[1] Basic Electronics

  ----------> >---                   
  |              |      Check the V(open circuit) of the 
|----|           |    solar cell.  Let's presume 24.0V.
|    |           /    Change the value of R1, beginning
| \  |       R1  \    with a high value.  You say this 
| /\ |           /    is a big cell, so we'll try 1K.
|    |           \    If the V(load) is still 24V-ish,
|----|           |    try with a smaller resistor.
  |              |      As we lower the resistor value, 
  ----------> >---    we are increasing the load.  At 
                      some point the voltage V(load) 
will sag to some lower value.  That shows you the limit 
of compliance of the solar cell.  If we say that 90% is 
our limit then we will stop testing when we reach 21.6V.

  So, now we have a resistor installed that drops the 
V(load) to 21.6V (our arbitrary limit.)   We know the 
Voltage (21.6) and the resistor value, (oh, let's say 47R)
so we just solve V=IR for I.

     V     21.6
I = --- = ------ ~= 460 mA
     R      47

  Now the numbers above are _completely fabricated_ so you 
will have to do the process yourself.  The above also 
presumes that the measurements are taken at some fixed 
level of light.  You will find that the numbers change 
with the light incident on the panel.

  Another point to ponder is impedance-matching.  Maximum 
power transfer occurs when Z(source) = Z(load).  Your 
homework for today (:-))  How does that affect the above?

  Watch your power dissipation.  You may need to change to 
higher _wattage_ resistors, as the resistor values drop. 

24V into 1000 Ohms gives 24 mA and 576 mW.

21.6V into 47 Ohms gives about 10 Watts ==>> heater!

P = IV = .46 * 21.6 ~= 9.9 Watts

Watch the resistor wattage values!




[ "What is a chip marked "TC 74HCU04P" can it be used 
in the MicroCore? ]

Q:  "What is it?"

A:
TC 74HCU04P

TC - manufacturer's code, possibly "Toshiba"

74 - popular collection of TTL ("transistor-transistor logic") 
     chips, most compatible in some way with others.

HC - sub family of "74 TTL"  Think of these as "High speed CMOS"
     with lower power consumption and slightly higher speed than 
     the baseline (original) family.

U  - special sub-family of the "HC" sub-family of "TTL."  The "U" 
     refers to "unbuffered."   Very slightly faster and greater 
     "fan-out" than buffered version.

04 - (74xx04) "Hex inverter"  
     (74xxU04) "Hex inverter"
     
Q:  "Can I use it in the MicroCore?"

A:  There are chips better suited to the MicroCore.  Look for a 
chip with a "Schmitt" input that is able to handle slowly-varying 
inputs.  Most TTL inputs (usually well behaved) will conduct large 
amounts of current in the transistion region between valid logic 
levels.
  The 74HC14 is ideal.  



>
>what is the difference between the fairchild's 
>
>74act139pc-nd   
>    &
>74act139sc-nd
>    ???


  The "ND"'s mean that you are looking in the DigiKey 
catalog :-)  The "74ACT139" part is the number that 
describes the function and interface type of the chip, 
and they are the same.  The "PC" vs. "SC", after the 
functional description generally indicates the type of 
package the chip is in.  In this case (the DigiKey 
catalog) the information is right there on the same 
line of the catalog;  The "PC" is a "16-dip" and the 
"SC" is a "SO-16".  So the "PC" is for mounting through 
holes or into a solderless breadboard; that is the type 
that most of the readers of this list will be using. 
A few will want the "SC" version which might also be 
called 'surface mount' which is short form for "your 
hands better be steady and your eyes and soldering iron 
tip better be sharp 'cause these puppies are tiny."



>How does the output pin work? If I write "1" to the data port, will
>the output pin go high? And if I write "0" will the pin go low?

Yes, if you have the LED connected to the processor and then to the
current-limiting resister and the resister is then connected to ground. 
BUT, If you have the resister connected to +5V and you flip the LED around, 
then the LED will light when a "0" is present and will turn off 
when a "1" is written to the port. Just be careful that the LED polarity 
is proper or it won't light at all!

Usually, you want to sink current so the second configuration is
actually better because the current will flow from the +5V 
through the resister and LED into the I/O pin.


How "big" is an amp and/or an mA? I mean, relative to a volt. 

  An ampere and a milli-ampere are related at 1000:1.  
The "milli" prefix tells you that the following unit 
is to be multiplied by 10^(-3), or one one-thousandth.

  A volt has no [size relative to] an ampere, but they 
are interdependant in a circuit.  For example, you will 
often here that a voltage of such-and-such, across 
so-and-so, gives rise to a current of blah-blah milli-
amps.

  These questions suggest to me that you are ready for 
a beginners' electronics book.  Radio Shack has one 
that is inexpensive, written by Forrest M. Mims III, 
"Getting Started in Electronics."  There are some 
smaller and less expensive booklets there too, but start 
with the large one.



> What is a millAmp?

milli = 1/1000th, or 0.001 of an Amp. For milliamp we write 'mA'.  
A mu would be for 'micro-', which is one millionth.  
The mu and the u are interchangeable, since they look alot
alike, both stand for 'one millionth', and there isn't a mu on most
keyboards.