Electrical Engineering V2 - Developers

[QUOTE=Shirky;46394871]Could anyone help me out with this question? I really dont know how to approach it. [b]A basic d’Arsonval meter has sensitivity of 50kohms/V and an internal resistance of 1.5kohms. Design a voltmeter out of this meter for a range of 10V[/b][/QUOTE] Since the sensitivity is 50,000 ohms per volt this means that full scale deflection current is 1V / 50k = 20uA. To scale the dropper resistor correctly is simply: 10V / 20uA = 500k, taking in to account the internal resistance this gives us 498.5k. [editline]3rd November 2014[/editline] Someone correct me if I screwed up.

I just realized I have to somehow power the ammeter and voltmeter separetely, if you just start plugging them in randomly, you might get wrong reading on ammeter because the LEDs on the meters suck up amps, and if your voltage is too low, the LEDs in the meters won't even power up. Interesting thing to think about. And also interesting to find cheap meters on eBay that have separate supply + measure wires.

[QUOTE=nikomo;46395184]I just realized I have to somehow power the ammeter and voltmeter separetely, if you just start plugging them in randomly, you might get wrong reading on ammeter because the LEDs on the meters suck up amps, and if your voltage is too low, the LEDs in the meters won't even power up. Interesting thing to think about. And also interesting to find cheap meters on eBay that have separate supply + measure wires.[/QUOTE] Most of the better ones do.

So I feel like I should stop making myself suffer with my $10 sparkfun soldering iron that I've had for 2 years. What's the cheapest iron that would be considered "alright" for hobby use? Eventually I'll be buying a proper station of some kind but I just need something to replace this fire hazard I've got currently.

[QUOTE=papkee;46396442]So I feel like I should stop making myself suffer with my $10 sparkfun soldering iron that I've had for 2 years. What's the cheapest iron that would be considered "alright" for hobby use? Eventually I'll be buying a proper station of some kind but I just need something to replace this fire hazard I've got currently.[/QUOTE] Just use your fire hazard a little longer until you have saved up. Buying an iron now just so you can buy another one later is a waste of money.

I've also got one of the sparkfun adjustable fire hazard. I've been eyeing a hakko but I'm not sure what the other options are.

I was looking at a Hakko 936 before someone pointed me towards [URL="uk.farnell.com/tenma/21-10115-eu/soldering-station-60w-220v-eu/dp/2062627"]this[/URL]. When I get money on Tuesday, I'm going to see how badly Farnell wants to screw me over with shipping etc., but if I can get it, I'll order it.

[QUOTE=HiddenMyst;46391338]I was playing around with my clock code some more and decided to test how fast it would run to see if I have enough headroom to add some additional animation on top of what I've already got. [media]http://www.youtube.com/watch?v=AY3aRIeAaiwp[/media][/QUOTE] I am pretty sure you could get some sick money for this clock at kickstarter :).

[QUOTE=Fourier;46397522]I am pretty sure you could get some sick money for this clock at kickstarter :).[/QUOTE] As cool as that would be, I think I'd prefer to just make them as one-offs and sell it at a market or something. Then again, I'd need to find a market where people are willing to shell out $100 for a clock or learn to create the PCB for it from scratch to cut costs.

Throw a schematic online, license as open hardware. [editline]3rd November 2014[/editline] I have to have a twilight switch done by Wednesday, but the LDR I got from the teacher is some second-hand piece of garbage that the school got cheap from somewhere. Trying to figure out the correct resistors to get it working is more like rolling a dice than science.

[media]http://www.youtube.com/watch?v=JM5aG1QAWlU[/media] Here's the current state of the clock animation. I'm pretty happy with how it looks and the colors are trivial to change if I wanted to do so. I still need to get an RTC integrated as the internal clock of the ATtiny85 isn't precise enough for actual timekeeping and I'll need to implement some way to change the time too. The overall goal will be to make a case for it, get a white acrylic faceplate to diffuse the light and give me a way to show the numbers on there. Once I get the code finalized I'll upload it for anybody that wants to mess around with it.

[QUOTE=nikomo;46396943]I was looking at a Hakko 936 before someone pointed me towards [URL="uk.farnell.com/tenma/21-10115-eu/soldering-station-60w-220v-eu/dp/2062627"]this[/URL]. When I get money on Tuesday, I'm going to see how badly Farnell wants to screw me over with shipping etc., but if I can get it, I'll order it.[/QUOTE] I have an identical looking iron I bought to keep at work for misc repairs called an Atten 938D [IMG]http://i.ebayimg.com/00/s/Mzc1WDUwMA==/z/1OcAAOSwnDZT9v5Y/$_12.JPG[/IMG] They were going for something like 45 USD shipped on ebay at some point so I bought two and gave one to a friend. They both have worked great with no issues other than putting a higher quality hakko tip on them. It is actually somewhat comparable to my more expensive iron at home. I think it's a great step up from those pencil irons.

Here's the current state of the clock animation. I'm pretty happy with how it looks and the colors are trivial to change if I wanted to do so. I still need to get an RTC integrated as the internal clock of the ATtiny85 isn't precise enough for actual timekeeping and I'll need to implement some way to change the time too. The overall goal will be to make a case for it, get a white acrylic faceplate to diffuse the light and give me a way to show the numbers on there. Once I get the code finalized I'll upload it for anybody that wants to mess around with it.[/QUOTE] How about making the minute and hour LED's fade into their position rather than jump straight into it?

