On Fri, 11 Oct 2019 19:03:34 +0100, Terry Casey
wrote:

In article dd6cec1d-1a3b-4331-bbf9-d7f6c268fcf3
, says...


Today we are running a lab titled Embeded Systems.
Which uses a freescale development board the first part shows them how to connect a push switch to the GPIO and then an LED off that.
The LED is shown in the diagram as connected to a 300 Ohm resistor the lecturer has even bothered to draw the resistor in place showing how to connect it up to a protoboard and the resistor with the colour code LtoR ! of Red Black Orange.

So I have 40 odd (very odd) students coming to me asking why I don't have any 300 ohm resistors.

Back in the late 60s I ran the service department of a Radio &
TV dealer and we alaso sold lots of components.

I stocked E12 series 5% carbon film resistors.

Lots of kids at the local school liked to build projects from
magazines such as Practical Electronics.

Lots of these articles were obvious written by engineers
working for likes of Marconi, STC, etc. and a lot of it was
obviouly 'back of a fag packet' design, so lots of values fell
into the 1, 2, 5, 10 sequence.

So the value 5.1k kept popping up which we didn't stock and I
would offer 4.7k as a substitute which seemed to cause some
confusion until I pointed to the note at the bottom of the
component list that said "All values ±20% tolerance unless
otherwise stated" and I could demonstrate that my 4.7k 5%
resistor was much more accurate that a 5.1k 20% possibly could
be (if anybody actually manufactured such a nonsensical 20%
value, anyway!)


One student was very proud of the fact of working out he could use 2 150R to make a 300R resistor, I did say he could use a 270R or a 330R and all that would do is change the brightness of the LED, I also said if he used the 330 then that would reduce the amount of current and help the world cut down on global warming and perhaps save trees too.
I've stopped a couple of them walking off with 330K resistors, as they weren't sure of the relivance of the R & K after the digits
But don't forget I'm not allowed to teach students, but I am allowed to and expected to impart my knowledge, which strangley enough doesn't warrent a salery scale increase.

When the subject of yolerance came up elsewhere a while ago I
devised this table to illustrate the tolerances of different
eanges of resistor. Feel free to show it your students.

https://www.dropbox.com/s/qdeyfp7lzzdsrdr/Resistor%
20Tolerances.png?dl=0

or https://tinyurl.com/y2jdjh7o

--

Terry

That's rather good!
--
Dave W