 I have this new oven thermometer. You can tell it's an oven thermometer because it says oven thermometer here on the label. What this is all about is I did try to order an oven thermometer of AliExpress and they sent me this instead and unfortunately despite proof that what they had sent me was in fact a multimeter they refused to refund me. So it only cost about three francs. So I thought it would be interesting or at least amusing just take a look at this and see how bad a three franc multimeter or as the manual says sunlight tester actually is. So what do you get? Well it's a fairly shoddy analog multimeter with fixed probes that are actually commendably pointy. That glare is in a very annoying place. A nice big analog meter. This rotary dial selects functions. You've got DC voltmeter. I don't know what that is. It says DC volts null. AC voltmeter off a capacitance and resistance meter. I'm not quite sure how that works. I could always look up the manual but I can't be bothered. The construction is a fairly shoddy single box held together by one screw. So let me just take the screw out well. Not much. Though interestingly the retaining strap which should be here was inside. That was very strange. In terms of actual components the battery is supposed to go here. Battery does not appear to be present. I'm not very surprised the battery is probably the single most valuable thing here but that means that the capacitance and resistance meter wouldn't work. There is a fuse. If you use this on anything where the fuse might blow you're in a world of pain. It says 5 amps 250 volts. Whether that's true or not I don't know. There is a single PCB held in by clips that shows the meter motion. It's not actually showing up on camera. You can see the coil there. The meter might be possibly useful. This is the potentiometer for the resistance adjustment. And there is all the works of an analog multimeter. Not a lot really. The way it will work is that this switch element this rotates moving these copper contacts. The copper contacts will then bridge these circular tracks and close particular circuits. That will connect the probe's output here through an assortment of resistors to the meter itself. To measure current it will just put the meter in series. With this to measure voltage it will put a big resistor across these and then measure the current through that resistor. So what is there here that we can actually salvage? Probably not very much. The most valuable bit is probably the box which may make a reasonable project box. But let me just disconnect the power wires. The power for these meters is used by the capacitance and the resistance meter. Oh sorry those weren't the power wires that was the meter itself. The power wires connect to here I believe. Yeah that's that three volt label in a second. This is connected to the lower terminal. If you notice these two terminals the top one is labeled plus and the bottom one is labeled minus and the red wire is connected to minus. So that frees up the PCB from the actual box itself. Let's just get that out of the way. So yeah that is in fact the negative connection. You see it is labeled minus here in the battery compartment. But they have used a red wire instead of a black wire which is very naughty. So if I'm actually going to use this again I do not want that. So let's just take these wires off put them back on the other way round. Because having miscolored wires like that means that the next person to use this box is invariably going to hook the wires up the wrong way round and explode something. So let's take a look at this PCB. Well we've got a couple of resistors on the back. Three in fact and a few bits of wire though I suspect that the most interesting bit is going to be the these probes. Because they are actually commendably sharp. It does say here 1000 volts. But if you try to put a thousand volts across these then I think you deserve everything that's coming to you. So can I desolder these? Yeah they are not actually far from done very well. They are multi-strand copper. I think copper. They might they might be something else like tin aluminium. But they did desolder reasonably well so I suspect they are actually thin and shoddy copper. But they will actually come in useful. I can easily put some like banana plugs on these and use them as oscilloscope probes. So that will come in a little bit handy. There's the potentiometer which it says is a 10k either log or linear I don't know. That will come off and go into a parts draw. That came off far too easily. I don't think that was actually soldered down properly. You can actually see from these pads here that the solder only goes part way around the pad. It's supposed to like cover it. So we have a pot. The fuse holder might come in handy. I think I've actually got a number of these but these are actually soldered down reasonably well. So these will be a pig to take off or not. I think that's very thin metal and it's not actually soaking up much heat. So what can I actually do with this? Well the two interesting bits are the meter which is like a perfectly serviceable if very cheap moving needle meter with a commendably big screen I have to say. And the and this switch now it would actually be fairly straightforward to isolate the various tracks here and use this as a selector switch. It's got a decent number of positions. It would just be a matter of either scratching off the track or more likely just removing and discarding some of these surface mount components which would then isolate the tracks. For example this position here connects together these two pads and these two pads. So removing this here and then connecting a like ground to this would actually allow me to easily select various things. The obvious thing to do is just feed all these into a microcontroller. In terms of the inner and outer rings the outer ring is just simple single positions. So I'm just looking at this where there is nothing. That's the off position isn't it? Off is here so the board must go in this way around. So in that position none of these contacts are actually touching copper anywhere. For the inner tracks we've actually got four pairs of contacts so that these two, these two, these two and these two. So the inner two right the inner two is simple power on off so the only position where they are not connected is in the off position. Right in fact it goes this way around. Okay so you've got off on this side so these things are actually connected down to the resistance section. So this ring has probably got to do with enabling the resistance meter circuitry. Interesting that there's a track here. Ah right it connects these two together so this will be some kind of common signal likewise here that is then connected