Short circuits Saber card User manual

INSTRUCTION MANUAL

SABER CARD

v0.92

The Kit

Printed Circuit Board

The Printed Circuit Board (PCB) is made of

resin and berglass, with a layer of copper on

the top and bottom. This is a 2 layer board, but

boards with more layers are common. We can

use PCB design software like the free KiCAD,

or Eagle, Altium etc to lay out our circuit. The

circuit is comprised of footprints that hold the

various components, and traces which are

copper connections between certain com-

ponents.

When the nished design is sent to the man-

ufacturer, they etch away the copper that isn’t

needed to form the circuit. The board is then

coated in a coloured layer that protects the

circuit. This is the mask. A different colour is

then applied. This is the silkscreen layer, which

is where the footprints, designations and oth-

er information are found. Short Circuit PCBs

have the traces drawn on this layer, to help you

follow the circuit.

Components

The kit comes with a variety of components.

They make the circuit function. Designing cir-

cuits can be incredibly complicated. Our kits

are designed to be simple to learn. They are

not always the most efcient or effective way

to design devices with these functions. They

are designed to teach various concepts that

can be applied in your own designs.

202

CD4017B

NE555

202

The Manual

There is quite a lot of information in this man-

ual, so here is a run down of each section and

what you are likely to nd.

The manual is a work in progress and we

would appreciate your feedback. Please head

to the forums if you have any ideas or con-

structive criticism. As the manual is digital, we

will be updating it regularly.

Bill of Materials

The Bill of Materials (BOM) is a list of parts and

their values. This can be used to nd replace-

ment parts or to check the datasheets for each

component.

Component Layout

This will show you where each of the com-

ponents, found in the BOM, will be soldered.

Schematic

The schematic shows the circuit design in it’s

simplest form. Each part of the circuit is shown

separately. Connections within these sections

are shown with white lines. You can nd tags

next to pins connected to other parts of the

circuit. Each tag has a corresponding tag in

the part of the circuit that it is connected to.

Circuit Explanation

The rest of the Circuit section of this manual

explains how and why the circuit works. You

can skip this bit and head straight to building

your kit, then check back later to learn how it

works. Or you can go head rst into the how

and why, then start building. It’s up to you.

Assembly Instructions

This is where you can learn how to solder

your components to the PCB. The tips at the

start are very useful. They may prevent you

from getting frustrated. The soldering iron can

be hard to tame. Make sure to keep that tip

tinned and shiny! The diagrams on the right

pages show the components mentioned in

the instructions. You can use this to match the

components to the footprints on the board,

and to make sure you are using the correct

resistor in the correct place.

Testing for Faults

Be sure to go through this section to minimise

any risk of breaking something. If you have a

short and you connect the board to power, you

might overheat a component, or burn out the

power supply.

Component Index

The Component Index will give you details

about each component (except some connec-

tors). We’ve included information about how

the component works, its construction, how to

nd out if it has failed and much more. You can

use this as a reference when designing circuits

or trying to x them.

MOTHERBOARD

Contents

The SABER CARD kit is designed to teach you how to solder and desolder surface mount com-

ponents without specialised equipment.

The circuit uses a 555 timer chip that can be set up to emit a clock pulse at a regular interval.

This is fed into a decade counter that turns each of it’s outputs on sequentially. This circuit can

run on 3-16V.

By changing a resistor or capacitor’s value, the clock pulse can be increased or decreased.

When experimenting with the different values available, you will become familiar with replacing

surface mount components.

A video tutorial showing the soldering technique and the build process can be found here.

