At a recent channel meetup on the isle of man uh, i was given a couple of ultrasonic pest repeller devices that are apparently faulty they used to light up. They don't light up anymore, having plugged them in. I can hear the click of the ultrasonics uh obvious ultrasonic. You can't hear it.

Let me just zoom down this. That's a good start. Obviously, you can't hear ultrasonic sound because it's beyond human hearing, but you can hear a click from this, which does suggest a sweeping frequency. Perhaps so these two units appear to be identical, except for the logo on them.

If i plug one of these in and let you hear it - i don't know if you're hearing me, hopefully it won't swamp the microphone out in the device, i'm not sure how sensitive it is to ultrasound. Well, let's find out, i shall plug it in and i shall hold it up to the microphone of the phone now, if you're hearing what i'm hearing, i'm not sure what will actually come through that it's a clicking noise, so that does suggest that they're active. Let's open it up and see what circuitry is inside, it's not going to fit um. I forgot what we got this one.

Oh dude, bright white, not great for filming. It just tends to swamp out a little bit. It's a tricky balance for exposure. Is it clipped? Well, it's not too badly clipped.

So there is an ultrasonic disc uh. There's the chip there's what looks like a capacitive driver. Capacitor right. Tell you what i'm going to take a picture of this circuit board, so we can get a much closer look and we'll reverse engineer it.

One moment please here we have the circuit board. So what i'm seeing here, let's just zoom down a little tiny bit. I will do some tests as well, so we get live and neutral come in and we've got another side of the circuit board, actually i'll whip the camera. No, i won't whip the cover off i'm about to plug this in that's a bit shitty, but there is a capacitor on the other side and then there's a couple of discharge resistors across it and there's a 100 ohm resistor tucked in under here, which goes from The neutral to the other end of the bridge rectifier - and that means it's basically a capacitive dropper on the output, there's a zener diode and then a smoothing, capacitor and then absorb decoupling capacitor over here, just to actually protect the microcontroller for from little transient glitches, uh Different countries have different names.

This capacitor, it seems to be a controversial thing. However, let's plug this in and to do that relatively safely. Let's bring the meter in. I want to measure the voltage in that i'm going to guess that it's going to be 5 volts because it's going straight to the microcontroller and that would make sense.

So i shall plug this in to the happy and it's not going to register anything. I tried it and i'll just zoom out for this a little bit. I shall plug it in and then i shall probe directly across the electrolytic in there. Noting this whole thing is live, including the piezoelectric sounder in there, so the this should be the positive, and this should be the negative we have just over five volts, okay, so i'm guessing.

This might actually be a 5.1 volt zener diode, which is a very standard value and there'll, be a bit of ripple which would make that look a bit lower again right now, i can take it out. I can unplug this i'll, give it time to discharge. Has it discharged yet? Yes, it has so a couple of screws holding the circuit board in and that will reveal the capacitor on the back and the resistor and the electrolytic capacitor. So here we have let's zoom down the dropper capacitor, which is a value of nano 560 nano 400 volt.

It's got the 100 ohm resistor brown, black brown, one zero and one as a additional zero and then uh we've got the electrolytic capacitor, which is 220 fired at 16 volts. It's only been used at about 5 volts, but 16 volts. This is sort of a sort of minimum value. It's a generic value for these, let's zoom back out and explore the circuitry, so we have the capacitive dropper.

We have a microcontroller here and it's only using two pins. What is under that black thing? I'm pretty sure. It's an led, it is an led uh. It's a little white led look of it.

I bet that's failed, where's, the meter. Let's stick the meter across that that should give an indication if it's still working or not. Usually you can make the led light. So the positive end will be here: a negative end will be there, it's showing an unusually low voltage across it, and it's not lighting.

That means it's. A dead white led how very typical of shitty white leds excellent. So that's an led under there i'll just draw that in on top there's an led under here, the piezoelectric sounder speaker, one speaker, speaker, minus speaker, plus, is driven by this transistor, which is a standard npn transistor um. Oh, what the heck! Let's cut straight to the schematic and i'm going to break with tradition here, i'm going to actually draw well.

It is actually it's going back to the beginning of this channel. When i just reversed engineer engineered things in the fly, this is looking a bit ferocious. That's all right, i'll use my special pen that stops a notepad, that's an idea, so we've got the incoming supply, live and neutral, there's a sine wave showing that it's the mains and the first thing it hits. Is this dropper capacitor with the resistors across it? So it comes in and there's a dropper capacitor and across that are two resistors.

