP shows how to drive these displays from logic level DC devices like microcontrollers.

Looking in my spare parts bin I found some old CD4049 hex inverters that I made into a simple RC based oscillator.

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!

We will need another way to generate the AC voltage! Metacollin from the MacroFab Slack Channel suggested using a low power comparator in a relaxation oscillator configuration. This will get us to around 1uA in current draw if we use a MAX9019 dual comparator chip.

I am also looking for a way to measure currents that low. My Siglent SDM3045x is a bit out of its league at this point!

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.

Ok, this is a project I talked about way back on Episode 237 of the MacroFab Engineering Podcast.

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:

• AEM10941
  • This is a Solar Energy Harvesting IC
• DSF505Q6R0JBG
  • Super Caps
  • 5F Capacitance!
• Solar Panel  AM-1816CA
  • 84µA at peak power
  • This is specced at 200lx
• LTC2956
  • 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.

Github repo for the project.