In our last adventure, we lit the IV-11 with benchtop power supplies. Nice to show that we know how it works. Now, it’s time to build a project around it. After some thought about what I would like to display with a single digit seven segment display, I chose a thermometer. Taking up a wall outlet doesn’t make much sense either, so it would have to be battery powered and fairly energy efficient. To drive the IV-11, I’d need a 25V supply as well. Fortunately, in my parts bin, I had a couple of tricks up my sleeve. First, I had a handy $2 boost supply from china. I also had a spare Murata 7805 replacement, which provides a drop in replacement for the classic LM7805 linear regulator, but at a higher efficiency (and without the strict need for the off board filter caps)!
The 5V rail would provide power for several things. Most obviously, it would supply power to the microprocessor (an Atmega 328P) and sensor (a DHT11 module from OSEPP, picked up at my local Fry’s — more on that in a later post). Additionally, the 5V rail would provide power for the filament — this would be more efficient than some sort of resistor divider down from 25V.
The final part of the power supply would be the soft power module. It’s a circuit I’ve used before, based off the Dave Jones soft power switch.
My modification omits the hardware “power off” functionality — the button latches the power supply on, and the microcontroller has the ability to switch the circuit off by pulling a GPIO pin low.
Using this power switch, the system can be turned on by a user pressing the power button, run its temperature measurement routine, and shut itself off, using absolutely no standby power.
I had originally hoped that I could use a couple of lithium coin cell batteries to power this, but I realized that I’d be taking a couple hundred milliamps at full draw. Clearly, this wasn’t in the cards for lithium batteries, which have high internal resistance. Nonetheless, a single 9V battery ended up being a reasonable compromise.