Circuit

Assembly Instructions

Component Index

Kit Contents

Tools Needed

1 x NE555 Timer

1 x CD4017 Counter

11 x 0805 LEDs (Blue)

4 x 0805 Resistors (2K)

2 x 0805 Resistors (4.7K)

4 x 0805 Resistors (10K)

2 x 0805 Resistors (20K)

2 x 0.1uF Capacitors

2 x 1uF Capacitor

2 x 4.7uF Capacitor

1 x Printed Circuit Board

Soldering Iron

Solder

Flux

0.3 - 0.5mm

+ Thin Tip

202

Transparent

Packaging

Transparent

Packaging

W/Black Pen

CD4017B

NE555

202

Designation

D1-11

Value

0.1uF

0.1uF/1uF/4.7uF

20mA, 2.5V, Blue

10K, 1/4W

2K, 1/4W

2K/4.7K/10K/20K

Name

Ceramic Capacitor

LED SMD

Resistor

Footprint

0805

Datasheet

NE555

CD4017

Timer

Decade Counter

SOP8

SOP16

Circuit - Bill of Materials (BOM)

Circuit - Identifying Components

Designation

C1, C2

D1-11

R1, R4

R3, R4

Value

0.1uF

20mA, 2.5V, Blue

10K, 1/4W

2K, 1/4W

4.7K

Illustration Packaging Description

White paper strip

C2 1uF Transparent plastic strip

C2 4.7uF Transparent with pen mark

Black plastic strip

White paper strip (1002)

White paper strip (202)

White paper strip (472)

R4 20K White paper strip (2002)

NE555

CD4017

In black plastic

202

CD4017B

NE555

202

472

202

2002

1002

202

1002

202

472

2002

CIRCUIT

Circuit - Component Layout

D1 R1

D10

D11

Circuit - Schematic

VCCGND

THR

DIS

NE555

QTR

CLK CLK LED1

LED2

LED3

LED4

LED5

LED6

LED7

LED8

LED9

LED10

4017

Q0CLK

VDD

VSS

CKEN

RESET

15 7

Cout

10K

1uF

100nF

LED1

LED2

LED3

LED4

LED5

LED6

LED7

LED8

LED9

LED10

D3 D4 D5 D6 D7 D8 D9 D10 D11

CLK

NE555 Clock Pulse Generator

CLK LED

LEDs

CD4017 Decade Counter

CIRCUIT

Circuit - NE555 Circuit

NE555 Timer IC (U1)

The NE555 is a very versatile chip that has

been around for 50 years. Some say the most

popular IC ever made...

The chip has 2 main operating modes:

1. Astable

In astable mode, the chip acts as an continu-

ous electronic oscillator. Common uses being

LED ashers, logic clocks, tone generators,

analog to digital conversion etc.

2. Monostable

In monostable mode, the chip produces one

pulse instead of a continuous oscillation once

triggered. The pulse ends when the capaci-

tor’s voltage reaches 2/3 of the supply voltage.

Any change to the trigger input will have no

effect on the output within the set timing inter-

val. This can be used to remove bounce from

switches, detect missing pulses, capacitance

measuring etc.

To provide a clock signal for the 4017 decade

counter, we are using the 555 timer in its as-

table mode.

Decoupling Capacitor (C1)

C1 is used to help smooth out variations in

the power supply that may cause the circuit

to malfunction. Capacitors take time to ll up

and empty. A bypass capacitor provides a res-

ervoir that adds voltage when some is missing

and absorbs some when extra is available.

R1, R2 and C2

The values of R1, R2 and C2 will determine the

frequency of the output clock signal.

The capacitor (C2) will charge up through R1

and R2 until it reaches 2/3 of the supply volt-

age. While this is happening, the output will be

high. When it reaches 2/3 of the supply voltage

it will start to discharge through R2 to the DIS

pin of the IC. This will cause the output to be

low. When the voltage of C2 reaches 1/3 of

the supply voltage, it will start charging again,

creating a continuous cycle.

Lowering the values of R1, R2 or C2 will in-

crease the frequency (speeding up the clock

pulse). Increasing their value will lower the

frequency.

You can use the following formula to calculate

the frequency:

Or head over to the Saber Card page on the

website to nd some helpful calculators.

To get the desired frequency you can exper-

iment with the different value resistors and

capacitors found in the kit. Removing and

resoldering these will give you valuable solder-

ing and repair experience.

ƒ = 1.44

(R1+ 2R2)C2

CD4017 Decade Counter

The CD4017 is a simple IC that turns on each

of its 10 outputs sequentially. The counter ad-

vances one count when the input clock signal

transitions to high. Each output will turn off

when the next is turned on.

There are 3 input pins. The clock pin, the clock

inhibit pin and the reset pin. The clock pin is

used to advance the counter. The clock inhibit

pin will ignore inputs from the clock pin if it is

pulled high. The reset pin will reset the count-

er to 0 if it is pulled high. We have connected

both to GND to prevent them from interrupting

the sequence.

Each of the outputs is connected to the anode

of an LED as the output is held high when on.

Power Pads

The 555 timer IC is rated for voltages between

4.5 and 16V. The 4017 decade counter IC is

rated for voltages between 3 and 18V. This

means we can apply any voltage between 4.5

and 16V to the power pads.

You can use clips that are connected to a

desktop power supply, solder wires connected

to a 9V battery, or snip a USB cable and solder

the black and red wires to the - and + pads re-

spectively. Breadboard (dupont) hookup wires

t perfectly into the holes, so you could use

these for a non-soldered alternative.

You could also power this from a Motherboard

kit. You could even wire the positive pad to an

IO pin on the Motherboard so you can control

when it turns on.

Circuit - Decade Counter Circuit

Circuit - Power

CIRCUIT

Circuit - LEDs

Main LED Strip (D2-11)

Each of the LEDs in the strip are connected

to an output on the CD4017 decade counter.

As the counter sends its outputs high when

on, and we want our LEDs to turn on one at a

time, we connect the outputs to the anodes of

the LEDs. The cathodes of the LEDs are con-

nected to GND via a current limiting resistor.

If we wanted all the LEDs to be on except the

one that is connected to the output that is

currently active, we can connect the outputs

to the cathodes of the LEDs. The anode can

then be connected to positive voltage through

a current limiting resistor.

Clock LED (D1)

To give some added visual feedback, we have

added an LED and current limiting resistor

to the clock signal coming from the 555 tim-

er. This is turned on when the clock signal

goes high.

Current Limiting Resistors (R3, R4)

These resistors limit the current going through

that part of the circuit. This protects the LEDs

from burning out. The higher the resistance

value, the lower the LED’s brightness and the

longer the LED will last.

Only one resistor is needed for the entire strip

of LEDs. This is because our circuit is de-

signed to turn one LED on at a time. If there

were more than one LED on at a time, then we

would need to use a resistor per LED.

To calculate the resistors minimum value, we

can use the following equation:

To nd the voltage drop across the resistor, we

need to take the voltage drop across the LED

away from the total voltage from the supply.

Vsource is the maximum Voltage we will con-

nect to the circuit. This will be 16V in our case.

Vled can be found in the LEDs datasheet.

Check the datasheet link found in the BOM.

The LEDs used in this kit have a typical for-

ward voltage of 3.2V.

The current (I) will be the LED’s recommended

forward current. The datasheet states it’s max

forward current as 30mA. The value used in

the datasheets measuring conditions can be

used as a good safe current. This is stated

as 20mA.

(16 - 3.2) / 0.02 = 640

640 is our minimum resistor value. After test-

ing the brightness of the LEDs with different

values above 640, we settled on 2k.

Check out the calculator on the website to

nd the LEDs forward current when using a

2k resistor.

R = V

SOURCE

- VLED

Assembly Instructions - Tips

General Soldering tips

1. ALWAYS KEEP YOUR TIP CLEAN

To ensure the soldering iron can transfer

enough heat from it to your solder/compo-

nent leg you must keep the tip clean and

shiny. A dull tip means the outside layer

of metal has oxidise. This oxidised layer

is a poor transferrer of heat. Because of

this, you will have to hold the tip against

the component for a longer period of time,

which can result in the component failing.

It’s also very frustrating.

To keep your soldering iron tip clean, wipe

it on a wet sponge or wire ball, then apply

some solder to coat it, then wipe it again.

Ideally, you should do this after every com-

ponent. At the very least, do it after every

4 components.

2. CONTACT

When soldering, make sure the tip of your

iron is making contact with both the leg of

the component and the pad on the PCB.

Apply heat to the area, then, within a sec-

ond or two, apply the solder to the point

of contact.

3. HEAT

It’s better to be too hot than too cold. As

mentioned earlier, when the iron tip isn’t hot

enough, you have to hold it on longer. This

allows heat to transfer into the component

and could cause a failure. It is better to set

your soldering iron a little hot so the solder

melts instantly and ows around the leg of

the component with ease. You can start at

around 350C and adjust from there. Too

hot and the tip will oxidise too quickly,..

4. SOLDER

Leaded solder is much easier to work with,

which makes it easier to learn with. It can

be hard to nd in some countries, but can

often be ordered from China. There are po-

tential health risks, but these are very low.

Make sure you have a fan pointing away

from your work area to blow the fumes

away. Work in a well ventilated area.

Thinner is better. Working with a thick wire

of solder can get messy. Use 0.4-0.5mm

solder for more control. You will have to

feed more into the solder joint, but you

have more control when there are other

pins close by that you want to avoid. This

is essential when soldering surface mount

components and Integrated Circuits.

5. SAFETY

350C is obviously very hot. Stuff catches

re at this temperature. Skin fries. Please

be careful. Remember to turn it off when

you are nished. This will prevent a poten-

tial house re and also save your iron tip

from continued oxidation.

6. ANGLE OF ATTACK

When soldering, make sure you position

the board so that it is easy to access the

area you are working on. It is easy to make

a mistake when you are trying to maneuver

your iron into position around some obsta-

cle. You may have to spin the board 180

degrees, or make sure you have snipped

the legs off the previous component. It’s

easy to be impatient so try to plan ahead.

Check the component position using the

images opposite. To help identify the compo-

nents please refer to the table on this page.

LEDs

The LEDs are polarity sensitive, so make sure

the LED is soldered the correct way around.

You will likely destroy the LED if you try and

desolder it, so make sure you get this right.

There are 4 extra LEDs in the kit, just in case.

The negative side of the LED is indicated by a

green mark on the top. There is also a symbol

on the bottom that “points” to the negative

side. This should line up with the “-” symbol

on each of the LED footprints on the PCB.

Be careful you don’t ping these tiny 0805

components across the room, nding them

will be tricky.

To begin, we will rst apply some solder to

one of the pads of each of the LED footprints.

This will enable us to solder the rst pad of

the LED without needing three hands. If you

are right handed, solder the right pad as you

are looking at the PCB. If you are left handed,

solder the left pad.

We will be holding tweezers in our non-domi-

nant hand (to hold the LED) and the soldering

iron in our dominant hand. Orient the LED in

the tweezers so it will be in the correct orienta-

tion when soldered to the PCB. Bring the LED

towards its footprint on the PCB and get it into

position next to the pre-soldered pad.

When ready, apply heat to the soldered pad

with the soldering iron and simultaneously

bring the LED towards the molten solder.

When the LED is in position and the solder is

covering the LED’s terminal, while still hold-

ing on to it with the tweezers, pull away the

ASSEMBLY

D1 R1

D10

D11

Assembly Instructions

soldering iron. If you pull away the soldering

iron without holding onto the LED, the LED will

likely come away with the iron.

If the LED is not at with the PCB, you can

apply downward pressure with the tweezers,

while temporarily heating the solder. When

you are happy with its position, solder the

other pad to the LED. Do this by apply heat to

the pad and the LED’s terminal while feeding

solder to the joint.

Repeat the procedure with all the LEDs. Again,

pay attention to polarity.

To desolder a surface mount LED you can ap-

ply a lot of solder to the area and use that to

heat up both pads at the same time, enabling

you to scrape the LED off with the soldering

iron tip. This will destroy the LED.

You can also try and heat one pad and wait

for the heat to transfer to the other through

the LED. This may take some time, and some

heat. This will destroy the LED and potentially

lift the pads on the PCB.

If you have a wide soldering iron tip, you can

try and hit both pads at the same time and

scrape the LED off like that. You can also use

the thinner tip lying at to achieve the same

effect in a more awkward fashion.

Resistors

R1 = 10K, R2 = You choose, R3 and R4 = 2K

The resistors will be soldered in the exact

same way as the LEDs but the polarity can

be ignored. It is advised to keep all the writ-

ing the same way up to prevent the universe

from imploding.

Capacitors

C1 = 100nF, C2 = You choose

The surface mount ceramic capacitors can be

treated in the same way as the resistors and

LEDs. They are not polarity sensitive.

When desoldering them, heat quickly con-

ducts through the capacitor, so heating one

pad and scraping the cap off when the heat

has melted the solder on the other pad is a

viable way to remove them with just a sol-

dering iron.

NE555

This is an 8 pin IC in an SOP package. That

means there is about a 0.7mm space between

the pins. You should denitely be using a thin

tip on your soldering iron. I use a 0.8mm tip.

All chips are polarity sensitive, so make sure

you align the pin 1 symbol on the chip with

the one on the PCB. SOP chips should have a

circle next to pin 1 on the top of the chip. If not,

the line and direction of the text can be used

(pin 1 is the bottom left pin).

Just like the 0805 components, we will solder

one pin to the PCB rst, to hold it in place. To

do this, add some solder to the pad closest

to your soldering hand. With tweezers holding

the chip in the right orientation, bring the chip

towards the pad with solder on it while heat-

ing the solder. Remove the heat when you are

satised with the chip’s position. If it is taking

a long time (>10 seconds), remove the heat

and try again when the PCB and chip have

cooled down.

Now you can solder the pin diagonally op-

posite while slightly repositioning the chip if

needed. Apply heat to the pin and pad while

feeding a tiny amount of solder to the joint.

Now solder each of the other pins. If you cre-

ate a bridge between two pins, make sure your

tip is clean and shiny, then drag it towards you,

between the two pins. This should part them. It

may take a few tries. If the solder doesn’t want

to behave, you may need to add some ux.

CD4017

The CD4017 can be soldered in the same way

as the 555 timer IC. This chip doesn’t have

the circle pin 1 indicator, so use the line on

the top. It should be the same way around as

the 555 IC.

To see a video of the assembly process includ-

ing desoldering components, please watch

the video here.

ASSEMBLY

D1 R1

D10

D11

1002

The green dot indicate a pin connected to

GND and the red dots indicate a pin con-

nected to Vcc. The closest GND and Vcc

pins are pin 15 and 16 of the CD4017 IC.

Polarity

Check all the components that are polarity

dependent. In this circuit, that would be the

LED’s and the ICs. The LED cathodes (faint

green mark on top of LED) should be con-

nected to the pad with a - symbol. If they are

connected the wrong way around, the LED

won’t work when the circuit is active. The

rest of the circuit should function though.

Make sure the ICs match the orientation in

the picture. Getting these wrong may be

fatal to the ICs.

Testing for faults

Before you power on the board, there are a

few things we need to do.

Visual Check

Firstly we need to do a visual check. We are

looking for solder bridges that connect two

pads that aren’t meant to be connected.

A magnifying glass is a good tool to have

when doing this.

If you see any solder bridges, bring your iron

up to temperature and drag it between the

two pads. You may have to repeat this a few

times. Make sure your iron is clean or the

solder won’t cling to it. Use ux if the solder

bridge isn’t parting.

Short Circuits

Now we need to check for short circuits (the

bad kind). If we have a short circuit some-

where on the board and we apply power,

you may destroy one of or both of the chips.

To check for shorts, get your multimeter and

put it in continuity mode ( ). Make sure

the black cable is plugged into the common

socket and the red cable into the red socket.

Touch the probes together and make sure

it makes a sound. Now press the black

probe on one of the GND contact (green

circle indicated in the diagram). Keep it held

there while you press the red probe onto the

Vcc(+) contact (the red circle indicated in the

diagram). Making sure they are both making

contact with metal, listen for a sound from

your multimeter. If there is none, excellent,

you don’t have a short between Vcc and

GND. You can jump over to the Power Test.

If you here a constant sound from your meter

when the probes are in contact with Vcc and

GND then you have a short. Check the IC

pins again for solder bridges. Focus on the

areas marked with boxes on the diagram.

Power Test

We can now connect the circuit to power.

Connect anything from 5-16V. The positive

lead/clip should be connected to the + pad

and the negative lead/clip should be con-

nected to the - pad.

If nothing happens, use a multimeter to

check the power supply. Check the IC ori-

entation again.

If one or a few LEDs don’t work, then check

their orientation.

Still having problems? Head over to the

forums on our website for help and sup-

port. www.shortcircuits.cc

Component Index

Here you will nd information about each component that this kit includes. We have included

some of the different sizes and shapes you may nd out in the wilderness, different uses for

each component, and important data that can be found in datasheets to help you design circuits

around them. We have also outlined what happens when these components go pop and how

to diagnose and replace them. This information can be used to x your household appliances,

rather than throwing them away. Make do and mend, as they used to say!

Capacitor - Ceramic 18

Integrated Circuit - NE555 Timer 21

Integrated Circuit - CD4017 Decade Counter 23

LED 25

Resistor 26

COMPONENTS

Capacitor - Ceramic

Overview

Capacitors are so named for their ability to

store a certain capacity of electrical energy

(Capacitance), measured in farads (F). A

certain amount of capacitance exists be-

tween any two conductors that are in close

proximity (this can sometimes be seen in

an LED matrix in the form of ghosting, and

can also cause problems in other sensitive

circuits). Capacitors use this in a controlled

manner, for various purposes. They usually

consist of two conductors separated by a

dielectric substance. A dielectric is an in-

sulator (does not conduct electricity) that

can be polarised (negative on one side

and positive on the other) by an electric

eld. In this case, the dielectric material is

ceramic. A dielectric substance increases

the amount of electrical energy that can

be stored compared to non-dielectric sub-

stances like air.

Physical Construction

Quick Reference

Common Varieties

Important Ratings

Check

Polarity

Positions

Type

Schematic

Symbol

PCB Symbol

Designator

passive

Single

Layer

High

Voltage

Multilayer Surface

Mount

SL = Single Layer

ML = Multilayer

Parameter Typical Values

Capacitance 0.5pF - 1uF (SL)

1pF - 470uF (ML)

Capacitance Tolerance

5 - 20%

DC Rated Voltage 10V - 20kV

Pitch 2.54 - 12.7mm

metal electrode

protective coating

ceramic dialectric

metal contacts

104J

multiplier

tolerance

(J=5%)

first digit

second digit

10 x 10000 = 100,000 pF

= 100 nF

= 0.1 uF

(picofarads)

(nanofarads)

(microfarads)

Identication

COMPONENTS

Capacitor - Ceramic

Common Uses

Decoupling

Capacitors are often used to protect cer-

tain components from interference from

other parts of the circuit. Most common

applications are right next to any integrated

circuit (IC). The capacitor acts as a storage

reserve. If the voltage drops below the re-

quired voltage, the capacitor will use it’s

stored energy to make up the difference.

