Design Brief - The manager of a local 'super store' has asked you to design a promotional display for the entrance to the store that can be used to promote a product of your choice.

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Design Brief

The manager of a local ‘super store’ has asked you to design a promotional display for the entrance to the store that can be used to promote a product of your choice.  The display needs to be interactive, so it will operate when someone enters the store or stands in front of the display.  You have been asked to design and make a working model of this, to show how it will work.

Notes:

  1. The final coursework deadline is: Easter
  2. This planner does not include half terms or holidays.

Integrated Circuits

A 555-timer chip can provide an astable (one state) or a monostable (constantly changing) output.  When the voltage between the trigger pin 2 and ground pin 1 falls (press switch) pin 3 voltage rises and discharge pin 7 is disconnected.  The capacitor charges until threshold pin 6 and ground reach 2/3 supply, pin 3 voltage falls and pin 7 reconnects.  As a monostable the timer returns to output 0V after a time set by the resistor/capacitor.  As an astable, pin 2 connects to pin 6 producing an automatic restart.  These clock pulses can drive a flashing LED, a loudspeaker, a transistor, an IC, etc.

A 741 operational amplifier or op-amp is used as a comparator or amplifier with one output and two inputs.  A voltage at the ‘+’ input causes a positive output (source) if it is more than the ‘-’ input and a negative output (sink) if it is less than.  The voltage gain is 100 000 and the output current is 10 mA.  As gain is very high resistors are used to produce negative feedback and give a lower but predictable and constant voltage gain.

Motors and Relays

  1. A relay uses induction to isolate circuits.  Other inductors include motors and solenoids.  A protective diode is used to prevent back emf.
  2. A two-pole relay is used for motor reverse control and latched (memorised) switching.

Formula

  1. For resistors in series the effect is increased:

R total = R1 + R2 + …        e.g. R total = 100 + 100 = 200Ω

  1. For resistors in parallel the effect is decreased:

 =  +  = R total =

e.g. R total =  =  =  = 50Ω

  1. Ohm's law states that if conditions of a conductor do not change then voltage and current are proportional.  E.g. an LED requires 20mA from a 9V battery, the voltage drop across the LED is 2V and as V / I = R, the resistor value will be 7V / 0.02A = 350Ω or 330Ω preferred value.
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A transistor is a semiconductor used as a high-speed switch or amplifier with three connections: base, collector and emitter.  A ‘Darlington Pair’ enables a larger current gain.  To calculate dc current gain (hfe):        hfe = Ic / Ib

Mechanisms

  1. A mechanism changes a given input motion or force into a desired output motion or force. There are four types of motion: linear, rotary, reciprocating and oscillating.
  2. A Mechanical Advantage (MA) is gained when the effort applied is further from the fulcrum than the load.  MA = Load / Effort
  3. Moments are the product of force (N) x distance (m) and ...

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