[mythtv-users] IR Transmitter Components, Need these double checked- Urgent!

[Pete Hartman]

At 04:30 PM 5/23/03 -0700, Chris Germano wrote:
>-The transistors mentioned on that schematic are NPN & PNP, now what kind 
>of transistors are they? I bought the generic "Switching Transistors" and 
>the larger transistors that have that heat sink which are for power 
>amplifying since the docs say low-medium powered transistors, but that 
>could mean both. So would it be the normal transistors 
>(http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F010%5F005%5F004%5F001&product%5Fid=276%2D1617) 
>or the powered 
>one(http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F010%5F005%5F004%5F005&product%5Fid=276%2D2020)?

You're going to make me drag out my old and rusty electronics training 
aren't you? :-)

They're both "normal" transistors.  Most transistor specs should indicate 
what type they are, pnp vs npn.  The reason for the nomenclature is this:

Most/All silicon components (transistors, diodes, chips) consist of 
sections of silicon doped in different ways so that you have areas of extra 
electrons and areas of extra "holes" (or lack of electrons).  In a 
transistor you have three sections (the three leads).  The "outer" two are 
doped the same way, and sandwiched between them is a section doped "the 
other" way.  One doping is "P" and the other doping is "N" and I believe 
"P" is extra electrons and "N" is extra holes.  You can make transistors 
either way:  P N P layers or N P N layers.  Hence the name.  They behave in 
different ways, and in diagrams you'll see the PNP with an arrow pointing 
in towards the center and the NPNs with it pointing out.  This is related 
to how they behave, but I sucked at analog electronics, so I can't explain 
it much better than that.

Your first link is NPN standard transistors.  Your second link is really a 
power circuit component, not something that you'd use as a general purpose 
transistor.  It seems kind of strange that Radio Shack doesn't appear to 
have any PNP transistors.

It might be a good idea to see if there's any LIRC forum discussion about 
specific models of transistors; there are current and voltage specs that 
are going to have significant impact on the practical reality of the 
circuit working.  I did a project in school to create a MIDI driver for a 
small modular computer system we had for labs, and the first two or three 
types of transistors we used were too low current to actually drive 
MIDI.  While MIDI is a special purpose thing and has higher current 
requirements than most, the point is that just because someone builds the 
circuit with one transistor and it works doesn't mean another transistor of 
the same type (PNP/NPN) won't have different characteristics that will 
cause it not to work.

>-1/2 watt resistors should be fine correct, I don't need 1/4 watt 
>(shouldnt make a diff anyway am I right?)

The wattage of a resistor is mostly about how much power they can dissipate 
without meelting.  If 1/4W will work, a 1/2 W should be fine.

>-The schematic states it needs a 4000uF capacitor, would something around 
>that uF work? Also does it matter if they are polarized or not?

No, generally you want the exact capacitance.  If you have problems finding 
a 4000uF cap, you can put different ones in parallel that add up to the 
same capacitance.  And yes, note the + on the diagram, that means polarized 
with the + where it's indicated.  If you put, say 4 1000uF caps in 
parallel, make sure they all have the + oriented in the same direction.

(By parallel I mean the positive and negative sides of each cap connect 
together, as opposed to one's positive connected to the next one's negative 
in series).

>Sorry for all the questions, the author of that schematic sure assumes a 
>lot, and I don't want to open all my components and put it together to 
>find out it doesnt work.

He assumes you are accustomed to doing electronics work :-).  Again, if 
you're new to transistor circuits and such, you should probably go check 
out the LIRC forums for discussion of specific part numbers for components 
that work.