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Class Notes
Week 2
Introduction to Common Electronic Components (2)
Logic Gates
Logic gates process signals which represent true or false. NORMALLY positive supply voltage represents
“true,” and 0V represents “false.”
Logic gates are identified by their function: NOT, AND, NAND, OR, NOR, EX-OR and EX-NOR.
Inputs and Outputs
Gates have 2 or more inputs, except for NOT gate which has 1 input.
Some gate symbols have a circle on their output which means that their function includes the inverting of the
output. We call these logic inverters.
Switches
Pole and throw
ON/OFF: single pole single throw (SPST)
ON1/On2: single pole double throw (SPDT)
Sparking: use a 4amp switch to turn a 2 amp motor on/off
Use “continuity tester” setting in your meter to figure out the on/off – it beeps if it finds a connection
Relays
A coil of wire wrapped around an iron core. When electricity runs through the coil the iron core exerts a
magnetic force, which pulls a lever that closes the connection.
Colt voltage (ideal)
Set voltage (minimum)
Operating current (milliamp – power consumption)
Switching capacity (maximum current you can switch)
Resistor + Resistor (1) Series or Parallel
When resistors are connected in series their combined resistance is equal to the sum of individual
resistances. R (total) = R1 + R2 + R3...
When resistors are connected in parallel their combined resistance is less than any of the individual
resistances. 1 / R (total) = 1 / R1 + 1/R2 + 1/R3...
Resistor + Resistor (2) Voltage Divider
A voltage divider consists of two resistances R1 and R2 connected in series across a supply voltage Vs.
The supply voltage is divided up between the two resistances, to give an output voltage Vo which is
the voltage across R2.
If R2 is much smaller than R1, Vo is small (low, almost 0V) (because most of the voltage is across R1).
If R2 is about the same as R1, Vo is about half Vs (because the voltage is shared about equally between R1
and R2).
If R2 is much larger than R1, Vo is large (high, almost Vs)(because most of the voltage is across R2)
Vo = (Vs X R2) / (R1 + R2).
Capacitor + Capacitor: Series or Parallel
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When in series, combined capacitance (C): 1/C = 1/C1 + 1/C2 + 1/C3...
When parallel, combined capacitance (C): C = C1 + C2 + C3...
Two or more capacitors are not commonly connected in a series, but parallel capacitors are useful to
obtain a very large capacitance.
Capacitor + Resistor
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The capacitor (C) in the circuit diagram is being charged from a supply voltage (Vs) with the current
passing through a resistor (R).
The voltage across the capacitor (Vc) is initially zero but it increases as the capacitor charges. The
capacitor is fully charged when Vc = Vs.
The charging current (I) is determined by the voltage across the resistor (Vs – Vc).
We refer to the speed of charge as a TIME CONSTANT = R x C.
A large time constant means that capacitor charges slowly.
R in ohms, C in farads, time constant in seconds.
(Remember our circuit bending exercise? We were essentially replacing the resistor and therefore tweaking
the time constant, and as a result affecting the speed at which the circuit oscillates.)
Buddha Machine Hacking
1. Unscrew and open up, you will find a range of components.
2. We want to hack the clock speed of the circuit. The majority of electronic toys are essentially a simple
computer dedicated to running one program. In most, a clock circuit determines the pitch of the sound and
the speed of blinking lights, etc.
3. Look for resistors, particularly those lying near an IC or near a capacitor.
4. Get the circuit making sound. With a slightly wet finger, touch the solder-side of the circuit until you find a
resistor that responds to your touch. The “right one” will cause the pitch to go up, and the tempo will also
speed up - gratz, you have found a resistor that is a part of the clock-circuit.
5. When you think you have found a hotspot, mark it on the circuit board.
[What's happening?]
In week 1, we learned that electric current flows through wire like water through a fat pipe. Resister are like
skinny pipes that resist the current flow.
When a resistor and a capacitor are combined in the “feedback loop” of an amplifier (we will learn about
this in the coming weeks), they resonate at a frequency that can be adjusted by changing either of the
components. Make the resistor or capacitor smaller the frequency goes up, make either large the frequency
goes down.
When you place one resistor in parallel with another you LOWER the net resistance (2 pipes > 1 pipe). Your
skin is a resistor. When you press your finger across the circuit you effectively decrease the size of the
resistor - more current flows, therefore the pitch goes up.
6. Now replace the clock resistor with a variable resistor – that is, the potentiometer.
7. Pick a potentiometer whose maximum value is greater than the resistor you have removed. Since must
clock circuits use rather large resistors (>100k) you will probably need a pot whose value is 1M or greater.
Reading Schematics
Schematics?
A schematic is a map showing the path the current takes through the various components.
Each component is represented by a symbol, usually with either a label or a value (or both).
The arrangement of the components on paper is chosen to make the function of the circuit clear, and usually
only vaguely resembles the actual construction of the device.
The current path is shown with lines drawn for maximum clarity, with little concern for the length or position
of the real wires.
General Conventions
If signal processing is involved it is often drawn from LEFT (input) to RIGHT (output) - think the logic gate
symbol.
This symbol represents the GROUND, and all ground points in the schematic are assumed to be connected
together. These GROUND points represents places in the circuit that are at 0 volts. Sometimes, the ground is
connected to the metal chasis of a device.
Each component in the circuit is represented by exactly one symbol in the schematic.
Different components that fall into the same component class are all described by the same schematic
symbol, e.g. all capacitors types share the same symbol.
Symbols
See attachment; Google if in doubt. Useful reference:
http://www.kpsec.freeuk.com/symbol.htm
Also check this useful software:
http://www.aaroncake.net/electronics/schem.htm
Simple Oscillator