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| Borderlands style jewelry box research thread |
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| Ian.M:
That would be a physically *BIG* capacitor. You'd probably be better off using at least two separate capacitors for a lower profile. You may also want a Schottky diode between the buck converter output and the Neopixel Vcc + bulk decoupling cap, with the servos + their bulk decoupling cap connected on the buck converter side of it so the servos cant glitch the neopixel supply. The Arduino Pro Micro RAW supply pin can tolerate up to 12V. If you want the convenience of using a 12V wallwart, you need to pre-regulate or drop the RAW supply so it can *NEVER* go over 12V. A 78L09 9V 100mA regulator is suitable or a string of four 1N4148 or similar diodes to drop about 3V. If you use a 9V wallwart, don't worry. N.B. the wallwart *MUST* be regulated - avoid old linear unregulated ones as their output can be as much as 50% high when minimally loaded which will blow the Arduino. |
| Youkai:
Alrighty so a few things: 1) The purpose of the diode is to prevent power "Backlash" from the servos starting/stopping causing trouble with the Pixels correct? If so then I would need D2 in my new diagram? Is there a reason to put D1 in there? Any reason to put one on the line that has the photointerupters on it? 2) Is the Schottky diode for the low forward voltage drop? Is there any other reason to use that instead of another type of diode? 3) What power rating should I use for the diode? A quick google seems to indicate that they are rated for voltage; and that the "breakdown voltage" is usually pretty high (like 100V). So just make sure I get a diode with a breakdown voltage way above 5V and I'm good? 4) NeoPixels seem fragile. Is that just me not being used to component parts or are they a little more finiky than normal stuff? 5) Figuring out power needs: * NeoPxel: 16 * 30 ~= 500mA (30 is the rough estimate of power use on an "average" pixel) * Servos: 200 * 3 = 600mA * Pro Micro: 200mA * Misc power loss: 200mATotal power needs: 1.5A. Does that seem correct? I'm thinking I'll try to find a 9V, 2A wall plug online. Will that be sufficient? 6) I found a 1000mF/16V capacitor that seems a reasonable size. Any reason I shouldn't use it? I know you said a couple smaller ones would work but if just one will do I'd rather keep it simpler. |
| Ian.M:
1. When a servo starts moving, the motor will momentarily draw its stall current which can be 10x its normal running current. This will cause the supply voltage to dip. If you feed 5V to the other devices like the Neopixels and the optosensors via a diode with a reservoir capacitor after it, when the supply dips, the reservoir capacitor after the diode maintains the 5V rail through the dip. D2 should NOT be on your diagram. D1 should feed *EVERYTHING* 5V except for the servos. You *need* a reservoir cap after the diode 2. A Schottky would be used exclusively for its low Vf drop. 3. Low voltage Schottkys typically have a lower Vf drop than high voltage ones. Also, at a fixed current, a higher current rating diode will typically have a lower Vf drop than a lower current rating one. The tradeoff is: higher current ones have more junction area so higher reverse leakage. A 3A 20V one should be suitable for this project. Don't go below 2A or over 40V. 4. They are LEDs + logic. They don't like over-voltage and/or ground bounce, or DIN going outside the supply rails AT THE LED so if you don't use enough bulk capacitance and your wiring inductance rings with the decoupling caps on the strip it can transiently over-voltage them, or your ground is too flimsy and their local ground is bouncing up and down by a volt or so, and you haven't used a series resistor in the data line, you can blow the internal input protection diodes of the DIN pin. Basically if your circuit is crap they can be killed by it. There have also been reports of quality problems with some suppliers (direct from China) of reels of the 'neopixel' style LEDs, with an excessive percentage being DOA. However Adafruit quality control should have weeded out defective ones after assembly so you shouldn't have any issues. 5. The Pro Micro will draw far less than 100mA as none of its outputs are directly driving heavy loads. The sensors will draw about 100mA for all their IR LEDs. 1.5A @5V is 7.5W. assuming 80% efficiency in the buck converter, that's 9.4W in, so fractionally over 1A at 9V. A 1A one will be right on the bleeding edge, but a 1.2A one should do nicely and will be a lot easier to find than a 2A one. 6. I'd advise two 1000uF caacitors, one before and one after the Schottky diode. |
| Youkai:
As always excellent information. Thanks Ian.M I updated my schematic with the provided information. The wires are a little more confusing but I rearranged the parts so that I could put everything on the second 5V rail together. Please let me know if I broke any cardinal tenants of schematic design. I tried to make sure that all the items are labeled correctly in this version. e.g. "Lid Servo" so I will know what pins they are attached to. Also I have all of the components labeled with their values so those can be verified. I did a little bit of looking online to find a wall wart that is 9V, 1.2A and didn't have much luck. Sparkfun has a 650mA version. Jameco has a 500mA. I maybe found a couple on some sites I have never heard of before. Do you have any recommendations for a site where I can get the wall plug? EDIT: After a little more searching I found this one at Jameco. It's 9VDC / 2A. Is that overkill? https://www.jameco.com/z/GS18U09-P1J-MEAN-WELL-2-Wire-AC-to-DC-Regulated-Switching-Wall-Adapter-18W-9VDC-2A-2-1mm-Plug_1952847.html |
| Ian.M:
The only thing its missing is some ceramic decoupling caps right at each of the Neopixel connectors - at least 0.1uf, but preferably larger, and a 2x 10K potential divider between 9V and Gnd, with the tap connected to an Arduino analog pin so it can sense the supply voltage and only enable the servo control and Neopixel data outputs if the 9V is present. For robustness, you may also want a pair of small Schottkys from DIN to +5V (resevoir), and from Gnd to DIN right at each Neopixel connector, cathodes positive so they are reverse biassed in normal operation - that will save your Neopixels if there is a fault in your code and it attempts to drive them when the 9V is not present. |
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