Voltage and Current measurements with the INA219 Sensor

Now that the Distributed Power Node (DPN) project is progressing on the software side I thought I better look into how the power management sensor will work for measuring the health of the battery. I got one of the Adafruit INA219 break-out boards back in December and had not had a chance to try it yet.

I wired it up exactly like the guide on Adafruit site and it worked with the library and sample code fine. I modified the code to sample every minute instead of every 2 seconds and wrote the output to the serial monitor in CSV format so I could cut and paste it into a text file for use with Excel. I connected a 12 volt 8.5 Ah SLA battery to a 400 watt power inverter running a 15 watt CFL and monitored the voltage and current for 2 hours or 120 samples. This is the results as plotted by Excel.

I was not happy with the current measurement since they varied by 500 ma from sample to sample. The voltage was not as bad. I was going to do some sample averaging in code and then thought that perhaps the power inverter is a "noisy" load and was causing the up and down readings. I decided to re-test with a static load of just a power resistor. I had just connected the power resistor load and had a couple of samples on the screen that seems much more stable and I reach over to check the heating on the load resistor and accidentally shorted the battery at the load. I notice the readings were way off now but it was still running so I powered it all down and checked everything. There was a slight smell of burnt resistor in the air so I checked the shunt resistor on the INA219 and it was open. The strange thing though was that the bus voltage was reading 32 volts constantly now. I could see the current reading being wrong since the shunt was open but the bus voltage is not dependent on the shunt. I made a temporary shunt with wire just to test it again and the current reading was still bad along with the bad voltage measurement. I think the large transient must have destroyed the op amp circuits on the chip but the I2C interface components survived because the Arduino can still talk to the board. This sensor design does provide good isolation from the MCU so I think the moral of the story is that if the battery would have been fused, it may have survived. I will need to study protection methods for the DC outputs anyway so this was a $9.95 plus shipping lesson!

2M TX PCB

Finished populating the 2M TX PCB, and found that the PICAXE is loading the oscillator in some way that kills it's oscillation with a common power supply but works with separate supplies. I plan to try using separate regulators and see if that works. I can have 6 volts for the transmitter and 5 for the MCU

Arduino LCD Rotary Encoder Menu

Development of a menu system has been started for use on the Arduino based Distributed Power Node (DPN). A simple method of using just a single rotary encoder with a push switch is the direction. The current idea is a single level menu using press and hold to change the value of the menu options. I got that working but I am having performance issues in updates even though it is using interrupts.

2 Meter TX PCB cut

I cut out the 2 Meter TX PCBs and began to populate one this weekend. It is a bit tighter than I hoped, but should work.

2 Meter TX PCB

Spent some time making a PCB for the small 2 Meter crystal oscillator transmitter. I have added a PICAXE to modulate it now. We will see how these boards work out.

Simple 2 Meter crystal transmitter

Using some inexpensive computer crystals from China at less than $2 each, I built up the basic oscillator portion of this Radio Locator Beacon and got surprisingly good results. I want a light weight 2 meter transmitter for a simple balloon project (cheap because it may get lost!). The oscillator ran fine from 5 to 6 volts and I keyed it with an Arduino and it remained stable. Need to couple a simple whip antenna and power and I will be set. I may try to make a PCB for the design.

More Progress on Tiny Packet Network (TPN) Protocol

Moving forward with the Tiny Packet Network (TPN) Protocol. Using the three reference platforms I setup a master and two slaves. The new code uses the protocol in a command response mode with the hybrid digipeating ability. The slave code can now respond to commands to change the color of the NeoPixel on the board. Next the query or answer back function will need to be implemented to read the analog pot and the command timer status. Once the protocol is proved out more it will be document better than the current rough notes.