Drive an LED (or two) from an Itsy Bitsy M0 Pin?

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WShawn
 
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Drive an LED (or two) from an Itsy Bitsy M0 Pin?

Post by WShawn »

How much current can an Itsy Bitsy output pin safely handle? Would I be able to light up a 20 mA "dumb" LED (with the proper resistor, of course)?

I looked at a data sheet for the ATSAMD21 Cortex M0, but I have little idea what I'm looking at.

For context, a friend and I are redesigning this PKE Meter prop I've built to use the Itsy Bitsy as the Teensy 3.2 we originally used isn't available for a while.

https://youtu.be/gct2NALXWS4

Thank you.

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adafruit_support_bill
 
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Re: Drive an LED (or two) from an Itsy Bitsy M0 Pin?

Post by adafruit_support_bill »

Not directly. The max is 7mA if configured for high drive strength. The default is 2mA.

To drive a 20mA load, you would need to add a transistor. https://learn.adafruit.com/transistors-101/example-uses

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WShawn
 
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Re: Drive an LED (or two) from an Itsy Bitsy M0 Pin?

Post by WShawn »

Thank you for the reply; I appreciate it.

We already use a ULN2803 to drive the 7 pairs of LEDs for the wings and 7 for the display. We have to drive three other single LEDs. I might be able to use the 8th output of the ULN2803 for one.

It would be swell if I could get the other two transistors in a single, through-hole package that's small and inexpensive. Any suggestions? The ones I'm seeing are either SMD or pretty pricey (like $7 for one).

Cheers!

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adafruit_support_bill
 
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Re: Drive an LED (or two) from an Itsy Bitsy M0 Pin?

Post by adafruit_support_bill »

Through-hole parts are gradually being phased out in favor of SMT packages. It is hard to beat the price-point of a commodity item like the ULN2803. Even if you don't need all the channels.

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WShawn
 
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Re: Drive an LED (or two) from an Itsy Bitsy M0 Pin?

Post by WShawn »

Thanks again for the quick reply; I really appreciate your help.

The space on my planned PCB is limited inside this small prop, so we're trying to use the fewest components possible.

I'm now thinking two of these high-power TPIC6B595 shift registers might be the most efficient solution:

https://www.adafruit.com/product/457

These would eliminate the need for a ULN2803 (or other transistors), right?

One would be programmed to chase the 7 pair of wing LEDs in one of two different patterns (Ghostbusters 1 & 2)
The other would drive a simple chase of the 7 LEDS on the display hood.
The two unused outputs from above could control one of two LEDs that light up when a touch switch is engaged.
That leaves one LED requiring a transistor to supply my 20 mA.

Cheers!

Shawn

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michaelmeissner
 
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Re: Drive an LED (or two) from an Itsy Bitsy M0 Pin?

Post by michaelmeissner »

I don't know whether it would work in terms of brightness, but one thought is to switch from separate LEDs to neopixels. There you only need one pin for data, and feed in VIN directly. On the Itsy Bitsy you would use pin 5, which is voltage level shifted to VIN. Of course, you would need to rewrite the code, but I imagine there is code out there that uses neopixels instead of separate LEDs.

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WShawn
 
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Re: Drive an LED (or two) from an Itsy Bitsy M0 Pin?

Post by WShawn »

Hi Michael:

Thanks for taking the time to reply.

This prop calls for LEDs of specific shapes and sizes. The wing LEDs use 3 mm round T1 LEDs. The display hood uses green T1s. Then there are some rectangular ones on the face of the prop. So it doesn't seem like NeoPixels would work here.
PKE_Grab.png
PKE_Grab.png (366.13 KiB) Viewed 172 times
My PCB footprint would allow for two TPIC6B595 shift registers, the Itsy Bitsy, various Picoblade headers, a handful of resistors, and one transistor.

Cheers!

Shawn

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Re: Drive an LED (or two) from an Itsy Bitsy M0 Pin?

Post by adafruit_support_bill »

The TPIC6B595 chips should work for what you want to do.

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WShawn
 
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Re: Drive an LED (or two) from an Itsy Bitsy M0 Pin?

Post by WShawn »

Thanks again for the advice. My cohort (who's handling the code) and I just ordered a batch of these from Adafruit, along with some sockets.

Have a swell holiday!

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WShawn
 
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Re: Drive an LED (or two) from an Itsy Bitsy M0 Pin?

Post by WShawn »

Sorry, one more quick question.

Will the Adafruit Circuit Python libraries for the 74HC595 shift register work the the TPIC6B595 (assuming it's wired up correctly)? We're hoping these are identical from a coding standpoint.

Thank you.

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Re: Drive an LED (or two) from an Itsy Bitsy M0 Pin?

Post by adafruit_support_bill »

I have not tried it, but it looks like you could probably make it work. The pin names from the datasheets are all different, but they do appear to have equivalent functionality.

https://cdn-shop.adafruit.com/datasheets/tpic6b595.pdf
https://www.diodes.com/assets/Datasheets/74HC595.pdf

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WShawn
 
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Re: Drive an LED (or two) from an Itsy Bitsy M0 Pin?

Post by WShawn »

Thanks for the reply. The TPIC6B595s are on the way, so I guess we'll see whether they work with CP.

Cheers.

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