Re: A few novice questions

On May 2, 8:52 pm, larwe <zwsdot...@xxxxxxxxx> wrote:

Developers are using whatever their designs and cost budgets call for.
Bipolar transistors are slightly cheaper than FETs (at least, the ones
we buy in millions per month). If you just need to switch on a high-
current LED from a low-current GPIO, there is no compelling reason not
to use a PN2222, BC547 or similar bipolar transistor. If there was no
longer a use for these parts, they would no longer be manufactured.


I figured they'd still be in production regardless of their use in
embedded systems because they're still used for analogue electronics
applications (e.g. a guitar amplifier).


1) What kinds of chips I should use with it, e.g. 74HCT family, or
4000 family, etc. ?

Selecting an external logic family depends on your circuit's Vcc, the
I/O level (CMOS, TTL, etc), the switching speed, the power consumption
etc. Many (I would guess most but I'm not sure about that) embedded
systems use no external discrete logic at all.


One of my lecturers told me today that 74HCT hasn't been used for
about the last 10 years, and that the 4000 family hasn't been used
since maybe the 60's.

So now I'm wondering what family of logic chips are actually being
used *today* in conjunction with today's microcontrollers, e.g. the
PIC16F684?


2) What transistors should I use a switches when I want to pass as
much as an ampere of current?

Depends a great deal. Is it AC or DC you're switching? What voltage?
Inductive or a resistive load? Would a relay be a better choice? How
much isolation do you need? Do you need to reverse the voltage?


OK here's an example:

I have 26 LED's. Instead of them all being on at the same time, I'm
going to light each of them for 100 microseconds and then move onto
the next one (i.e. display multiplexing).

The LED's I'm using have a maximum current rating of 25 mA, but since
they'll only be on for one twenty-sixth of the time, I'm going to put
about half an ampere thru them. To achieve this, I'm not even going to
use a resistor, I'm just gonna go straight from Vcc to LED to ground.
(I've tested this in my project this year and it works perfectly).

Now my microcontroller pin won't be able to supply half an ampere, so
I'll use the microcontroller pin to turn on a transistor which will
allow the half ampere to flow from Vcc to the LED to ground.

So my question is, what's the best transistor to use? The supply
voltage is 5 V and it's powering an LED.


Also, very quickly without getting into too deep a discussion, I'd
like to ask what microcontrollers I should be using? We've used the

For learning, you should be using something simple to use, with a
modern, clean architecture. I would recommend MSP430, AVR or ARM. Eval
hardware for the first two can be obtained for under $20; for the
second, under $40.

Neither PIC nor 8051 qualify as easy learning micros. However they can
be the right part for a real project.


To be honest I'm finding the PIC16F684 quite handy to work with; its
instruction set is really easy to learn and understand, plus the
PICKIT is great for hooking up to my laptop.

.