The resistors are 470k each. This is this actually hints at goodish design, actually so 470k times two gives roughly say close to one mega ohm, but uh close enough 560 nano farad, 400 volt classic capacitor on the neutral we're going via this resistor. Here this little metal film resistor here, which doubles up as both inrush surge limiter and also a fuse when things go horribly wrong. Although, having said that, i looked this up online to see if i could find if they were still about and uh someone had a picture of one on amazon, which was clearly exploded and it was very suited it hadn't uh survived very well at all the whole Water, just basically splattered everywhere nice.

I come to think of it that disk dropped out on it. It's possible. The disc had dropped out and shorted out the uh yeah. That would have been exciting.

That would have worked. That would have done it. So here's my lazy, rectifier, here's positive here's negative here's, the 100 ohm resistor the output, then, is the positive rail and the negative rail which we shall refer to as five volts because it is clamped down to that and zero volts. The zero volt is the circuit reference.

It's not zero volts with respect to earth because it's actually directly referenced the means here, and i know the first thing it actually hits. Is this zener diode? So let's draw a zener or zener diode in depending how you like to pronounce it. Apparently the person invented it is family referred to him as zener, but it's a controversial area again. It's the world is full of controversy.

Then there's a smoothing capacitor a little electrolytic, which is rated 220 mega fart 220 micro farad at 16 volts and that's the old we'll see this is just 5 volts. This is in our diode, then there's a little decoupling, capacitor i'll just squeeze in here, and if i probe through on this circuit board from this side to this side, these power rails run straight underneath so there's actually a 1k resistor across that as well. This is probably to ensure a good, solid reset when the power's turned off. I would say that 1k is actually quite low.

I think 10k would have been better flat, particularly given its base in a capacitive dropper. But after that it goes to the micro controller. Doesn't everything it's like microcontrollers have become they've, taken a lot of the fun out of electronics. So that's 1k, because they're boring m c: u the little black box, it's all the software and that is doing the work on the output of the microcontroller.

We have two outputs: we've got the one for the led and it's going from the positive rail through a resistor, 470 ohm, four, seven one, four, seven and one zero 470 ohm through that little resistor led to the chip so we'll just draw up here. It's going through a resistor and through an led, a dead led to the chip and that's 470 ohms and the other output, because there are only two outputs from this. Chip goes to a uh transistor uh, npn transistor and a uh 1am, which is uh the 3904. Probably 2n.

3904. Really so that's uh hold on i'll just tilt this at an angle. That seems inappropriate to worry it's what i did and that goes to my controller. That's an output, and that is a 1k resistor, driving that the 1k resistor is a clue that it's not a mosfet or something like that.

But then again so is the name of the transistor. Then we have a bit of an oddity from the positive speed. The negative connection of the speaker is connected to the transistor, but there's also a resistor connect across that 390 ohm for a very specific reason, i'll get into in a moment, but there's also a 100 ohm resistor, probably to limit the current spikes. So 100 ohm resistor, 101, 100 ohm and then it splits and goes through a resistor in parallel with the sounder.

The piezoelectric crystal the resistor value is 3.391, so that's 390 ohms. Ah, that's it. I think i've drawn everything that is the circuit. That's all we're getting here right here so to explore this, i'm just going to zoom down a little bit standard, capacitive dropper with inrush limiter there.

This basically comprises a 5v power supply, but also when you turn the power off, this resistor will make sure it goes right down to zero very quickly, ensuring a nice clean reset of the microcontroller. If you don't do that, sometimes they can crash this uh led. I don't know, does it it's microcontroller control, does it flash we could? We could replace that we could uh yeah we'll do that afterwards, we'll whop that led off because it wasn't working the other one either and we'll put another led across and see what happens. So.

The microcontroller is pulsing, this uh transistor, which then pulses this crystal. This limits the inrush, because that crystal acts a bit like a capacitor, and that's also, why there's a resistor across it, because if it wasn't across it, basically it would kind of put a charge across that disc, a ceramic disc and it just it would initially make A noise, but then that would be it so it needs a sort of load resistor across either that or you drive it push pull button for simplicity here. They've just simply used basically a discharge resistor across that not very efficient, but it will do the job um and that's it it's not much to it. Right.

Tell you what um i'm going to get this oscilloscope out, i'm going to scope this and see what it's producing, because i think that is possibly a ramped high frequency waveform. But the only way we're going to find out what frequency it covers is to grab the key site and connect it to it. One moment please - and this is what the keysight oscilloscope shows us. If you look at the frequency down here, you'll see that it's gradually stepping you can see it's stepping the waveform, but it's stepping up higher and higher to 66 kilohertz, and then it will suddenly reset down boom to 24 kilohertz, and it will start the cycle again And it gradually sweeps the tone over the course of approximately one minute and 36 seconds and every so often you'll see a slight glitch in the waveform.

