PCB Schematic Review Request

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Schematic_Steamer-v3N_2025-07-18
Schematic_Steamer-v3N_2025-07-181169×827 27.6 KB

Hi Everyone,
I always use this forum to get a peer review. I am slowly expanding my expertise in PCB design. And have tested each component piece by piece. However, I would still appreciate it if you could review the schematic and highlight any shortfalls or issues with the design.

@jim-p @6v6gt - you guys have helped me in the past. A big shoutout to you for helping again.

VIN - used to connect the output from a 12V DC SMPS

SCT is used to connect to SCT-013-030 Non-invasive AC Current Sensor Clamp Sensor

U10 - connect to a K-type thermocouple

Pressure - connect to a pressure transducer 0.2mpa (link below) https://robu.in/product/pro-range-pr18-a002-pressure-sensors-00-2mpa-420ma-g1-4/

CN1 - Connect a Nextion display

OUT2, OUT3, OUT4 - connect solid state relays

LIM1 - connect a float sensor (link below)
https://robu.in/product/anti-corrosion-type-ball-float-liquid-water-level-sensor

DOOR - connect a proximity sensor (link below)
https://robu.in/product/inductive-proximity-sensor-lj18a3-8-z-ax-npn-non-shieldedsetting-distance-6-4-mm/

1 Like
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What was the idea behind the use of R10, R11, C14, C15 and C3 ?

How did you come to choose 33uH for the inductor?

I don't think the door sensor interface is correct.

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The voltage divider at DS looks inverted. Anyway, ensure it delivers no more than 5v to pin A5.
Try to keep pins A4 and A5 free in case you want later to add an I2C device.
Use Schottky diodes for D1 and D6 instead of the 1N4148 to ensure that the voltage on CS is always in the defined range.

2 Likes
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33uH Inductor is suggested in the datasheet. Attached herewith.

"In the majority of designs, a 33-μH, 3.5-A inductor is the best choice."

lm2596.pdf (4.8 MB)

For MAX31855 -
C6 is recommended in the manual.

R10, R11, C14, C15 and C3 - these were not recommended in the manual. After your post, I believe I may have overevaluated it. Pull-up resistors are added to reduce noise and improve accuracy.

Door Sensor - Re-evaluating it. Let me come back to this.

One question: Am I right in connecting the 3.3V output from the Arduino to power the MAX31855?

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Schottky dioides in place of 1N4148 - duly noted. Thanks

Door Sensor - Re-evaluating it. Let me come back to this.

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Schematic_Steamer-v3N_2025-07-19
Schematic_Steamer-v3N_2025-07-191169×828 26.9 KB

Corrected voltage divider. Thanks for pointing it out.

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What does "majority" mean?
Do you consider your design to be in the majority?
What is the minimum and maximum current your system will need?

C6 is recommended in the manual

Looking at the most recent datasheet, I see no mention of putting a 0.1uF cap across the thermocouple lines.
I suggest you download the latest datasheet and carefully read the application information section. Do as they recommend.

Pull-up resistors are added to reduce noise and improve accuracy.

They could damage the IC, will probably add noise and will certainly keep the automatic fault detection from working. Remove R10, R11, C14, C15 and C3.

Door Sensor - Re-evaluating it. Let me come back to this.

If you have it , measure the resistance between the brown and black wires.

One question: Am I right in connecting the 3.3V output from the Arduino to power the MAX31855?

Well let's find out.
How much current can the nano 3.3V supply?
Is the 3.3V supply from the nano well regulated?
How much current does the MAX31855 need?

I would make C11 10uF.
What is the purpose of D2, D23 and D4?

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That could be worse.

Explain to me how the limit switch circuit is supposed to work.

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For MAX31855

Arduino nano 3.3V max output 50mA. And the output is regulated.
MAX31855 consumption is just 1.5mA

Door Sensor
The resistance between the brown and black wires is 2.5 ohms

Voltage at DS 1/3 x 12V = 4V (approx)

Removed R10, R11, C14, C15 and C3

MAX31855 Datasheet

From the datasheet

"Noise Considerations
Because of the small signal levels involved, thermocouple temperature measurement is susceptible to powersupply coupled noise. The effects of power-supply noise can be minimized by placing a 0.1μF ceramic bypass capacitor close to the VCC pin of the device and to GND.
The input amplifier is a low-noise amplifier designed to enable high-precision input sensing. Keep the thermocouple and connecting wires away from electrical noise sources. It is strongly recommended to add a 10nF ceramic surface-mount differential capacitor, placed across the T+ and T- pins, in order to filter noise on the thermocouple lines."

MAX31855(1).pdf (561.9 KB)

D2, D3 and D4
The idea was to suppress inductive kickback. However, SSR does not do kickbacks. :sweat_smile:

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Revised schematic

Schematic_Steamer-v3N_2025-07-19
Schematic_Steamer-v3N_2025-07-191169×827 26.2 KB

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You actually made this measurement or are you guessing what it should be?

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I made this measurement on a small prototype board.

The SMPS is adjustable in the range of 10V to 13V. In both extreme cases, digitalRead() throws the result HIGH.

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It's an NPN output. Some have a 10K pull-up inside to V+ so don't. In either case your interface circuit would be wrong

Does it go low when an object is detected?

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Does it go low when an object is detected? Yes. Correct

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You have not made any of the changes recommended in the MAX31855 datasheet. Do you understand what they are saying?

I also see that you have not made any changes to the Buck or Limit switch circuit.

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Well something is not right. The AX version is NPN NC so it should normally be LOW and go HIGH when an object is detected.

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I do understand bits and pieces. I do google on many instances. I thought the buck convertor and limit switch circuit were fine.

Limit switch is On when water level is full. Otherwise its off. Code Appended below.

 //CHECK WATER LEVEL
    if (analogRead(level_sensor) < 20) {
      //water full
      if (low_water_previous_value != 0) {
        myNex.writeNum("home.low_water.val", 0);
        low_water_previous_value = 0;
      }
    } else {
      //water level low
      if (low_water_previous_value != 1) {
        myNex.writeNum("home.low_water.val", 1);
        low_water_previous_value = 1;
      }
    }
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I'm not worried about the code.
Let's stick to the schematic.

I'm not going to design this board for you but I will point out your mistakes. Whether you decide to take may advice or not it's up to you.

If you are going to design a PCB then you need to read and understand the datasheets for all the components you are using.

I looked at the datasheets/info for the proximity switch and limit switch and I can't see how they operate the way you describe.

Also I suggest you read post #7 regarding the buck

2 Likes
19

Understood. I went through the datasheet again and found my mistake.

The 5V current consumption is in the range of 200-300mA, not including the components running at 12V.

Qty Min (mA) Max (mA)
Arduino nano 1 20 30
MAX31855 1 1.5 1.5
SCT 013 Sensor 1 1 1
SSR (3nos) 3 3.5 13
Nextion 4.8inch 1 170 220
203 292

Screenshot 2025-07-20 142849
Screenshot 2025-07-20 142849419×377 16.1 KB

As per the graph above, I should be using a 100uH inductor.

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On the other hand, this post (link below) recommends having an inductor less than 15uH. I am confused again.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/875782/lm2596-output-capacitor-and-inductance-determination