Water Lvl Monitor w/ Latching Solenoid Controller

I've been working on the REV1 enclosure and had to brush off some old CAD skills. Whenever I do 3D modeling I try to do lots of simple test prints... Measure once, print twice I guess. It's much faster than printing the final enclosure and really helps tighten up the design feedback loop. Below are 4 simple test prints, followed by the first prototype of the full enclosure.

The enclosure will need to be water tight. Water might leak at the (1) lid-case interface, (2) solenoid-case interface, and (3) through the case itself. To avoid (1) I'd like to use either a o-ring or some type of silicone/epoxy in the groove along the top of the case. For (2) I can easily put an o-ring around the base of the solenoid. And to avoid (3) I'll need to coat the cavity with some kind of a sealant, since PLA isn't exactly water tight.

The prototype was successfully deployed, and just in time too, as you can see the well was flooded again when I arrived. The setup is very make-shift, but only needs to work for 2-months. I’ll continue to work on a more polished design for next summer.

There were a couple integration headaches to deal with, mainly finding couplers to connect the hose to the ½’’ pipe threading on my solenoid. Even with the couplers, the hose was fairly leaky so I’m a bit worried about water getting into my enclosure. Luckily I had some marine sealant/epoxy.

For the next (non-prototype) version I think I’ll try to mount the PCB directly to the solenoid and place a gasket against its side to seal it off. I’ll also need to find a better way to set the float switch’s position. I’m designing a PCB on the side here, so once I’m about to order I’ll post an update. I expect it’ll take awhile though, especially since I have to do some 3D modelling.

How will I power this board? I need at least 5V for the solenoid, and the ATTINY can run off 1V8 to 5V with varied restrictions to its performance. Ideally, I’d just include a low quiescent buck-converter to converter a 9V battery to 5V for the whole system. Sadly, I don’t have a converter handy in my parts bin, so here we are…

The solenoid was handled in by the original circuit posted here. 9V Vbat will be current limited to ~500mA regardless of the input voltage (within reason!). That leaves the ATTINY and a 9V battery, with no on-hand switchers or even LDO’s that fit the bill. All my LDO’s either didn’t support >7V input, or had a minimum load requirement that would squash the battery life of the board.

What I did have on-hand was a boost converter (BU33UV7NUX) on the same dev board as my ATTINY. So  I decided to add another battery, two actually (2x AA). This new battery will feed the boost converter that will generate a 3V3 supply for my ATTINY. The boost converter can operate all the way down to 0.6V!

Sadly, this plan didn’t last long...

So, I immediately killed my boost converter dev board *face palm*. I hummed and hawed for a while and decided to do something dirty, but easy. Why even bother regulating the supply! Two double A’s will get me around 3.2V -> 2.4V during their discharge. That’s well within the operating range of my ATTINY. I tested the circuit at the new range of Vin, and everything was still functional. 

Here’s the system running off battery. Everything’s working okay, though at higher quiescent currents then I measured previously. Originally I measured <1uA on the ATTINY, but now I measure 300uA. I suspect I tweaked something in my code during debugging. I’ll hunt this down later. (even at 300uA the current draw isn’t enough to drain the device over the summer, so I could leave it as is… seeing as this is just a prototype).

I always regret going down the perf board route. Too many flywire’s and last minute bodge jobs. But since this project is on a timeline, I don’t have much of an option. The circuit hasn’t changed from the last picture, but now includes an ATTINY1616 to tickle the various control lines. See my first project log for the circuit.

I decided to first sketch the circuit out to roughly floor plan the parts & wiring. The final result looks a bit like the ramblings of a madman, but thanks to the floor planning there were no major miswirings or debugging required.

And here’s what that looks like in reality…

I quickly ran into an issue with the ADC leakage current. The leakage current from VBAT to PB3 through the 1Meg resistor, pulled the measured signal down to the almost nothing. In the future design I’ll include a buffer to resolve this issue… but for this perf board version I might just ignore the feature. TBD.

I have the hardware figured out and breadboarded, next thing to flesh out is the FW. It’s a VERY simple program. Here’s 4 main basic states.

• Measure Battery Voltage
• Poll water gauge
  • To save on battery, I’m going to “poll” the float switch (no pullups!)
  • Send signal on COM and look for signal on either of the two remaining wires
• Set Solenoid
  • Short 30ms pulse
  • Polarity of pulse dictates whether solenoid opens or closes
• Sleep & Repeat
  • Go to deep deep sleep

• If voltage is too low we might decide to shut off the solenoid as a preventative measure… we don’t want to risk losing power while the solenoid is open!
• We could also monitor battery drain to gauge faults in the system

I wanted the main data structure to be circular. So, I made a linked list that sort of feeds into itself… this way I don’t have to know the exact location of any data node, and I navigate the structure using node->next or node->last.

This way I (1) don’t have to keep track of which element I’m on and (2) I get to retain some history of events that I’ve logged. I suspect there’s a less obtuse way of storing this data, but for now this will work (I’m just a HW guy after all!).