And you can actually audibly hear that from the sounder. As i click because it's pre it's rescaling in the software, because each of those sections of this square wave actually has a reducing multiple of repeat cycles, and once they get down to a certain level, then they have to decrease the length of each of those cycles. And then increase the total number of them again then start counting down through them again. So you'll see a big glitch when that happens.

However, it's not a nice round binary multiple. It may have been done as an algorithm, but it's possible. It was just done as a lookup table and you can see that glitch and hear the click when it happens, but one of them had about 44 steps and another had 23. So it's kind of almost like manually done just to actually get a fairly linear frequency change over a broad spectrum, and i guess the point.

This is they're trying to say, for instance, a cockroach or whatever has a peak of frequency at 33 kilohertz. At some point. It will hit that frequency and it'll be really annoying, but each particular critter they're aiming at could have a random frequency that it hits that it is more sensitive to and by gradually sweeping uh through all those frequencies they're, hopefully going to find one that annoys the Rodent, but there we go no, i was expecting. That is how it works.

It's about to reset again click there. It goes. Okay, let's get close, put an led in to replace this surface mount one. I don't have one this size i might have somewhere, but but they're very tiny.

You know very hard to find. Most of mine are one two, oh six size, and this is not. This is much smaller, so let's desolder it by throwing a bit of sodium one end, the a little bit of solder in the other and then heating, both at once and just sliding it off like that. Put a bit more fresh solder onto the pads, then i'll get my donor led, which is most certainly not surface mount.

I shall crop the led leads down and tin them. So i'm just going to grip this carefully. So i don't put my fingers on the pins while i'm heating them up, i'm going to put a bit of tinning in that one and on this one a bit reckless, but not to worry it's fine, so just double check polarity. So i don't get the wrong way around and make a full of myself.

I make a fool of myself quite a lot. That's fine! Uh! Now i shall sort of that end and this end okay, ugly, but i just don't care right. What can i use uh? Well, that's cooler! I show you as long as power to just gently bend that led up without actually snapping it off. Well, actually, i'm more concerned about snapping the tracks off talking of snapping tracks off nice enough circuit board, good separation, anti-tracking slots, nice, clear design.

It's not bad, absolutely horrific power factor. The hoppy refuses to even measure the power factor talk of the hobby. Let's bring it up plug this into the then plug the hoppy in so i shall just dangle it like this, so we can see the led does make sure it's not near anything live, and i shall plug that in noting that this is live. So i won't go near it.

The led is dimmed up. Oh and it's dimming out again. Oh is this one of those apple style, breathing leds oh hold on. Let me turn the light off here.

It's getting brighter and then it's getting dimmer it's one of those breathing type, led's effects. Oh that's! Quite interesting, watch your eyes. The light is coming back. That's quite interesting because that how did they implement that in software? Oh, i know what it would have been.

The steps of the pulse of modulation are probably also along the lines of the frequency output, not sure about that. Oh, that is quite complex. That is a sneaky piece of software. This is unplugged before i unplug this that's quite complex.

Now these things are controversial. Uh, the manufactures of them have been hauled over the cold in the past and some are warned and just kept putting them out the same claims. In reality, independent tests have shown that they have at the very best, a temporary effect on repelling rodents, because imagine, if you're watching the facts with a lot of noise or he works in nightclub or in a theme park with the same loop of music. Going on all the time after a while, you just get used to it in the background and you ignore it and it's just your brain, just mutes it out and that's probably what happens those as well.

That's certainly what they that's, what they thought the researchers, but when you look at listings for these, you find people glowing enthusiastic about how wonderful and how they just turned on and all the rodents disappeared. So i'm not sure about that. Well, look! Look at this where they've sewed onto the back of this they've damaged the surface, but it still works. It's actually gone they've taken some of the metallization off.

You have to be really careful with that. The soldering onto these discs pretty much. Maybe a low melting point solder, but pretty much soldering, someone and just basically dabbing the iron on just to get a pool and then getting the wire on just dabbing it just to actually just reflow as quickly as possible without damaging the the conductive layer. But there we go, it's uh very minimalist, it's not going to be allowed.

Goodness knows it's going to have an effect, but i suppose ultimately, if you plug it in you'll, get a a plug that does what they said. It puts out ultrasonic ramping frequencies and it's got a little led that just glows up and down apple stylee, so um yeah. I don't know, let me know in the comments down below if you've ever used, one of these and if it had any effect or if you think it's a worthy technology or if it's a scam, keep in mind that i'm pretty sure i've opened some of these. In the past that had nothing in them other than led that had the disc in the front, but not connected to anything, it just seems to be a common scam on ebay,.

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