GCSE DT

Rotary Motion

Going around and around; think wheels, cogs, tape decks etc.

Oscillating Motion

Swinging backwards and forwards; think about the pendulum in a clock.

Linear Motion

Movement in a straight line; think abiut a guillotine.

Reciprocating Motion

Movement backwards and forwards; think about a sewing machine needle.

State the relationship between:
- Moment
- Force
- Distance moved

Moment \= Force x Distance moved

State the relationship between:
- Anticlockwise moments (ACM)
- Clockwise moments (CM)

ACM balances CM \=\> ACM \= CM

Class 1 lever

- Load and effort on opposite sides of the fulcrum.
- Example: Crowbars.

Class 2 lever

- Load and effort on the same side of the fulcrum, with the load closer to the fulcrum.

- Example: Wheelbarrows.

Class 3 lever

- Load and effort on the same side of the fulcrum, with the effort closer to the fulcrum.

- Example: Tweezers

Mechanical Advantage

The ratio of the output force to the input force of a mechanism.

Velocity Ratio in levers

The ratio of the distance travelled by the effort to the distance travelled by the load.

Gear Ratio

The ratio of the number of teeth on a driving gear to the number of teeth on a driven gear.

Mechanical Advantage formula

Mechanical advantage \= Load / Effort

Velocity Ratio formula

Velocity ratio \=
Distance moved by: effort / load

Gear Ratio formula

Gear ratio \= Driven teeth / Driver teeth

Gear output speed formula

Output speed \= Input speed / Output speed

Reverse motion linkage

This creates an exact opposite motion to the input motion.

Parallel motion linkage

This creates an identical parallel motion to the input motion.

Bell crank linkage

This changes the direction of movement through 90°.

Efficiency of a mechanism formula

Efficiency \= Mech. adv. / Vel. ratio x 100

Screw thread uses

- Provide powerful movements (car jacks).
- To hold things in place (screws and bolts).
- To position things accurately (binoculars).

V-thread screw

- Thread that produces lots of friction.
- Used to hold things in place.

Square screw thread

- Thread that allows for high applied forces.
- Not very much friction compared to V-threads.

Butress screw thread

Thread that allow a force to be applied in one direction and quickly released.

Acme screw thread

Thread used where the nut engages the thread whilst still moving, which it is ideal because of its sloped sides.

Pulley mechanical advantage

There is no mechanical advantage when one of these is used but there is with more than one.

Pulley mechanical advantage calculation

Simply count the number of ropes that pass between the top and bottom sets of pulleys to find the \_________ \_____________ \_______________.

Pulley output speed formula

Output speed \= Input speed / Vel. ratio

Torque formula

Output torque \= Input torque x Vel. ratio

Round belts

Belt that is used where small forces are required or where the belt must twist (vacuum cleaners).

Jockey wheel

This wheel provides tension in belts that are transmitting a force over a long distance.

V-link belt

This belt is used in situations where a continuous belt is not feasible.

Grub screws

These headless screws go down a threaded hole and press against a shaft, keeping it in place (used with small forces).

Keys and keyways

A coupling that uses a grove and a corresponding tapered key that when interlocked create a very secure hold.

Cotter pins

These are tapped down into place with the flat side fitting to the flat side of the shaft and is held in place by nuts on the other end.

Splined coupling

This coupling is used where the length of one shaft must change.

Interference fit

This is where the shaft is too large for the coupling so is simply forced into it and held in place by friction.

Flanged / Muffed coupling

This coupling consists of both bolts and keyways and is used when shafts are aligned.

Flexible coupling

A coupling consisting of a rubber disk bolted to both shafts used when the shafts are misaligned.

Universal joint

This is used where shafts are misaligned by up to 20 degrees, a pair of which are used to compensate for the different velocities the shafts would end up rotating at.

Potential divider

The basis on which other sensing circuits are built.

Potential divider: more than half the supply voltage

When R2 in a potential divider is more resistive than R1 the output voltage will be...

Output voltage formula

Vout \= R2/( R1 + R2 ) × Vsupply

Light sensing LDR placement

LDR placed in R1 position in potential divider.

Dark sensing LDR placement

LDR placed in R2 position in potential divider.

Thermistor

A component who's resistance drops as the temperature increases.

Light Dependant Resistor

A component who's resistance varies with light intensity.

Touch sensor

A component which changes its output voltage when a finger bridges two copper contacts.

Moisture sensor

A sensor that relies on the fact water's presence in varying amounts changes its resistance. It is comprised simply of two rods inserted into a material.

Switches

Mechanical components that when pressed can make a circuit complete or break one.

Switches: poles

This is the number of circuits the switch can break or make at the same time.

