THS Robotics - Basic Electronics

This page contains a basic electronics primer with an emphasis on bringing all THS robotics students to a common level of understanding for continuing work on robotics control systems. It is not intended as a general electronics primer.

Getting Started

Ohm's Law

We will start with the first thing presented in almost any basic electronics course or primer. Ohm's Law - know it, love it, memorize it!

V = IR and P = VI
I is current in amperes (or amps)
P is power in watts
R is resistance in ohms
V (or E) is voltage in volts

Here are some basic algebraic permutations:

I = V/R
R = V/I
P = I²R

Please commit these to memory - everything that follows will go a lot easier and smoother.

Using Ohm's Law to light an LED

A quick Brentism: Keep one eye on the roadmap and one eye on the road. Please remember that anyone can take some items with explicit instructions and connect them together to make something. The first challenge of the robotics club is to enable everyone to use engineering principles to develop what they want, based on what they have or can reasonably get.

Case in point - if you are handed the following items:

two 1.5 v D-cell batteries

a 10 cm length of 22-guage wire with a half centemeter of insulation removed from the end

a 2 v LED that works optimally at 20 mA (milliamps) and has a 1 v forward bias voltage
a 100 ohm resistor
celophane tape

and are given the following instructions:

Tape the two batteries together, end to end, so there is good contact between the positive terminal of one battery and the negative termal of the other.
Tape one end of the wire to the negative terminal of the combined batteries so there is good contact.
Twist the long lead of the LED with one end of the resistor so they stay together and there is good contact.
Tape the other end of the resistor to the positive terminal of the battery so there is good contact.
Touch the exposed end of the wire to the short LED lead.

you will be able to light the LED.

Amazing, right! It is interesting, but it would unlikely impress anyone with your engineering knowledge or skills.

OK, so then you have to return the battery, but you get a 9 v battery you can keep. You can keep the LED and the resistor (or grab another resistor out of a resistor "grab bag"). The "grab bag" is from a good 'ol Radio Shack 500-piece 0.25 watt carbon-film resistor assortment.

After a few minutes, you think the LED is defective because you reconnected everything just like before with the new battery, and the LED just stopped working after lighting for a moment and burning your hand. THe instructor then looks at you with what my wife refers to as the "cute retarded puppy look" and hands you a new LED, suggesting you should find a more suitable resistor using Ohm's Law.

A couple of notes about light emitting diodes (LEDs). LEDs (along with diodes in general) are polarized -- the long lead is typically positive with the short lead being negative. LEDs also require a certain voltage threshold before they will conduct electricity. This is called the forward-bias voltage (Vfb), and it needs to be taken into consideration when constructing a circuit (subtracted from voltage available to the circuit).

This exercise deals with resistors having fixed resistance values but it is worth mentioning that devices are available that provide variable resistance within a range. These devices are called potentiometers (commonly called pots). Since potentiometers discrete ranges of resistance values, all of the information from this exercise applies to them as well. As you can imagine, pots are useful for their flexibility, but they are only used where adjustable resistance is needed, because they are substantially more expensive (less than $0.10 for a 0.25 watt, fixed carbon-film resistor against about $3-4.00 for a 0.25 watt pot).

Exercise

OK, so you had to learn some algebra-looking stuff. If you are like me, if you don't use it, you will lose it, so aree some simple exercises using Ohm's Law.

Please find the resistance needed to provide 20 mA to the LED with the following common voltages:

2.4 v
3.0 v
3.6 v
6.0 v
7.2 v
9.0 v
9.6 v
12.0 v

Reference

Resistor Color Coding for 3 and 4-band resistors

Color	1st	2nd	3rd	Mult	Tol	Failure Rate (% per 1000 hrs)
None					±20%
Silver				0.01	±10%
Gold				0.1	±5%
Black	0	0	0	1
Brown	1	1	1	10	±1%	M = 1.0%
Red	2	2	2	100	±2%	P = 0.1%
Orange	3	3	3	1000 (1k)		R = 0.01%
Yellow	4	4	4	10k		S = 0.001%
Green	5	5	5	100k
Blue	6	6	6	1000k
Violet	7	7	7
Gray	8	8	8
White	9	9	9
3-band example: 62k ohms ±10%