What do the Schottky's do here? (Adafruit NeoPixel circuit)

[Youkai]

I'm working on a circuit for a project I'm in the middle of. I was instructed to put these Schottky diodes in my circuit right next to an Adafruit NeoPixel. I understand that the NeoPixel doesn't want to get a data signal when it's not powered and that these diodes are supposed to help protect it for that scenario. But I don't understand how they do anything. Relevant portion of the diagram attached. Here is what I think is going on based on my limited understanding of electronics. Please help me figure out what I'm missing.

If there is power connected:
Then the top diode has 5v in the Yellow and is getting 5v pulses in the Green. I assume that since Yellow is saturated with voltage then the signal goes to the data pin instead of through the diode. The bottom diode never has a positive voltage on Black so nothing ever goes through the diode.
If the power is not connected:
Yellow is an open circuit so no current flows through the top diode. No thing happens ever on the bottom diode.

Is the capacitor in the diagram "eating" the pulses when power is disconnected? Or perhaps the 1000uf capacitor in the full diagram is somehow allowing a complete circuit and draining the signal?

Other thread with full circuit diagram: https://www.eevblog.com/forum/projects/borderlands-style-jewelry-box-research-thread/msg1858662/#msg1858662

[aheid]

My first impression on looking at the schematic, before reading your post, was that they were input polarity protection, in case someone plugged in the 3-pin header the wrong way around.

But there's a good chance I'm mistaken

[Mr. Scram]

I'd expect a transistor switch type deal to prevent data from being clocked in, not this.

[Paul Moir]

Usually you see the "do not put in a data signal if not powered" in a datasheet as the maximum voltage an input can have with reference to Vcc.  For example you'll see something like ViH max = Vcc+0.3V (voltage-input-high maximum = Vcc + 0.3 volts).  What the upper diode does is drag Vcc up when there's a postive voltage on the data line, making things safe for the data signal input voltage to go up too.

It would be good if a resistor could be provided for limiting the current on the data line.  Sometimes that's hard or impossible though.  And there are better ways to do it...

[HackedFridgeMagnet]

1 & 2.  The 1000uF electrolytic capacitors have a fairly high impedance at high frequencies due to their size and wound foil construction.  A 0.1uF or 1uF ceramic has a much lower impedance at high frequencies so you need both in parallel, with the ceramic as close as possible to the load that will be drawing pulses of current.  You don't need one for the Arduino as it already has some on-board.

3.  The Neopixels could be damaged if data is applied without power.  If the 9V is missing its preferable to stop and not do anything (except possibly blink the Arduino's on-board LED to indicate its waiting for 9V power), so that's why I recommend sensing the 9V in.   If you want to test the board while its hooked up to your PC by USB, supply 9V as well.

4. For the DIN protection Schottky diodes, use low current ones. If you are building it using SMD parts BAT54S would be very suitable as it gives you two series connected diodes in a single three pin SOT-23 package.  If you are using through hole parts, BAT43 wuld be suitable.   Ideally solder the DIN protection Schottkys direct to the Neopixel LED strips, as there they can protect against loose power or ground wiring. If you mount them on your main board, they should go as directly as possible to the Neopixel connector pins.

5. I'd build it on Veroboard, with the Arduino soldered down and a full width set of strips for the 5V rails and the Gnd rail just beyond the end of the Arduino to mount the big caps and big Schottky to.

6. Your new diagram looks good.  Ceramic decoupling and DIN protection Schottkys should be as close as possible to the Neopixels - ideally soldered direct to the strips, otherwise at the three pin connectors for the neopixels on your main board.

From the other thread.

Basically I guess to protect the device by ensuring the data line does not go further than about 0.3V beyond the power rails.

[ArthurDent]

If the 5 volt power is on, the data is fed through a 470 ohm resistor and the top diode limits or clamps the chip input to 5+.3 volts and when the signal goes low it is clamped by the lower diode to zero-.3 volts. If the input to the 470 ohm goes to 6 volts, the upper diode limits the chip input to 5.3 and .7 volts (6-5.3) is dropped across the resistor. If the voltage tries to go negative the voltage is clamped at -.3 volts and if it is higher, the 470 ohm resistor will drop any voltage less than -.3 volts.

If the power is off the same happens only the voltage is now clamped to +/-.3 volts. With the power off the +5 line will stay at essentially ground or zero volts.

https://qph.fs.quoracdn.net/main-qimg-b1ab15f46c32aa64751ec2cf02eb5cd4

[Youkai]

Usually you see the "do not put in a data signal if not powered" in a datasheet as the maximum voltage an input can have with reference to Vcc.  For example you'll see something like ViH max = Vcc+0.3V (voltage-input-high maximum = Vcc + 0.3 volts).  What the upper diode does is drag Vcc up when there's a postive voltage on the data line, making things safe for the data signal input voltage to go up too.

It would be good if a resistor could be provided for limiting the current on the data line.  Sometimes that's hard or impossible though.  And there are better ways to do it...

I see so basically the upper diode is making sure that if power goes to data; then at least an equal amount of power goes to Vcc. This covers the scenario when the 5v rail is unplugged. There is a 470ohm resistor on the data line. It's at the top of the cropped image.

That makes sense. Thanks guys.

[David Hess]

This is a pretty standard I/O protection circuit which is also effective against ESD.

The diodes clamp the signal level so that it remains within ground and power.  Schottky diodes are used because their lower forward voltage drop clamps the signal before any similar silicon diodes built into the I/O pins conduct which can sometimes cause problems.

[saike]

I had to repair several bits of kit recently that didn't have these diodes fitted. If the data signal was already present while the power rails were connected, it sometimes killed the first led in the string stone dead so stopping the whole string from working.