For our purposes we are going to explore hobby electronics. We will examine just the components you are likely to encounter starting out. We hope to demystify some of the magic of technology while showing you exciting possibilities.
Fundamentals of Electronics
There are only 3 things you can do to an electronic signal: turn it on, turn it off or vary its amplitude. All electronic devices consist of a source and a load. Every source has a potential difference: a positive pole and a negative pole and some amount of voltage between them. Alternating current (AC) switches these poles 60 times a second while direct current (DC) remains fixed. Most DC loads, however complex, may be represented by a resistor to express the overall demand upon the source.
The source may be a battery pack or a power supply. The load could be a light bulb, or a computer motherboard. The path that connects the source to the load is called a circuit; because it departs from the source and must return to it. In general, the worst thing to do to a source is to short-circuit it without any load. In a short-circuit condition 100% of the source’s current capacity (the rate of flow of electrons) is unleashed; resulting in damaged equipment, possibly a fire or explosion (like the lithium-ion batteries of late).
A good load is suppose to provide some resistance to the flow of current so that it consumes power from the source at a controlled rate. It is the difference between drinking from a water fountain and a fire hose. The current flowing through the load generates heat. Unless the load is supposed to be a heating element; the heat that is generated is just waste. Heat is the enemy of electronics (causing premature wear and destruction); and waste is the enemy of our environmental resources. Good circuit design aims to be energy-efficient and only consumes the power necessary to do the job.
Ohms Law and the Power Wheel
Ohms Law states that a 1 volt source connected to a 1 ohm load results in 1 amp of current. Voltage is the potential difference of the source; we commonly handle 9V batteries and many electronic devices run on 5.0V or 3.3V. Current is the rate of flow of electronics and is measures in amps; however, for hobby projects and electronics, most of our circuits consume milliamps (mA) or 0.001 to 0.500 A. Resistance is the property that restricts the flow of electrons and is measured in ohms.
The rule of a series circuit is that current is the same in all points of that circuit. The resistive loads in a series circuit add up; in this illustration it would be 100 + 300 + 50 = 450 ohms. the current would be the same if the load was connected to a single 450 ohm load. In that case, all 9V would be dissipated across that load resulting in a current of 9V/450 ohms = .02A or 20mA. However, in the circuit illustrated; R1, R2 and R3 would have different voltage drops that would add up to 9V. Can you compute the voltage drop of each resistor using Ohm’s Law?
The rule of a parallel circuit is that the same voltage is dissipated across each element. R1, R2 and R3 each dissipate 9V; however, the current through each resistor is different. The current through R1, R2 and R3 annotated by IR1 + IR2 + IR3 = IRT or the total resistance.
One way of calculating the total resistance is using this formula:
(I would not attempt without a good calculator!)
Another way would be to figure out the individual currents:
IR1 = 9V/10Kohm =
IR2 = 9V/2Kohm =
IR3 = 9V/1Kohm =
IT = IR1 +IR2 + IR3 =
RT = 9V/IT =
2 equal resistors in parallel result in 1/2 the total resistance; as in 2 10K-ohm resistors in parallel would measure 5K-ohms.
Series Parallel Circuit
For extra credit; what is the voltage and current for every element of this circuit? How would you attack this? (Solve the parallel branch first, then the equivalent R2:R3 will be a part of a series circuit.)
The Power Wheel
Besides voltage, current and resistance an overall measure of the circuits consumption and efficiency is rated in terms of the power it consumes. The basic formula is P = IE. Power is an important consideration in the rating of components; especially for resistors and transistors. It is measured in Watts (W).
The resistors in your kit are 1/4 W (.25W). If more than that amount of current goes through that rating of resistor it will become hot and ultimately burn up. A conservative margin is at least twice the wattage you expect in the circuit for the component specification.
Resistor Color Codes
Our kits have the 4-band resistors; the 4th band being the tolerance (typically 5%); 5-band resistors are used where higher precision is needed.
Your breadboard should have 3 different values of resistor installed. Using the resistor ‘colour code’ above, what are their values?
Brown Black Orange
Orange Orange Brown
Brown Black Black
What would be the colors of a 100K-ohm resistor? (Kilo)
What would be the colors of a 1K-ohm resistor?
What would be the value of a 1M-ohm resistor? (Mega)
In the circuit on the left, if R1 = R2, and the input voltage is 5V, then the divided voltage will be exactly half, or 2.5V. For the circuit on the right, which is a potentiometer or variable resistor; if the wiper is set to the middle you would get exactly the same thing. It would not matter whether the pot is 5K, 10K, 50 K or 100K; the middle point would provide exactly half the input. The pot, however, can be rotated to output any voltage in between the supply voltage and ground.
Of course, this holds true for what would be known as a high-impedance load (high -Z); a low impedance load, especially one that is lower than R2, would drop the voltage to a lower value; essentially ‘overloading’ the circuit.
Variable Brightness LED
This test circuit will only require adding a jumper wire above the red LED to the center leg of the potentiometer; and also connecting the voltmeter probe to the same center led of the pot. Plug in your mini USB and adjust the potentiometer while observing the change in brightness and the voltage readings.
In electronics values range from very large to very small; metric prefixes make it much easier to describe a range of values than keeping track of a bunch of zeros. These prefixes change every 3 zeros:
Ohms Law ‘Computer’
While it is technically feasible to measure the voltages and do the math to know how much current is running through a circuit technicians have found easier ways. The so-called ‘Ohms Law Computer’ is nothing more than a 10 ohm resistor added between a circuit and ground (the 10 ohms is considered negligible). The voltage monitored at this point will actually read the approximate current in milliamps if we mentally move the decimal point to the left. Let us try it. Add the blue wires indicated:
Current Through an LED
Just move the blue wire from the 330 ohm resistor to the bottom of the red LED. What current do you measure?
A ‘pure’ resistor (as in theoretically perfect) provides the same impedance to a signal regardless of its frequency (DC to light). There are 2 other basic components that provide very different impedances based upon the frequency of the signal: capacitors and inductors.
The 2 plates of a capacitor are separated by a dialectric (sometimes air) and these plates store a small charge. They completely block direct current (DC) and pass alternating current.
Inductors easily pass DC; but offer a growing impedance to AC as the frequency increases due to the magnetic fields in adjacent windings. In fact, the properties of an inductor so reject a change in current that (according to Lenz’s Law) when the applied electric field collaspes the inductor generates a reverse-polarity inductive ‘kick’ that can measure thousands of volts for a brief instant (a voltage spike).
Series and Parallel Reactive Components
As you can see, the properties of capacitors and inductors are opposite of each other. A very interesting occurs in a LC circuit at the frequency at which the reactances of C and L are equal: Resonance!
There is a voltage peak for a parallel resonant circuit and a voltage dip for a series resonant circuit. This is the basis of radio communication and how one station can be tuned in while rejecting others above and below a given frequency.
RC Time Constant
Sometimes a circuit needs a little time delay; even milli or microseconds can make a difference.
Assemble the RC Circuit below. You will need to add a 100K-ohm and a 1-M-ohm resistor as well as a 47uF electrolytic capacitor.
I made the values really high so you could see what the circuit was doing over seconds rather than milliseconds. The momentary switch is to drain the capacitor to ground in order to restart the time constant circuit. In this lab, you will notice the voltage slowly rising, but never reaching the full level of the source. Pull the voltmeter probe wire for a few seconds and reattach it to see the voltage has risen. This is due to our little panel meter voltmeter not having a high impedance (it loads down the circuit a little). A good digital multimeter will not do this so badly. Substitute the 1M-ohm resistor for the 100K-ohm and repeat the experiment.