Automatic Faucet Board Schematic Review

Hey. I posted a thread a while back about an automatic cat faucet that I have been designing. I’ve gone through and gotten everything working but want something a little more professional and finished for the board.

Basic idea is to have a weight sensor (HX711 as the amp) that detects a change and opens up a motorized ball valve using a TB6612FNG as the motor driver. I was looking for a little help in checking over the schematic I’ve put together to make sure everything is all set before I go ahead with the board design.

My concerns aside from any others notice are:

  1. Whether Q2 will need a Zener diode and resistor to limit the voltage from gate to source. The gate to source max in the Datasheet for the MOSFET is 12V. Since the circuit will never be powered with more than 12V is it ok to leave this as is? This is going to be a battery powered circuit so I don’t want to add the zener if it is going to waste power unnecessarily.

  2. Are there sufficient bypass caps and adequate values?

  3. Any concerns with analog and digital in the load cell amp? I’ve been trying to read up as much as I can on this and it seems one article contradicts the next on how to approach this.

Any other issues I would love to hear about before moving on.

Thanks.

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How about a power and heartbeat LED ?

Going to have any manual control switches or plug-in LCD ?

Good call on the power LED. By the heartbeat, do you mean tx and rx leds for programming feedback?

I didn’t plan on any manual controls, but it might be a good idea to throw a power switch on it now that you say it. Thanks

… no, a heartbeat LED is an indicator that twitches (low current), every second or so, in order to indicate the main loop() is running (not hung).

It can do more, but that’s a good start.

(sorry @larryd)

Nice drawing! I prefer two caps on the in and out of a regulator, Hi frequency bypass and bulk. 10Uf is fine but maybe a 10 to 100nf on the in and out. My preference on the micro is to terminate the unused pins with resistors. Overall it is a good design as far as I can see. If you do not terminate them program them as outputs, logic stare doesn’t matter.

How is power provided and what are the power requirements of the motor.? Would a 5volt to 12 volt boost converter, switched on as required by the MCU, be an option?

Thanks! I went by the recommendation of the datasheet for the regulator capacitors but I’m gonna throw the extra 100nF caps in as well.

As far as unused pins if I tie them each to ground through 100k resistors do you think that would be sufficient?

Power is provided by a 12v battery. The motor uses only about 1/2 watt so it’s nothing crazy. I tapped off the main 12v supply to the tb6612.

Will you always use ISCP to program the controller ?

Maybe add an FTDI option too.


Edit:

Don’t forget to add corner mounting holes in the PCB.

Better to use some led lights at every stage to understand the flow and the connections over this large and wide network

I debated this also. I really only plan on programming this one time and not really touching it again. Is there any advantage I haven’t thought of to adding the ftdi?

I like the idea of adding the leds, but with it being battery powered I’m trying to keep the current very low. I have the atmega running checks for any change in the weight sensor and then going back to sleep. So the more I think about leds that was a big reason I kept them off the board.

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Much easier to upload code, you will need to buy an FTDI interface for ~$10.00. Then it can be used for other projects too.

You will have access to Tx and Rx too, might be useful for debugging.

Might be nice to have access to SDA and SCL A4/A5.

True. I’m obviously going to have to debug the code here and there. I can just use a little header for sda and scl in case it’s needed also.

Yes.

It’s always best to add pads for things that might be needed rather than soldering wires to components.

You don’t need to populate these optional items, just make allowances for possible use. This may include: LEDs, test points, possible switches, headers for i/o, front panel i/o, SMD footprints that you may need (example a SOT-23 MOSFET for an off board relay).

BTW, modern SMD LEDs can be run at very low currents, milliamps. You can also have series headers to enable these LEDs when you want to check things i.e. add a jumper to enable the LED, pull the jumper to disable.

Jumpers for the leds. That’s a good idea!

So I’ve made adjustments to the schematic for FTDI and resistors to get the unused pins to known states. This is most of the board done as long as I haven’t forgotten anything. Just wanted to make sure everything looks good before finalizing. It is going to be a 4 layer board. Any input would be greatly appreciated.

  • I need to add a connector for the 12V input.

PCB_PCB_Cat_Fountain_2_2021-05-09_3.pdf (148.4 KB)
PCB_PCB_Cat_Fountain_2_2021-05-09.pdf (341.1 KB)

Also, the schematic

Not sure why it needs to be 4 layers.

Where are the mounting holes ?

Suggest you make the traces 25mil wide.


No LEDs:
:cry:

It definitely doesn’t need to be 4 layers but I wanted to see going into it with the intent of 4 layers. I’ve never done a 4 layer board and it only adds $6 to the total cost. Is there more harm in doing the 4 layers with the additional vias?

Haha I honestly was sure I wasn’t going to forget the mounting holes!

I do think I still will add leds.

25 mil traces will be really tight for the tb6612 footprint. The power traces are around 20 mil. I can definitely bump them up elsewhere

Can be tapered to what’s needed.


Personally I use 5v set to 50mil, no less than 25.


Personally, I use signal traces 12.5 to 25mil.


Vias 13mil.

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