Switches: throws

This is the number of positions each switch can be set to.

Integrated circuit: monostable

A system that only has one stable state, that can be changed but will always revert back to that stable state.

Integrated circuit: bistable

A system with two stable states in such a way it can remain in either position.

Integrated circuit: astable

A system with no stable state, it changes constantly.

Create a time delay

A resistor, capacitor and transistor in series can be used to \______ __ \_____ \_______.

Transistor trigger voltage

0.4-0.6V

Logic gate symbols

Relay

- A device used to separate two circuits. - The control circuit energises a coil.
- A magnetic field switches the contacts.
- Double pole versions can latch.

Light Emitting Diodes (LEDs)

These only emit a light when the current is flowing in the right direction. They can only handle a small current so must be used in series with a resistor.

Block diagrams: input box

This part of a block diagram should contain the variable you are measuring or a component to measure it.

Block diagrams: process box

Should make a comparison / component capable of doing so. It could also reference a threshold being reached / component activates at that threshold.

Block diagrams: output box

Should reference a component that will generate a visible / audible output.

Feedback

This is the sending of information back into a system to control the process. This can be positive (greenhouse regulation) or negative (reamplified speaker sound).

Microswitches

Extremely useful with regards to motor reversal circuits, they indicate when an object has reached its movement limit.

Reed switch

Extremely useful switch for presence detection consisting of two contacts that are connected when attracted to a magnet and released when not.

Testing a completed PCB

Check visibly that all components are correctly aligned and of correct values then check for short circuits / breaks in the circuit with a multimeter.

Friction

A disadvantage of this in mechanical systems is that energy loss to forms such as heat and sound causes a reduction in efficiency of the system.

Lubrication

A method that reduces friction by smoothing the area of contact.

PCB layout rules

1. Drawn tracks must not touch additional pads / tracks.

2. They must be neatly drawn and of correct thinkness.

Ohm's Law

Voltage \= Current x Power

Transistor

- Can be used as an amplifier.
- Can be used as an electronic switch.
- Can be in NPN or PNP silicon.

Resistors

Electronic sompnents that increase the reistance of a circuit and are usually made from carbon film.

Resistance values: Band 1

The first number in a resistor's resistance value.

Resistance values: Band 2

The second number in a resistor's resistance value.

Resistance values: Band 3

The number of 0s in a resistor's resistance value.

Resistance values: Band 4

The tolerance of a resistor's resistance value.

Prefixes: Tera (T)

1 x 10^12

Prefixes: Giga (G)

1 x 10^9

Prefixes: Mega (M)

1 x 10^6

Prefixes: Kilo (K)

1 x 10^3

Prefixes: milli (m)

1 x 10^-3

Prefixes: micro (µ)

1 x 10^-6

Prefixes: nano (n)

1 x 10^-9

Prefixes: pico (p)

1 x 10^-12

Analogue signal

- Infinite states between given voltage values.

- Their information can be reduced by interference.

Digital signal

- Two states (On or Off 1 or 0 etc.).
- Information is sent in pulses of uniform amplitude.
- Easier to remove interference over longer distances.

Back EMF

Unwanted voltage values created when current flows the wrong way in a circuit.

Capacitors

Components that can store charge for a period of time.

Planned obselesence

When products are designed to become outdated by means of having software too demanding to run on old hardware (oh apple sorry didn't see you there).

CNC

Computer Numerically Controlled

Idler gear

Gear that reverses motion.

PCB production: Milling

- Used for small quantities of PCBs.
- Low start up cost.
- Machine is slow and is expensive to operate.
- Good for prototyping as it is easy to reproduce failed versions.

PCB production: Acid bath

- Used to make large quantities of PCBs.
- Higher start up cost but is cost effective.
- Still is slow per PCB but allows many simultaneous ones to be made.

Pros of Computer Aided Design

- Editing designs is easy and fast.
- Designs can be instantly sent anywhere.
- Numerous versions can be copied and stored.
- Parts and components can be tested.

Cons of Computer Aided Design

- Software licenses can be expensive.
- Requires expensive, powerful hardware.
- Confidential designs can be leaked.
- Designs could be inadvertently replaced.

Pros of Computer Aided Manufacture

- Process can be repeated numerous times.
- It can operate almost continuously.
- High start up cost but cheaper long-term.
- Products can be made to a uniform standard.

Impacts of E-waste

- Contains toxic chemicals and pollutants.
- People salvaging appliances can suffer long-term health consequences.

Solenoid advantages and disadvantages

- Solenoids are small and easy to power.
- They have a short throw and are noisy.

Breadboard advantages and disadvantages

- Easy to change components.
- Fragile, poor contacts.