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

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.

The Battery

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
During one iteration of the power section of the Ladybug Blue Lite I designed for a LiPo battery (note: I found Adafruit’s description of Lithium ion polymer and Lithium ion batteries very helpful).  The Ladybug Blue Lite included the ability to recharge the battery through a USB micro port.

LDO and DC/DC Included

Then I took a closer look at the nRF51822’s data sheet (link to download the data sheet).  And lookey-lookey!
 
Pasted Image 7 29 15 1 48 PM
 
According to Table 20 in the data sheet (link to download the data sheet):
VDDValuesFornRF51822
A battery can connect directly to the nRF51822 then go through either an LDO or Buck-Boost converter provided by the nRF51822.
 As noted in the nRF51822 product specification (link to download the data sheet):
The nRF51 DC/DC buck converter transforms battery voltage to lower internal voltage with minimal powerloss. The converted voltage is then available for the linear regulator input. The DC/DC converter can be disabled when the supply voltage drops to the lower limit of the voltage range so the LDO can be used for low supply voltages. When enabled, the DC/DC converter operation is automatically suspended between radio events when only the low current regulator is needed internally.This feature is particularly useful for applications using battery technologies with nominal cell voltages higher than the minimum supply voltage with DC/DC enabled. The reduction in supply voltage level from a high voltage to a low voltage reduces the peak power drain from the battery. Used with a 3 V coin-cell battery, the peak current drawn from the battery is reduced by approximately 25%.
 
The reference manual goes on to state (link to download):

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.

Good to know….Importantly – the heck with including these circuits within the power design of the Ladybug Blue Lite.  This will save additional BoM costs since one of the parts I was considering for the Buck-Boost converter is the TPS63030 which currently costs $2.50 on Digikey.com.  Not to mention the price of a LiPo battery and any connectors needed.

Not Designed for LiPo or Lithium Ion

The power supply must be between 1.8 – 3.6V (LDO) or 2.1 – 3.6V (DC/DC).  The LiPo battery I am testing with is rated at 3.7V.  After charging my DMM measures the voltage at 4.2V.  Even at 3.7V, plugging a LiPo battery directly into the chip will damage the chip.  If I were to continue down the path of a LiPo battery, I would explore using the advice given by Stefan in this Nordic Devzone post:
Since a lithium battery voltage is up to 4.2V but the supply voltage for the nRF51822 is 1.8V-3.6V, then I guess an efficient option is to have an external LDO drop the voltage down to 3.6V for the nRF51822, and have the DCDC enabled. Lithium batteries normally have voltage range of 3.0V-4.2V, so you should bypass your external LDO as your battery voltage drops below 3.6V.

Low End Voltage Requirements

That’s the high end of the power source.  What about the low end?  How much voltage is needed?  
    • 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. 
Thus the minimum voltage needed is ~ 2.1V
 
To be conservative, I will design for the absolute minimum voltage to be 2.4V.

Which Battery

I decided to go with what is used with the nRF51 DK – a CR2032 coin cell battery.

EnergizerCoinCellBattery

Looking at the data sheet for the CR2032 that came with the nRF51 DK (the Energizer CR2032), the voltage starts off at 3V:

ContinuousDischargeCharacteristicCR2032

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.

 

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