Logic level converter - Question


#1

Hi folks, I’m looking for some help with the following component https://shop.pimoroni.com/products/adafruit-4-channel-i2c-safe-bi-directional-logic-level-converter

I was looking to use with a teensy 3.2 to drive some 12v WS2811 LEDs

If I use 5v to supply the LEDs they work correctly, but when I try a 12v supply, they then begin to flicker. I had put this down to the logic level of the teensy being 3.3 not 5v so thought this logic level converter would help.

I had assumed I could just connect up the 3.3v supply to the LV and 5v to the HV and then connect the teensy to one side and the LEDs to the other, but this doesn’t appear to work. I also noted that the pins for the converter that are not connected all appear to be high at all times - e.g the lv side are all 3.3 and he hv side are all 5v. Are they high by default?

Any help or guidance would be much appreciated as I have only a few days to turn round this project


#2

Hello there,

I’m sorry but this cannot work. As it says on the product page of the Level shifter:
“Works down to 1.8V on the low side, up to 10V on the high side”

so low side at teensy (3.3V) should do the job, but 12V on the HL side are just to much for this little chip.

Think it would be the easiest to buy 5V LED Strips or just use your 5V Power Supply. Are you sure the LEDs need 12V? If they work with 5V i would think they need 5V… ;)

but would be best to have a look at the datasheet of your LED Strip!

In case you didn’t know there’s a special Adaptor for Teensy to drive up to 8 WS28 Strips. We don’t have it in store right now, but you can get it from Adafruit: https://www.adafruit.com/products/1779
But it is also only for 5V LED Strips.

Maybe i’m not getting you right, you’re saying it works with a 5V Power Supply connected to the LEDs and you bought the level shifter to connect your teensy to LEDs that work when they get 5V. Should be fine just don’t use the 12V Power Supply.

Best Regards,
steffen


#3

Sorry, I should have been clearer. The LEDs use 12V to drive the LEDs, but the actual data pin expects 5v

This is why I was trying to step up the 3.3V logic of the teensy to the full 5V

Do I have the right chip to do so? Is it right that the unused pins all register as high by default?


#4

P.s they work at lower voltages as they have a voltage regulator, but they can’t go to full brightness


#5

I think you’ll find the data pin expects 0.7VDD, which would be 8.4v, which may be why sending them a 5v signal results in nothing at all. See the datasheet for the pixels here: https://cdn-shop.adafruit.com/datasheets/WS2811.pdf

In a pinch, you should be able to voltage divide an 8.4v supply from your 12v supply. As long as it’s roughly between 8.4v and 10v it will fall between the maximum voltage of the logic level converter, and the minimum voltage that the ws2812s consider to be an “on” state.


#6

Thanks for the tip. Are you suggesting the chip I have is correct and I’m using it right, but that I just need to supply 8.4v not 5v to the hv side of the level shifter? Can I just use a resistor?


#7

Yes, you would need to supply somewhere around 9v ideally. You could do this with a voltage divider, 9v from 12v is relatively easy to get, you can just use a 10k R1 and a 33k R2 resistor: http://www.raltron.com/cust/tools/voltage_divider.asp

This would give you 9.2v, which is greater than 8.4 and less than 10, so it should work!


#8

Thanks for the tip I’ll give that a try later