I gave the transparency + iron (as in, the clothing-related tool, not the mineral, or soldering iron) method a try with a PCB I had absolutely no need for anymore. [t]http://i.imgur.com/08ZiE4V.jpg[/t] Jesus, that's actually good, considering I pretty much half-assed it. I might do that properly some day. All you have to do is not pussy out in the middle of it, and just really murder it with pressure for like a good 15 minutes.

Can someone explain how coax cable gets the 75/50 ohm designation? Since its not attached to a unit of distance I assume its not inherent to the cable but to the type of connector, but how?

[QUOTE=Cakebatyr;46400541]Can someone explain how coax cable gets the 75/50 ohm designation? Since its not attached to a unit of distance I assume its not inherent to the cable but to the type of connector, but how?[/QUOTE] Capacitance and inductance.

[QUOTE=Cakebatyr;46400541]Can someone explain how coax cable gets the 75/50 ohm designation? Since its not attached to a unit of distance I assume its not inherent to the cable but to the type of connector, but how?[/QUOTE] The reason why distance isn't a factor is because a coax that is terminated with its characteristic impedance (I.E 50 ohm) can be treated as a transmission line of infinite length where none of the signal is reflected. Characteristic impedance is determined by the capacitance and inductance of the cable, since Z0 = sqrt(L/C) the length of the cable doesn't matter since it's a fixed ratio, the capacitance and inductance is controlled mainly by the diameter of the conductor and insulator as well as the dielectric constant of the insulator and series resistance so the characteristic impedance does shift with the length of the coax and frequency giving some loss but it isn't significant unless you're talking 20+ meters or super high frequencies.

[QUOTE=Chryseus;46402790]The reason why distance isn't a factor is because a coax that is terminated with its characteristic impedance (I.E 50 ohm) can be treated as a transmission line of infinite length where none of the signal is reflected. Characteristic impedance is determined by the capacitance and inductance of the cable, since Z0 = sqrt(L/C) the length of the cable doesn't matter since it's a fixed ratio, the capacitance and inductance is controlled mainly by the diameter of the conductor and insulator as well as the dielectric constant of the insulator and series resistance so the characteristic impedance does shift with the length of the coax and frequency giving some loss but it isn't significant unless you're talking 20+ meters or super high frequencies.[/QUOTE] Usually a cable has a loss coefficient with the unit db/m.

... Fuck Farnell. Farnell UK only ships to UK, Farnell Finland requires a bloody VAT number, and Farnell Export refuses to acknowledge that there's both businesses and humans on this planet. I now have 3 Farnell accounts, and there's a non-existant company named exactly after me. But I got an order through.

do you also have to order for a minimum equivalent to 50 euros? I tried ordering something but they refused to do so as it was below that.

Depends on country, they used to have a minimum order here but now they just charge you £5 postage if it's below.

I didn't get charged for shipping on Farnell Export, and the item was 34GBP. Worth mentioning, they still haven't actually charged my debit card, it's possible they'll cancel the order. Will have to see.

[QUOTE=nikomo;46408048]I didn't get charged for shipping on Farnell Export, and the item was 34GBP. Worth mentioning, they still haven't actually charged my debit card, it's possible they'll cancel the order. Will have to see.[/QUOTE] As far as I remember, Farnell ( Lithuanian one atleast ) includes the postage in the price of every part. You see how much it totals to when they bill you and send you the invoice. Also, I think slapping in a random VAT number should work, as it is only used to write invoices.

I'll try that in the future, though it wasn't showing tax on checkout on the Finnish site, so I have a feeling they actually use the VAT number. Stuff's easy to integrate here, nowadays, so maybe they've taken the effort.

Do any of you have tips for a color-blind EE enthusiast? I've been trying to build some basic circuits but I'm severely red/green color blind and picking through resistors is almost impossible for me.

[QUOTE=Meepbob;46410568]Do any of you have tips for a color-blind EE enthusiast? I've been trying to build some basic circuits but I'm severely red/green color blind and picking through resistors is almost impossible for me.[/QUOTE] Could use a multimeter to measure the resistors.

[QUOTE=Leestons;46410741]Could use a multimeter to measure the resistors.[/QUOTE] Yeah that's what I've been doing unfortunately, very time consuming when I have a bin of them but I suppose that's my only option. Thanks :)

PC is turning off after 5 seconds or so (but still manages to start loading the OS) - safe to assume it's a blown cap in the PSU or something?

[QUOTE=Dorkslayz;46413736]PC is turning off after 5 seconds or so (but still manages to start loading the OS) - safe to assume it's a blown cap in the PSU or something?[/QUOTE] Is it cycling on and off itself or does it just shut off completely?

[QUOTE=Meepbob;46410568]Do any of you have tips for a color-blind EE enthusiast? I've been trying to build some basic circuits but I'm severely red/green color blind and picking through resistors is almost impossible for me.[/QUOTE] [t]http://www.electro-tech-online.com/imgcache/2778-storage_units.jpg[/t] a cabinet like this will be even more helpfull to you than they are normally. Sorting the resitors out initially will be a pain or you, but it will help a fuckton afterwards. [editline]5th November 2014[/editline] and in combination with these strips [t]http://www.penguintutor.com/electronics/reference/resistor-strip-01.jpg[/t] you only have to measure one to know like 20 of them.