Egg Timer

The below table helps us to prioritise the requirements for our egg timer, this table is known as a SPECIFICATION.

Must	Should	Could
Have the ability to start sounding after a certain amount of time	Use a PCB mounted slide switch selector	have a custom designed and made housing
Use light as well as sound to show the alarm	Be able to set the time for brushing teeth and egg cooking.
Be inexpensive i.e. no microcontroller	Low power so it lasts a long time (battery does no go 'flat' quickly).
Loud enough to be heard from at least 5 metres away	Have a method of choosing the amount of time

Boiling an egg requires perfect timing.
There are many ways to create a timing circuit, but one of the simplest ways is to use a 555 timer circuit.
The 555 timer circuit relies on a basic electronic timing circuit which uses resistors and capacitors to use time in different ways.
A capacitor takes time to charge. We can use this, along with a resistor to create different time delays.
It's output pin (3) can be made to change its behaviour with these two simple components.
This is the basis of this project.

A 555 timer can be made to work in a few different ways by changing the circuit slightly.

The circuit diagram above is called astable. You can see it running live below.

The term 'astable' means that the output pin has no stable state; it will constantly flip between being on and off.
We can calculate how long it stays on (high) and off (low) for by using a few formulae.
We can calculate how long the output will stay on (high) for in seconds, with:

Finally, we can calculate the frequency (number of on/off cycles per second, measured in Hertz) with:

In the example above, resistor 1 has a value of 1k Ohms. 1k is shorthand for 1000, so R1 = 1000
Resistor 2 has the value 10k, so R2 = 10000
The capacitor (C) has the value 100 μF. μF is shorthand for 'microfarads'.
You may already know from science that putting a μ before a measurement means that what we are actually writing is that number x10^-6.
So C = 100 μF = 100 x 10^-6 = 0.000100 = 0.0001
We can now calculate the characteristics of our circuit.
Time High = 0.7 x (1000 + 10000) x 0.0001
Time High = 0.77s

Another way is called 'monostable', as the circuit has one stable state: Being turned off.

In this setup, the output pin (3) stays off until an input (e.g. a switch) is triggered.
Once triggered, the output is turned on for a set time, then goes back to being off again.
We can calculate the time delay using a simple formula, T = 1.1 x R x C
In the example above, R is 10k (so R = 10000)
C is 220μF. Written in Farads, this is 220 x 10^-6 = 0.000220.
T = 1.1 x 10000 x 0.00022
T = 2.42 seconds.
These are handy for all sorts of applications where you want something to happen for a set period of time. E.g.
- A patio heater which is turned on for 2 minutes when a switch is pressed.
- A security door for a block of flats, where the door lock is released for 5 seconds when the owner 'buzzes you in'.
- A door bell, where the bell rings for 3 seconds when someone pushes the bell switch.
- A hand drier, where the heater and fan are energised for 15 seconds when someone waves their hand under the sensor.
- An outdoor security light, where a floodlight is energised for a minute when someone walks past.

Try building the Monostable circuit above using circuit wizard.
Change the value of R and see how it affects the speed at which the LED flashes.

PART 1: Create the circuit diagram, as per the task above.
PART 2: Set the resistor and capacitor so that when triggered, there is (roughly) a 1 second delay. Show your calculation.
Hint: You can use a calculator like this one if you're struggling, to check your values.
Which type of circuit do you think our egg timer will use? Monostable or astable? Why?
Extension: Create the astable circuit, and make it flash twice in a second.