 The great search brought to you by Digikey made for it. This is the time of the week. Every single week in Lady 80 is a power of engineering to help you. Yes, you with your vision pro meat edition to find the things on the digikey.com site. Lady what are you looking at? The vision pro to just have like the digikey site like all the okay. No, I don't know. I don't know the projects I'm working on right now is I want to make a board using a LTE module sim 7080 which is an L N B I O T slash LTE cat M cellular module to replace the old 2G and 3G phono boards because these are way past their expiration date and part during the part shorter can get them and then of course now they're totally discontinued. So moving to a new module you know I'm familiar with this family of modules and so not a lot of stuff isn't too new. One of the things I have to do is there is a reference schematic maybe I'll I'll go down and I'll find it. There's reference schematics for you know every implementation of you know the hardware so it's like oh here's how you hook up the you know external PCM setup here so you can do I squared C you can do SPI for connecting to like SD card or something. But one thing that is interesting about these modules is they they tend to have like you know a fairly advanced you know whatever not a narrow band but like a net band processor whatever it is that communicates with the cellular network that chip usually has an IO voltage of like 2.2 or 2.8 volts it's not 3.3 and so if you want to have it for example light up an LED especially if one that's blue the 2.8 volts won't be able to do it. Now if you happen to only want red LEDs like maybe it would kind of work but the chips the chip is not intended to have high output high pin strength outputs and so in order to turn on this LED for like network status they're like hey you know here's really how you should do it you should have this transistor which is biased and you know this when this turns on this transistor syncs current it turns on the LED powered from V-BAT which is a 3.8 volt or 3.4 volt you know battery input. Why don't they use n-channel FETs you know I'm assuming is because maybe the IO voltage could be even lower sometimes maybe it's like 1.8 volts and so like we want to make sure that you can turn on a transistor transistors turn on at 0.7 volts and some FETs maybe they don't turn on to like 1.5 I don't know really why this is the reference circuit we follow the reference circuit because it's probably for a good reason. So the only thing that's annoying is it's like okay well I want to turn on the LED and like before you know it now I need one two three extra components in addition to the resistor and the LED itself and I want to keep this board small. Now one thing I learned is there is such a thing as a pre-biased VJT transistor which means that you get one three pin component that has all of these parts built in together so very handy especially if you're well you're doing cellular modules where you want this reference circuit but really anytime you're like oh man I have like in a large number of transistors I need like by the way six of these pre-biased transistors because one for each LED and then there's one for like another input or an output or the power key like there's a bunch of places that they use these transistors that are pre-biased in the reference circuit so you know I don't mind picking up maybe an array later but definitely singleton ones of these to save me from having to also have all the resistors just like less components to pick and also don't have to worry about the components being wrong because it's like cake it's built into the transistor itself so let's look for transistors and if we just click on uh yeah okay here you go so a single bipolar transistors are standard MMBT 222 PN2907 whatever but they have a whole category just for pre-biased bipolar transistors such that I don't know where else these are used I mean I've definitely like know that they're used a lot but I've only really seen them referenced in the circuit the data sheets the hardware design data sheets for cellular modules so let's check it out first off we're only going to go for active we also don't need a diode I'm assuming a diode is maybe a flyback diode so if you're using it to like power a uh a motor or something we just want it pre-biased NPN let's verify that okay I clicked away hold on it was net light there you go yeah so NPN transistor and then let's make sure that this is surface mount I don't want a through-hole version in stock and I'm going to exclude marketplace for now just to make it super easy right so the next thing is you get to pick the collector emitter breakdown so all of these transistors are going to run at no higher than like four or five volts so I don't care about this because all the more higher than my minimum sorry my maximum next up I will pick the resistor base and the resistor gain so um this is the base so the base resistor is 4.7 k so let's go here base resistor 4.7 k and then base emitter is 47 k so it gives us a gain of 10 uh right maybe not no I don't remember okay uh 47 and then apply okay cool great so now I've got 51 options um so looks like there's quite a few families so the only thing I think I would watch for is uh you know hopefully they all have the same pinouts that they're like cross compatible make sure that that's true before you like start secondary sourcing because I'm not getting you know I'm not convinced that these are all going to be the same pinout nothing else here is that important I mean you know I needed to be able to handle like 10 milliamps but all of them handle 10 milliamps these are the gain but the gain you know the gain isn't relevant for uh and these are the saturation also not relevant um okay so let's look at pricing so let's say I'm going to get a thousand a piece looks like uh the mmun 22233 is kind of the cheapest but there's actually quite a few which is nice one thing I'll notice that there's a few in this family uh LT 1g and T1g you don't know the difference between those probably just packaging though and then yeah so this is making sure this is collect this is set up the right way looks good and looks like it hasn't they actually have it in uh as little as a sot 1 1 2 3 so you can get them quite tiny I might actually you know what's funny is that normally I would say go for a sot 23 but um if they have it in sc 70 let's see what packages they've got they've got sc 70 sc 75 that's teeny so let's maybe look at one of these I think the rest of these are a little bit too small sot 70s a little bit small in the sot 23 sc 75 looks okay but these are I think these are a little too tiny for me to deal with so let's look at these okay cool so yeah we've got the SMUN 5233 also got this DTC same family just smaller package size all you know easily yeah handle 100 milliamps easily so yeah these are some good options so I think you know if I'm going with the sot 23 oh looks like the naming determines the the package size so yeah the s s mun is the sot 323 sc 70 um the DTC is the name for the sc 75 but they're all the same group so yeah either one of these uh would be good check this one out um this should save me a lot of space because I'm going to need like six or seven of these um so I'm going to pick these and make sure that I get them laid out on the board maybe by next week I'll show you the board layout for my new cellular module and that's the great search