Hello there,

Building my first PCB as an electronics enthusiast and would appreciate some input from the community. As an overview, I aim to replace an existing PCB in a battery-operated smart water device to remove it from the cloud. The device has 2 outlets, each with a latching solenoid and flow meter.

Some other details about this build:

1. I'm reasonably limited in depth/height of the board and its components
2. The circuit consists of the below power-consuming devices:
   1. 2 x latching solenoid (each 3.3v)
   2. 2 x water flow meter (each 3.3v)
   3. 1 x ESP32-S3-WROOM-U1
   4. 5 x SMD LEDs
3. The input voltage of the circuit is 6v supplied via 4 x AA size batteries
4. Being a tap timer/flow meter, this unit will live outside. In my case, it faces the west and cops the harsh afternoon sun (I hail from Australia). I mention this because I'm not sure if it will impact the thermal consideration of any components discussed here.
5. Testing via breadboard and a benchtop power supply, using an ESP8266 in the absence of owning the ESP32 (32 uses more energy), the total load was approx 280mA.

Where I would like some feedback is around finding a voltage regulator. I don't know if I can achieve what I want through a single regulator with an always-on and switchable output or if I need to stick with 2 or more.

1. The first regulator would provide a constant 3.3v to the ESP32-S3-WROOM-U1. This chip will use no more than 355mA peak, at idle I am not 100% sure but believe it would be approx 20-30mA. I was looking at the AMS1117-33 for this application.
   1. The chip would remain "fully active" when one of the solenoids is opened, and wake up for 10-15 seconds every 15 minutes to check and confirm there is no water flow when the solenoids should be closed.
2. The second regulator would be controlled via a GPIO pin. The idea here is that the flow meters should only get power when a solenoid is open or for a brief period every 15 minutes as noted in the above dot point. I've found a few options online such as the ST LD39200/LD39100 which are likely overkill.
   1. The second regulator would also provide power to the h-bridge controlling the solenoids and the solenoids themselves. The h-bridge is something I have never worked with and want to post separately to stop my brain from exploding.
   2. The solenoids only need to be active for 500ms at most to open or close.

Hopefully I've captured everything relevant here, let me know if I can provide any further info!

 

The 1117 might have rather too much quiescent current. It will typically add an extra 5mA to the constant load on the battery and reduce battery life. If that’s OK for you, it will do the job.
How much current does the flow meter need? At a guess it is just an LED and a phototransistor, so just enough to light the LED. Perhaps you could run it straight from the GPIO.
How much current does the solenoid need? I’d suggest Toshiba TB67H450 to drive it, but don’t put it through a regulator.

 

The 1117 might have rather too much quiescent current. It will typically add an extra 5mA to the constant load on the battery and reduce battery life. If that’s OK for you, it will do the job.
How much current does the flow meter need? At a guess it is just an LED and a phototransistor, so just enough to light the LED. Perhaps you could run it straight from the GPIO.
How much current does the solenoid need? I’d suggest Toshiba TB67H450 to drive it, but don’t put it through a regulator.

Thanks! I knew there was something I was forgetting.. the flow sensors are hall effect sensors. I did test one individually but clever me didn't note it down before I put everything back together so I've got to take it apart again and check. From memory though I want to say maybe 50mA tops?

The ESP8266 uses about 170mA when in full operation so if I subtract that from the ~280mA when everything was connected it would seem safe to say the flow meter and solenoid combined would use around ~100mA. Again, I need to double check this.

Also, would like to ask - you mention not to put the driver through the regulator - does this mean the driver would drop the 6v in to 3.3v out to the solenoid?

 

Thanks! I knew there was something I was forgetting.. the flow sensors are hall effect sensors. I did test one individually but clever me didn't note it down before I put everything back together so I've got to take it apart again and check. From memory though I want to say maybe 50mA tops?

The ESP8266 uses about 170mA when in full operation so if I subtract that from the ~280mA when everything was connected it would seem safe to say the flow meter and solenoid combined would use around ~100mA. Again, I need to double check this.

Also, would like to ask - you mention not to put the driver through the regulator - does this mean the driver would drop the 6v in to 3.3v out to the solenoid?

A simple Hall-effect switch (TLE4905L) takes 8mA. That might be within the capabilities of a GPIO pin, or it might not, depending on your micro. If not, just switch it with a transistor, or a logic gate such as 74LVC1G04.

The TB67H450 will limit the current, and it does so efficiently by using the inductance of the solenoid.
If it is a latching solenoid, you can drive it with 6V. It won't come to any harm on a short pulse. It may even work better.

 

A very simple trick that might work for you is to reconfigure the battery pack to provide 3 volts instead of six volts.No regulator required.