Review of schematic

Projects
1

Is there anywhere to place a schematic for review and input?

I am a beginner and have a design and am looking for feedback.

Thanks

Adam

2

Are you asking for a critique of the electronic circuit design, or comments on your drafting style and format for presenting information?

Strictly speaking, this Forum isn’t focused on either of those topics but both of them get addressed from time to time when users post their documentation when asking other questions. Post your EESchema schematic file (and symbol libraries that are not in the standard GITHub list), or plot your design to a *.pdf file, and identify one (or a small handful) of items where you are especially concerned.

Dale

1 Like
3

Thanks Dale.

I’m looking for a review of the design. This is a PCB design for an automated saw I am building. It is controlled by a Raspberry Pi and I’ve tried to isolate (protect) the Pi as much as possible. I’ve put this together based on what I could find and some help from others at my local maker space.

As a new user I can’t upload a file, but here is a link to the PDF of my schematic.

https://dl.dropboxusercontent.com/u/1619624/saw.pdf

Areas of concern are:

  1. The transistor circuit controlling the buzzer
  2. The 4 relay control circuits from the Pi (40 Pin header)
  3. Power control

I’m looking for anyway to improve the design and my knowledge and appreciate any and all comments!

All the best,

Adam

4

First thoughts:

  1. It seems you put component values as component designators! This is the ultimate bad approach.
  2. Use power symbols when drawing power wires. At least GND.
  3. You have Relay_1 LED short by wire.
  4. 330k for Relay_5 LED? This LED has no chance to light up in this case! (Unless your RPi has min. 400V DC output).
2 Likes
5

A 1A switching regulator, is that enough?
LEDS in series with the optoisolator infrared LED. Remember a Pi has 3.3V "1"s and quite limited I/O drive

2 Likes
6

You mean those LEDs that are back to back? :wink: I would move the LEDs to the output side, after the transistor.

I agree with @keruseykaryu, your schematic would be a lot cleaner and more readable if you used power symbols for all your power and gnd connections. As well as labels for any connections that would otherwise travel a long distance across the schematic.

The usual nomenclature for resistor, capacitor and even voltage values uses the letter to replace the decimal point. That is 1k5 = 1.5k, 510R = 510 ohms, R1 = 0.1 ohms, 4n7 = 4.7nF, 4u7 = 4.7uF, 3v3 = 3.3 volts.

For your power input capacitors, I would keep the 100u and remove the 10u. There’s also no need for the 1n (0.001u) capacitor.

Give all your resistors and capacitors unique reference ids as you have with other components.

You might also want to consider LEDs on your power input to indicate you have power and your fuse is intact. And maybe even a fuse on the 24V such that a blown 24V fuse doesn’t interrupt power to the processor, the processor could even monitor the 24V.

2 Likes
7

No, bottom centre. The LEDs in series with the optos diodes Relay 1,2,3&4 will block any current driving the optos.
Led “Relay 1” also appears to have a wire shorting it out.

2 Likes
8

Yes, exactly, those LEDs are back to back (anode to anode). To go on and discuss the Pi’s outputs as being 3v3 is rather moot.

That line running through the diode symbol does make it rather difficult to spot that wire though.

2 Likes
9

… unless one notes the junctions at each end…

10

sigh Yes, true in this case the junction dots do catch your attention. But it’s entirely possible to have the dots without a wire across the diode, or have a wire with no dots. The line through the diode just makes it more difficult to spot the wire.

1 Like
11

Sure, absolute necessity. The optocoupler has a forward voltage drop of 1.2V (1.4V max). I do not know which LED shall be implemented, but they typically have forward drops around 2V. So you are done with any “3V3” output.

12

I’n not even going to reply to that one (other then to let you know I’m not going to reply). :slight_smile:

2 Likes
13

…SO SAD! :grin::grin::grin::grin:

14

First of all, I get the bit about the sum of the forward voltage drops amounting to more than the 3v3 output from the Pi.

Secondly, I would wager that 3v3 is sufficient for that circuit to work, with a much smaller resistor and the LED turned around. The 1.2V you quoted from the datasheet is for a If of 20ma. Clearly it does not need 20ma. Dropping the If to 5ma brings the Vf closer to 1V. LEDs are available with a wide range of Vf but for a typical Vf of 2V, again at an If of 20ma, if we reduce that to 5ma the Vf is down around 1.9V. 1V + 1.9V = 2.9V leaving 0.4V for the resistor which would be around 80R. Having said that, it would probably even work with less than 5ma.

My original point was that discussing only the Vf without mentioning that one LED is backwards is rather moot (ie. pointless). And I would still move the LED to the output side in parallel with the relay coil.

Perhaps now we could get back to reviewing the op’s schematic instead of me? :wink:

2 Likes
15

Ok. I hit the wrong reply button. Just wanted to quote you. Sorry for that.

1 Like
16

Hi Everyone.

Thank you all so much for the feedback and comments, I really appreciate them! I’ll work on these changes while I work though each message.

Best regards,

Adam

17

Thanks keruseykaryu.

Does this mean all components share a common ground (5V and 24V circuits)?

I was thinking it may be a good idea to at least keep the RPi GPIOs on their own ground circuit. My reasoning was to isolate the RPi from any issues with the other circuits.

Adam

1 Like
18

On your microswitch inputs (J7) I would increase the resistor values from 220R to 1K. And those 5.1V zeners are not of much use. Just use a regular diode if you want to protect against reverse voltages.

1 Like
19

You can have more than one ground, just name the ground symbol accordingly. Something like GND_IO, GND_PI. Same goes for your other voltages.

1 Like
20

Depends on your PMIC module… if it is of the non-isolated type all GNDs will be the same.
If it was isolated you’d have different GNDs.
PS: I use these non-isolated DCDC converters to run RPis: APXW003A0X3-SRZ (9-32Vdc input, 27W, adjustable output).

You’re powering the RPi via it’s header - if there ever is a chance that the USB power supply for the RPi is plugged in at the same time that you power it via the hat you want this between Pin 2/4 and your 5V source:

640Ă—660 92 KB

PPS: create a special symbol for the RPi 40pin header with the pins named, like in the screenshot, makes it easier to read the schematic and causes less errors.

2 Likes