# Coursework [help] - egg timers

Discussion in 'Homework Help' started by Treznax, Sep 28, 2009.

1. ### Treznax Thread Starter New Member

Sep 28, 2009
1
0
Hey,
Can someone PLEASE get 3 egg timer circuits and break them down into input/out/process please?
[I have googled, I post link to an image of egg timer and resonce I get "This is not an egg timer :\ "]

Also: Explain how it works please
Also 2: Make the first one a like simple circuit, and then the second one a sort of harder curcuit and the 3rd one an even harder one (Not to hard, just slowly get harder please)
**ELECTRICAL EGG TIMERS ONLY!**
I'm really stuck :s

-I think best way to do this is paint? You an easily use their starit line tool thing to cut input/process/output and u can use their text box thing to explain how it works
Thanks

2. ### JDT Well-Known Member

Feb 12, 2009
658
87
So, you want a electronic device that after being turned over (or switched on) it beeps or flashes a light after a few minutes?

Simple electronic timers usually involve charging a capacitor through a resistor. A circuit switches something on when the capacitor gets to a certain voltage. The time is approximately = C x R (seconds). For example: charge a 1uF capacitor through 1M ohm. The voltage on the capacitor will get to approximately 63% of the supply voltage in 1 second.

To get times greater than a few seconds requires large capacitors or large resistor values.

A more sophisticated way is to use an accurate oscillator (quartz crystal) and count the cycles using a logic counter. When the count is up, buzzer sounds.

If you want to make your timer function like a real egg timer you could build in a mercury switch so that the power is applied when you turn it over.

Start Googling!