PCA20014 solar beacon fimware and HW questions

[MANGO]

Hi, I'm deeply interested about the Nordic's PCA20014; a.k.a solar beacon.

This HW uses

• Amorton AM-1417CA solar cell, 1.5V, 12.5uA at FL-200lux

• 2 channel PFET(FDG6304P, typical gate threshold voltage = -0.82V) and NFET(ALD110914SAL, typical gate threshold voltage = 1.40V)

My main concern is using the solar cell with the nRF52 MCU.

In my case, I was thinking of combining the PCA20014 with an I2S Amplifier (MAX98357A).

These are my questions for my investigation before purchasing the PCA20014;

0.Is there a reason for drawing the solar cell as an LED?

At first glance while looking at U2 and the solder bridge, I thought there was an LED that was connected backward.

1.Calculating the voltage dividing resistors, I think the solar cell needs to be charged up to 3V to exceed each gate threshold voltages.

Is there a reverse polarity issue while using the solar cell?

Using my shallow knowledge, I learned that PFETs are mostly used for reverse polarity protection.

I'm curious of the purpose for using 2 PFETs and a NFET.

2.Do these MOSFET works as a voltage regulator?

After opening PFET's gate 2, the divided voltage, 1.5~1.8V will be fed to VDD_nRF, I guess.

I don't see any DC-DC converter or voltage regulators in the schematic.

Does this mean that the voltage provided to the nRF MCU and the pressure fluctuates if the light level changes?

If so, I'm curious whether the MCU can do the job.

3.Since I will be adding an I2S amp, using a bigger solar cell and adding a buck converter will be better for me instead of using the above MOSFETs, right?

4.Switch on/off the external crystal.

The aforementioned statements are written in the Nordic's Infocenter.

Does this mean that this firmware doesn't use the Nordic's SoftDevice and turns on and off the 32MHz, the high-frequency clock, for power saving in the application?

I did check that the GitHub code uses RADIO_IRQHandler.

-Best Regards, MANGO

[Bjørn Kvaale]

I will sum up the comments as one answer:

Is there a reason for drawing the solar cell as an LED?

Yes, this is quite common, as the LED refers to a photodiode (see this article). A solar cell is just a large area photodiode.

Is there a reverse polarity issue while using the solar cell?

There can be a reverse polarity issue if multiple solar cells are connected in series (see this forum & this link). Since the solar cell you refer to is most likely a single solar cell, I believe this will not be an issue for you. But just to be certain, check with your supplier or the manufacturer directly.

Do these MOSFET works as a voltage regulator?

I believe not because the nrf52832 has an integrated DC-DC converter (see here under Section "Flexible Power Management").

I don't see any DC-DC converter or voltage regulators in the schematic. Does this mean that the voltage provided to the nRF MCU and the pressure fluctuates if the light level changes?

No, see above answer.

Since I will be adding an I2S amp, using a bigger solar cell and adding a buck converter will be better for me instead of using the above MOSFETs, right?

You might want to use a bigger solar cell. That is up to you. Just make sure the solar cell is within the supply voltage of the nrf52832 (i.e. 1.7-3.6 V). If you are using multiple solar cells in series, you might want to checkup on any possible reverse polarity issues. I am unsure whether you will need a Buck converter for the I2S amp. This is probably a good idea since the input supply will fluctuate due to changing light intensity (the I2S amp you refer to can handle 2.5-5.5 V input supply voltage).

As for these questions: Can you check other things such as

• is PCA20014 using one or multi-solar cell(s)
• need for DC-DC converter for nRF MCU and I2S amp
• PCA20014 firmware code

I believe the PCA20014 is a single solar cell, but check with your supplier or manufacturer to be certain. There is no need for a DC-DC converter for the nrf52832 (see answer above), but this might be useful for the I2S amp. Talk to Adafruit tech support to confirm this for the I2S amp. The PCA20014 solar beacon firmware code can be found here.

Does this mean that this firmware doesn't use the Nordic's SoftDevice and turns on and off the 32MHz, the high-frequency clock, for power saving in the application?

I am quite certain that this firmware does not use the SoftDevice. In the Infocenter, I saw these statements at the bottom: "The beacon saves power by putting the CPU to sleep between every step in the firmware sequence shown above. It also saves power by switching off the crystal oscillator and the power to the sensor chip and the TWI."

I hope I answered all of your questions. :)

[MANGO]

What are you planning to do if there is no sunlight, i.e. no input voltage from solar cell? Do you want to hook up a battery or will no sound play?

My purpose was to create a battery-free product; I planned for the system to be shut down when there is no light; no beacon and no sound.

I learned a lot from you! Hooking up a battery sounds cool (if you meant that battery like CR2032 is used if there is no sunlight).

I will consider that scenario!

-Best Regards

[Bjørn Kvaale]

I would start with the battery free solution first, if I were you. A little less complex to start with. Should be able to charge up a coin cell battery (not a CR coin cell battery, as this is a disposable battery, see here). Not sure how difficult it is to charge a coin cell battery, but it would be a cool project! Good luck with your project!