Controlling coil power supply for cat water fountain

[symbianas]

Hi, I have bought cat water fountain which has wireless powered water pump. It is not very "Smart" fountain, just plugs into USB and it is ON forever. However it somehow gets the state of the "water empty" from the pump, maybe it measures coil power draw?.. I would like to make this fountain "Smarter" by attaching ESP32 or STM32 and controlling its ON/OFF by motion sensor.

I thought I could attach transistor to one of the coil wires and it should trigger coil ON/OFF if I power transistors Base? I had 2N5551 transistor + 240ohm resistor available so I have attached it and powered Base with 3.3v, but pump does not power on and indicator blinks Blue indicating "water empty". Its the same if I cut one of the coil wires, I get Blue light blinking indicating "water empty". I have attached water and its not empty - so It does not work...

Is it OK, to use transistor for this? Or should I use something else? Why it does not work with transistor? Any help?

Thanks.

[Manul]

Why switch the coil in the first place? Why not simply switch 5V supply to the entire circuit board which controls the coil? Seems like more straightforward approach. In that case you can be sure, that you can use ground as a reference point for your control signal, which is convenient. Cause coil output might not be referenced to ground and if it's not, that's an issue. Furthermore you won't potentially disturb coil sensing.

[symbianas]

Why switch the coil in the first place? Why not simply switch 5V supply to the entire circuit board which controls the coil? Seems like more straightforward approach. In that case you can be sure, that you can use ground as a reference point for your control signal, which is convenient. Cause coil output might not be referenced to ground and if it's not, that's an issue. Furthermore you won't potentially disturb coil sensing.

Its an option, but sadly on startup it switches coil on and off several times for some reason, wanted to avoid that, so decided to control coil instead. Is there any other way? Maybe a relay would work instead of transistor?

[Manul]

Is one of the two coil output points directly connected to either gnd or +5?

[symbianas]

Is one of the two coil output points directly connected to either gnd or +5?

No, it does not seem to be connected directly to gnd or +5v. It seems to be connected to first chip and capacitors. maybe somewhere else too. So I guess transistor wont work, yes? how about relay then? Is optocoupler an option?

[Manul]

Optocoupler is not an option. Small SSR - maybe. Try with a simple electromechanical relay. Also you might be able to tap somewhere into the existing circuit and control it, but that requires reverse engineering and some expertise. The PCB looks like conformal coated, so it's not very inviting to mess with. Just try a relay.

[Kim Christensen]

I suspect that the coil is driven with an H-bridge, or similar configuration, which means that it has AC current flow through it. Hence, a simple bipolar transistor switch isn't going to work.
Probably the easiest thing to do would be to scope the drive signal to the coil and then recreate this signal with your own circuitry.

What's the part numbers on the 2 ICs on the original PCB?

[symbianas]

I suspect that the coil is driven with an H-bridge, or similar configuration, which means that it has AC current flow through it. Hence, a simple bipolar transistor switch isn't going to work.
Probably the easiest thing to do would be to scope the drive signal to the coil and then recreate this signal with your own circuitry.

What's the part numbers on the 2 ICs on the original PCB?

I was interested to see those number too, but it seems that those 2 8 leg chips are without naming/numbers.
I was gooling how to power the pump but it no luck. I found some documentation about pump, but maybe thats not much:
https://device.report/manual/6938016
From documentation it seems that input is 5V DC?

[Kim Christensen]

The 5v spec is for what feeds the little PCB that you see.

Looks to be something similar to a wireless charging coil for a smartphone. Except the energy ends up running a pump motor rather than charging a battery in a phone.
So yes, simply putting an oscilloscope across the coil (When it's connected in it's original configuration) and measuring the waveform there, when the pump is running, would tell you how to drive the coil directly.

[symbianas]

The 5v spec is for what feeds the little PCB that you see.

Looks to be something similar to a wireless charging coil for a smartphone. Except the energy ends up running a pump motor rather than charging a battery in a phone.
So yes, simply putting an oscilloscope across the coil (When it's connected in it's original configuration) and measuring the waveform there, when the pump is running, would tell you how to drive the coil directly.

Sadly I don't have oscilloscope. What would be the cheapest and decent Oscilloscope?
Do you think a SSL relay would work? I need to buy DC-AC relay?

[Kim Christensen]

A solid state relay might work. The problem is we don't know the frequency at which the coil is pulsed but a typical frequency range is 110-205Khz for wireless charging. A solid state relay will have an upper frequency limit, so you'd need to make sure that the one you select will work.
Or you could use a mechanical relay.

[symbianas]

It works with relay! But it works exactly the same as on regular startups (it switches coil on and off several times). So there is no relason to cut it at the coil, its the same as cutting initial power 5V at beginning with transistor... I hop cat will get used to it and wont be scared.

[symbianas]

would be nice to learn to drive that coil. Also would need to get the status if water is empty...

[symbianas]

So yes, simply putting an oscilloscope across the coil (When it's connected in it's original configuration) and measuring the waveform there, when the pump is running, would tell you how to drive the coil directly.

Would this be possible with logic analizer? I have: Kingst LA1010 - Input voltage range: -50 ~ +50 V

I guess I could build oscilloscope from STM32 is input values are <5V?

[Kim Christensen]

No, a logic analyzer won't help you here.

It would be better for you to buy one of those cheap MCU based scopes rather than trying to make your own.
But if you're planning on doing more sophisticated work in the future, spending money on a more capable scope would be worth it.

Not a recommendation, but here is an example of a cheap handheld scope reviewed by our host: