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WS2812B low voltage behavior??
Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.

WS2812B low voltage behavior??

by iqasi096 on Wed Dec 16, 2020 12:19 pm

Hello,

I have recently purchased a solar charger, a 6000mAh battery, and a Powerboost 1000 basic 5V converter from adafruit to run a battery powered data acquisition system. I added a voltage comparator between the charger and converter to prevent my battery from overdischarge by triggering the enable pin of the regulator at a voltage of 3.2V. The output of the regulator is to a USB output, 15 WS2812B LEDs, and an arduino nano.

I however, noticed an issue such that if I've drained the battery overnight and I leave the device to charge on its own in the low solar energy available in the morning, my device would fluctuate between my 3.2 V threshold and 3.3 V which would present low voltage values to both my microcontroller and LEDs. These low voltages would cause my LEDs to do some random things like turn 7 leds on with a green color which is not something I specified the leds to do in my code.

That being said, once enough charge has been supplied to the drained battery, the device would start functioning normally again.

Is there a way I would be able to overcome the rough startup of my device?? Would maybe adding a mosfet to keep the 5 V line to the LEDs open until my arduino sends a signal to close the open circuit be the way to go?? What is happening to the LEDs to cause them to malfunction like this??

On a side note, removing the connection between my voltage comparator output and the 5 V converter enable pin would also allow my device to run properly. This comes at the expense of my overdischarge protection however...

Thank you,
Ibrahim

iqasi096
 
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Re: WS2812B low voltage behavior??

by adafruit_support_bill on Wed Dec 16, 2020 1:16 pm

Neopixels are designed for 5v operation. We have found that they work reliably at 3.7v. Below that level, you will generally start to see the blue channel drop out and operation will become less stable.

The LiPo cells sold in our store all have integral protection circuits which will cut off output at about 3v to prevent over discharge.

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Re: WS2812B low voltage behavior??

by iqasi096 on Wed Jan 06, 2021 1:47 pm

Hello Bill,

I have been doing some testing on the Neopixels and I am looking at possibly adding an nMOS transistor to disconnect the ground of the neopixels and opening the circuit between my LEDs and battery. I noticed that the neopixels would still light up when the ground is not connected while only the digital input and +5V pins are connected. What is causing the LEDs to light up and why are they not completely off once the circuit is opened??

Here is a simplified image of my connections:
Attachments
Untitled Sketch_schem.png
Untitled Sketch_schem.png (72.61 KiB) Viewed 73 times

iqasi096
 
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Re: WS2812B low voltage behavior??

by adafruit_support_bill on Wed Jan 06, 2021 1:58 pm

I am looking at possibly adding an nMOS transistor to disconnect the ground of the neopixels and opening the circuit between my LEDs and battery.

Not a good idea. Please read the Best Practices page of the guide:
https://learn.adafruit.com/adafruit-neo ... -2894472-1
Avoid connecting NeoPixels to a live circuit. If you simply must, always connect ground first, then +5V, then data. Disconnect in the reverse order.


Clearly current is flowing and current can't flow unless there is a circuit. If the only connections are +5v and Signal, if must be flowing back via the signal pin.

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Re: WS2812B low voltage behavior??

by iqasi096 on Thu Jan 07, 2021 10:19 am

Hello,

The issue I'm describing is best represented by the video found here: https://www.youtube.com/watch?v=JUYwuR2aObA

If I decided to connect the ground and open circuit at the 5 V line to the neopixel, I notice that there is a massive voltage drop. I measured the voltage coming into the LEDs when the MOSFET closes the circuit and it turned out to be 2.53 V which is nowhere near enough to power the LEDs properly.

Is there a way to implement a hardware on/off switch without requiring a signal coming from the arduino??

The issues only occur when the battery is at a low voltage which does not allow the arduino to function properly and clear the neopixels, so a hardware solution is best.

NOTE: I have attached the schematic for the design I previously mentioned below here:
Attachments
Untitled Sketch_schem.png
Untitled Sketch_schem.png (72.41 KiB) Viewed 63 times

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Posts: 25
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Re: WS2812B low voltage behavior??

by adafruit_support_bill on Thu Jan 07, 2021 11:48 am

N-channel devices are best suited for low-side switching. For high-side switching you should use a P-channel.
https://electronics.stackexchange.com/q ... ide-driver

To avoid damage to the pixel, be sure to put the Arduino signal pin into high-impedance (input) mode before you cut power to the device.

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Re: WS2812B low voltage behavior??

by iqasi096 on Fri Jan 08, 2021 10:57 am

Perfect, I shall implement the design with a high side pMOS switch.

Thanks for your help bill!

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Posts: 25
Joined: Tue Apr 14, 2020 5:08 pm

Please be positive and constructive with your questions and comments.