If the voltage increases above the required

voltage, the capacitor absorbs the excess

voltage. The microcontroller, or other sensi-

tive device will see a much more even volt-

age because of this. Decoupling capacitors

are placed as close to the sensitive parts

of the circuit as possible, for maximum ef-

fectiveness. To smooth higher frequencies,

you would use a low value capacitor (like a

10nF - 0.1uF ceramic capacitor). To smooth

the lower frequency noise, a higher value

would be used (often a 1-10uF electrolytic

capacitor). Decoupling capacitors feature

in most of our kits.

Filtering

Unlike resistors, who’s resistance stays

constant no mater what frequency of sig-

nal that’s passing through it, Capacitors

are reactive devices that resist higher fre-

quencies less and lower frequencies more.

Because of this, they will block DC signals

(very low frequencies) and allow AC signals

(alternating at higher frequencies) through.

This is useful when you need AC and DC

in a circuit, but only want AC signals in a

certain part. A common example of this

is a microphone circuit. The microphone

needs a DC signal to power the device,

but records AC signals (sound) from the

environment. When we pass the AC noise

through to an amplier circuit, we want the

AC signal, but not the DC. Adding a capac-

itor in series will remove the unwanted DC

signal. (See the Sensor Array for a working

example of this)

ATMega328P’s Decoupling Capacitors

AC Filter on the Sensor Array

(electrolytic Capacitor)

Troubleshooting

Unlike electrolytic capacitors, ceramic ca-

pacitors rarely fail in normal use. However,

if the voltage exceeds the datasheet’s rec-

ommended maximum values (breakdown

voltage), they can and will produce magic

smoke. If they look burnt, or smell burnt,

then the charge may have arced across the

dielectric layer and will have overheated

considerably.

A simple test to see if the capacitor is

functioning as it should is to measure it’s

resistance. As the capacitor charges, the

resistance will increase. Before all tests

make sure you discharge the capacitor by

bridging the leads with a screwdriver. Set

your multimeter to Ohms and attach your

multimeter probes to each lead. If the resis-

tance climbs to innity, then the capacitor

is functioning as it should. If the resistance

reads 0 then the dielectric layer has been

compromised and the capacitor will need

replacing. This method unfortunately

doesn’t check its capacitance.

The only reliable way to test a capacitors

capacitance is to remove it from the circuit

and test it with a multimeter capable of

reading capacitance. Set your multimeter

to capacitance (Look for the____symbol)

and connect the multimeter to the legs of

the capacitor. If the value is no longer within

the stated tolerance, replace it.

Capacitor - Ceramic

Another form of lter that uses capacitors

is an RC lter. The most common RC l-

ters are low-pass and high pass lters.

As the name suggests, the low-pass lter

lets low frequency signals pass but not

high frequencies. High pass lters simply

achieve the opposite. This is a passive

lter as it uses passive components (resis-

tors and capacitors). The following are the

simplest of low-pass and high-pass lters,

the equation that governs their properties

and a graph to show the typical relation-

ship between frequency (Hz) and amplitude

(given in volts).

Time Delay

In an RC circuit, capacitors take time to

reach their maximum store of electrical

energy when exposed to a voltage source,

and to deplete that store when the voltage

source is removed. This can be used to cre-

ate a time delay between turning on a volt-

age supply and a component receiving the

voltage it needs to turn on, or read a logic

level high for on a microcontroller for exam-

ple. We can use a simple equation to work

out how much time it will take the capacitor

to reach approximately 63% of the supply

voltage. This is refereed to as the RC time

constant and uses the symbol tau (T). In the

example circuit the LED will gradually get

brighter until the capacitor reaches capaci-

ty, following the curve of the graph.

Low-Pass Filter

High-Pass Filter

Time Delay Circuit

Input

Output

cutoff

= 1

2πRC

Volts

Frequency

cutoff

= 1

2πRC

Input

Output

Volts

Frequency

cutoff

1000uF

10k

Input

(seconds)

= R(Ohms) x C(farads)

= 10,000 x 0.001

= 10 seconds

Volts

Time Constant

(

)

LED

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