Here’s the main data structure.

struct DataNode{
  //Debug Data
  bool Init_FLAG = false;              // If initizlized is false, data is potentially NULL
  uint8_t ArrayElem;                   // Defines which element of the array this is

  // Data
  uint16_t BatteryLevel;                    //  Measured battery voltage [RAW]
  SOLENOID SolenoidState = UNDEFINED;      //  State that was written to the solenoid

  // Pointers
  struct DataNode* last;                  // Pointer to the lastelement
  struct DataNode* next;                  // Pointer to the nextelement
};

 I have to do some initialization leg work to link the “last” and “next” pointers as shown.

  // ============ Variables ============
  static struct DataNode HistoricData[8];    //MUST BE SIZE=8
  struct DataNode* DataPtr = &HistoricData[0];
  uint8_t i = 0;

  // Initialize HistoricData[]
  for(i = 1; i<8; i++){
    HistoricData[i].last = &HistoricData[i-1];
    HistoricData[i].ArrayElem = i;
  }for(i = 0; i<7; i++){ 
    HistoricData[i].next = &HistoricData[i+1];
  }
  HistoricData[0].last = &HistoricData[7];
  HistoricData[7].next = &HistoricData[0];

I originally had aspirations to make this a nicely polished project, but I'm a little too busy to make this a fully fleshed out design before the summer heat really hits. The core of this design is a latching solenoid, and a water gauge. A latching solenoid was critical since this widget will be running off batteries. It allows you to toggle the state of the solenoid by sending a short 5V pulse, once the state has changed the device can be unpowered and it'll hold wherever it was left. 

The water gauge, or float switch, is pretty simple; its a buoyant cavity on the end of a wire. Inside the cavity is a heavy ball bearing that opens/closes a switch based on the orientation of the cavity. When the water rises, the object floats up allowing the ball bearing to roll to the opposite side of the cavity.

All that I'm missing is a cheap micro to control everything. Luckily, I have an ATTINY1616 on hand so I'll use that... to be honest, it would be a fun project to replace this entire design with 74 series logic, but given my time crunch I went the "keep it simple stupid" route.

H-Bridge!!!

Who'd of guessed.... I need an H-bridge. My supply of P-channel mosfets is VERY low, so I'll have to get by with BJT's instead. It's not ideal, but given the solenoids relatively low power requirements I should be fine (5V @500mA for 30ms). I had to dig fairly deep for a good npn/pnp pair, but I finally found the SS8050/SS8550. They can handle the current, but I’m a little concerned by their Vbe rating… I’ll definitely need to scrap together a breadboarded prototype. Here's the somewhat finalized circuit.

I’m running it off an unregulated 9V battery, so care to be taken. I added a current limiting circuit (Q7/Q8), to better maintain the 500mA load across the batteries discharge curve.

Since I’m worried about power consumption, I added a low side switch that completely disconnects the circuit when its not in use (I might remove this later, depending on how tests look). The hope being it would stop any leakage through this circuit, but I’m suspicious it may do more harm then good.

Lastly, I flipped the polarity on my upper transistors using NPN’s. I wouldn’t be able to drive the PNP’s since my MCU can only output 3V3. The emitter of Q1/Q2 will be somewhere b/w 9V to 5V, so I’d never be able to turn either one off. This has the added benefit of nicely matching the 4x bases of my H-Bridge. Q1 and Q4 can now have their control signals tied together (thanks to Q5’s inversion), and same goes for Q2 and Q3.

Current Limiter Test

A quick breadboard test of the current limiter. I connected “VBAT” to a PSU at 9.5V, and connected the output of the current limiter to an ELOAD. At this point I wasn’t sure if I wanted R5 to be on the smaller side, so I tested a couple values shown below. I expected ~70mA current limit, and that’s more or less what I saw. I’d like to do a quick sanity at temperature before I deploy the widget… I know Vbe can vary with temperature, but I forget with what sort of magnitude. (TODO - Investigate)

The automation only took a view lines of code thanks to an SCPI library I’ve been working on. If you haven’t poked PyVISA yet, I’d HIGHLY recommend it!

#Overview:
# Testing the current limiting of my BJT circuit - July2024

from DriversPy.ELoad import *
USER_Eload_ip = "192.168.1.13"

eload = ELoad(USER_Eload_ip, DBG_Print = False)
eload.LOAD_OFF()

# VARS
CR_Values = list(range(0,200,5))
CR_Values  = [x / 1000 for x in CR_Values ]

# Test CC
eload.SET_MODE(LoadType.CC)
eload.SET_CR_VAL(10000)
eload.LOAD_ON()


for R_VAL in CR_Values:
    sleep(1)
    eload.SET_CC_VAL(R_VAL)
    sleep(0.5)
    I = eload.GET_Current()
    V = eload.GET_Voltage()
    print(str(R_VAL) + " " + str(I)+" " + str(V))

eload.LOAD_OFF()

End Of Day 1

Next Steps (1) Rough Power Analysis, (2) Finalize Code, (2) Perf Board It!

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