Please give me some tips for my pcb design
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RubenNanda 1 year ago
Hey, I am trying to make a pcb for a motion detector. Here is the link of the project: [https://easyeda.com/RubenNanda/bewegings-melder](https://easyeda.com/RubenNanda/bewegings-melder) the 4 pin female header is a power regulator: [https://nl.aliexpress.com/item/Ultra-Kleine-Size-DC-DC-Step-Down-Voedingsmodule-3A-Buck-Converter-Verstelbare-1-8-V-2/32880983608.html?spm=a2g0s.9042311.0.0.32874c4dy28M0C](https://nl.aliexpress.com/item/Ultra-Kleine-Size-DC-DC-Step-Down-Voedingsmodule-3A-Buck-Converter-Verstelbare-1-8-V-2/32880983608.html?spm=a2g0s.9042311.0.0.32874c4dy28M0C) And the 3 pin female header is a motion detector: [https://nl.aliexpress.com/item/Good-H-quality-human-Body-Infrared-PIR-Motion-Sensor-Detector-Module-HC-SR501/32536393041.html?spm=a2g0s.9042311.0.0.32874c4dy28M0C](https://nl.aliexpress.com/item/Good-H-quality-human-Body-Infrared-PIR-Motion-Sensor-Detector-Module-HC-SR501/32536393041.html?spm=a2g0s.9042311.0.0.32874c4dy28M0C) I am wondering if this pcb is good. It works on a breadboard but maybe I need some more components, but since im quite new to this I don't know what. Just ask me if you need more information. Could someone please help me? Robin Camfferman
Comments
MikeDB 1 year ago
Usually you would add a proper reset circuit - 10k from RESET pin to +3.3v and 0.1 uF to GND. I'd personally use a linear regulator - LT1117-33 or similar.  But if you stick with the regulator you have check the output capacitor is at least 10uF as WiFi modules can take quite large peak currents.  I'd put the holes in the board for one from VCC to GND close to the ESP even if you don't actually end up using it.
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andyfierman 1 year ago
Although the modules you are using may already have some power supply input decoupling capacitance built in, I would add at least 10uF across VCC and GND where it comes in to the PCB. Don't forget to allow at least a 30% upper margin on the voltage rating of any electrolytic decoupling. So for example if your maximum VCC is 12V then rate the electrolytic for at least 16V. If you use ceramic caps then basically fit the highest voltage rating part you can get in the biggest package size for the capacitance you have chosen, that you can fit in the space available. The reason for this is because the (very poorly documented) Voltage Dependence of Capacitiance (VDC a.k.a. Voltage Coefficient of Capacitance or VCC) of almost all ceramic multilayer ceramic chip caps (MLCC) with dielectrics other than NP0 or C0G significantly reduces their capacitance with applied DC (i.e. supply) voltage across them. This VCC/VDC effect is often not documented in the datasheets for many makes of MLCC but here's some background: [http://www.elmac.co.uk/Voltage_coefficient.pdf](http://www.elmac.co.uk/Voltage_coefficient.pdf) [https://www.edn.com/design/analog/4402049/Temperature-and-voltage-variation-of-ceramic-capacitors--or-why-your-4-7--F-capacitor-becomes-a-0-33--F-capacitor](https://www.edn.com/design/analog/4402049/Temperature-and-voltage-variation-of-ceramic-capacitors--or-why-your-4-7--F-capacitor-becomes-a-0-33--F-capacitor)
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RubenNanda 1 year ago
@MikeDB for the reset circuit. do the resistor and capacitor need to be rated for 3.3 volt? And i chose that regulator because its easy for me to use and i already have them here. And what do you mean with the last part?
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RubenNanda 1 year ago
@andyfierman I added the capacitor, but do you always need to decouple your input voltage? And are there more places you would want to decouple it?
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MikeDB 1 year ago
Andy explained where to put the 10uF capacitor after me which hopefully clarified my last part.  Just use a standard 0,1uF 50v for the reset.
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andyfierman 1 year ago
@RubenNanda, "do you always need to decouple your input voltage? And are there more places you would want to decouple it?" The trouble with modules like the ones you are using is that they are poorly specified and/or have poor quality datasheets. They are also probably produced to a price rather than a peformance so may well be stripped back to the minimum component count. Therefore you cannot assume that they have adequate supply input decoupling for use at the end of a wired connection to the power supply. Therefore you must assume that additional supply input bulk capacitance must be added at the supply input to your PCB. For a more robust design, you could also put something like 100nF - 1uF MLCC across the supply pins of each of the modules on the main PCB. If you are making this for a commercial product then, to comply with EMC regulations, you should also have some form of supply input EMC filtering (common mode + differential mode chokes + caps) to reduce the conducted emissions exported back up the supply input wires from the (mainly SMPS switching + some RF content) currents exported by the input to the SMPS module. This is another reason for using a linear rather than a switching regulator on the input. You haven't said what the input voltage range is but as long as it's under 48V then AFAIK you should be OK for the Low Voltage Directive and other aspects of electrical safety. :)
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RubenNanda 1 year ago
@MikeDB Oh okey so you just meant the cap between the input voltage and ground. Thanks for the help :)
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RubenNanda 1 year ago
@andyfierman oohh okey. So would the 10uF cap on the input be enough for this? This is just for personal use so I can skip the EMC part or is it still important? And the supply is probably going to be between 5V and 12V. Thanks for the help btw :)
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MikeDB 1 year ago
10uF is more than enough.
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RubenNanda 1 year ago
Thanks for the help guys. :) So this is a good circuit now?
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