to one of these other things. Now you notice there is in fact a connection across there. I would need to actually sit down and map all this but I can imagine this being of use. The inner two rings can be used to power up a microcontroller. The outer two rings for selecting one of several GPIOs maybe although there's enough of these wiring and GPIO up to each one will be a waste. You'd want to use some kind of port multiplier. Anyway enough of that. What can we do with the meter? Ha drop proof it says. I'm sure that this hard plastic thing is drop proof in much the same way that Faberge eggs are drop proof. Let's take a look at this meter. This is a standard moving coil meter. It's part of a whole world of analog electronics I'm not really very good at but there's this moving coil here around a magnet. Current is induced in the coil that causes torque against the magnet. The needle is fastened to the coil. Now as far as I'm aware these things measure current not voltage and they're also very delicate so they measure small amounts of current and if you put too much through it the coil burns out and is now useless. So I want a source of low current electricity to try this with and turns out multimeters are a very good source of this. In resistance mode which I see this has been left on I hope the battery hasn't gone flat it will generate a current limited voltage that does seem to be working and this is used to it measures the voltage drop and that's used to measure the resistance whatever you connect it to so there we go. You can see the coil move or less. So the tiny amount of current this thing is producing is getting most of a full deflection so I need to figure out how much that is. Here I have my real multimeter hooked up in series to this so by connecting this one up this by the way was my old multimeter before I went to this digital one I see a current of about 40 microamps that's not a lot. So if I want to work the meter from something that takes say five volts I need to make sure that the current going through the meter at five volts doesn't go above about 50 microamps. So time to do some maths. Simple resistive devices like this meter are governed by Ohm's law which is v equals i r saying that the voltage dropped across a resistor is equal to the current times the resistance. We actually want to know the resistance given the voltage and the current so if we write it like this v i r in a triangle this gives a really easy way to reform this into other things. We want to know the resistance we have the voltage and the current so that is r equals v over i so our voltage target voltage is about five volts and our current is 50 microamps so that should be a fairly simple division. Pulling out the calculator it's five divided by 50 e minus six is 100,000 ohms or 100 k so if we put a 100 k resistor in series with the meter and then apply five volts across that we should get a current of 50 microamps which should be enough to move the needle. I've soldered on a 100 k resistor in series of meter from my stash and hooked it up to my bench power supply it's currently set to 20 millivolts let's turn this on nothing happens which is good so let's crank the voltage up let's turn that up to up there it's moving that's one volt okay this means we're running roughly the right range so two volts three volts four volts five volts excellent and we are seeing more or less full deflection five four three two one zero moving very slowly good i'm quite relieved i was a little worried that i'd get my math wrong and burn the meter out so that's great the reason i've done this is it has occurs to me that this might actually make quite a nice little project box we have the selector switch and we figured out that the PCB here can be used both as an on-off switch and to detect individual settings and we have the meter which works nicely at five volts inside we've got a battery compartment for two double A batteries giving three volts there's actually quite a lot of spare space it's moderately thick the meter here seems to be the highest part of it there's in fact a special little meter lid here to protect the moving parts but there's space here including these two little standoffs and space around here on which a PCB can be put a small one there's this thing here which was used for the zero ohm calibration this plugs into this pot that goes through from underneath and this could be very easily swapped out for a shaft encoder thing i've got one there's plenty of space for putting more buttons on and so on but it occurs to me that if i've wanted to build a meter for something this would be a very useful box for doing it on for example a wi-fi signal strength tester select channel here and it displays it on the analog meter i would want to change the label this plastic lid should just clip on like so and this reveals in a nice bit of detail the actual meter you can see here's the needle attached to the coil it's all very delicate this thing here is used for adjusting zero position there's a thing here that drops a pin into that slot and you can then move it backwards and forwards and that will recalibrate the meter i'm not going to fiddle with that the label here is under this so replacing it would be a little bit fraught because you end up having to remove part of the meter which as i said is very delicate but it would be easy enough just to stick a label on top here obviously under the needle so i think that this is going to go into a draw in case i ever have a project that wants a meter because it's going to actually go back on again yes it does because i think this could be quite useful if i can figure out how it all goes back together again of course that way up and so i think i will keep my nice new oven thermometer it will come in useful if ever i need a project that involves making a meter of some description because it is a meter it's got all the user interface bits there's a decent amount of space inside that's even this compartment here which could be used to put more stuff in i can imagine like a 3d printed thing that goes here with sockets in it i don't know at all what i would actually do with it at this point but it'll go into the parts box to come in useful at some point in the future anyway i hope you've enjoyed this rather shambolic tour through the innards of a very cheap multimeter i should add this is get essentially the same thing except slightly better made it will be a single pcb meter adjustment pot probes there's nothing particularly exotic about this these could well have worked perfectly well as a cheap multimeter but i've already got one and i have a much better but still quite cheap multimeter so i hope you enjoyed this video as always please let me know what you think in the comments