I decided on two Ladybug Blue models – one with pumps and one without. The one without pump measures the pH and EC. It doesn’t require a lot of current. I’ve decided to make a PCB I’m calling the Ladybug Blue Lite that runs off of a battery. This way, the pH and EC can be measured wherever there is a container of nutrient bath.
But which battery?
The goal of this post is to pick a battery topology to power remote measuring of the pH and EC of a nutrient bath. Results can be read on a smartphone.
Thanks to Those That Went Before
I am very grateful for all I have learned from Chris Gammell and Contextual Electronics. His mentorship style has given me the confidence to keep at electronics and embedded systems programming even though I have no background in these fields. I look back a year and get excited thinking about how much I have learned and been able to accomplish from designing a PCB to soldering the chips on the board to figuring out expectations on how the circuits should work.
In Chris’s Contextual Electronics course, he mentored us through two topics:
- battery topologies to drive sensors + microntrollers + 5V motors
- buck, boost, and buck-boost converters
LDO and DC/DC Included
The DC/DC converter only reduces the power consumption used by the radio, it does not affect the powerused by the Flash, System, and Peripheral.
Enabling the DC/DC converter will not turn it on, but set it in a state where it automatically gets turned on when the radio is enabled and goes off again when the radio gets disabled. This is done to avoid wasting power running the DC/DC in between the radio events where current consumption is too low.
Not Designed for LiPo or Lithium Ion
Low End Voltage Requirements
- VDD to the ICs: There are only two chips to look at – the nRF51822 and the MCP6244 op amp. The nRF51822’s minimum supply voltage is 1.8 (assuming LDO conversion). The minimum supply voltage needed to power the MCP6244 is 1.4V.
- there are three signals the rails of the op amp need to accommodate:
- ph: +/- 415 mV
- Wien Bridge: ~ +/- 550mV
- Gain Loop: the amplitude of the gain loop is controlled by pre-scaling the Wien Bridge wave form using a voltage divider. Typically, the max gain for the scenarios the Ladybug was designed for (discussed in earlier posts) is 6. If the Wien Bridge is shrunk to +/-150mV, the max amplitude of the gain loop is +/- 900mV.
- using the on-board DC/DC (buck) converter requires a minimum of 2.1V.
I decided to go with what is used with the nRF51 DK – a CR2032 coin cell battery.
Looking at the data sheet for the CR2032 that came with the nRF51 DK (the Energizer CR2032), the voltage starts off at 3V:
The CR2032 provides more than enough current at 240mAh.
Now I’ll update the design and also put together a circuit that checks the voltage level. It certainly rings true to me that peeling an onion exposes a bag of more onions..
Thanks for reading this far. I hope you find many things to smile about.