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Timing is complicated in electronics, and there are only a few goodsolutions.

You should use a microprocessor called the LM555 timer (or just the 555 timer). While seemingly

daunting at first, this small 8-pin device affords a lot of features and can perform one-shot timing

(press a button and keep a light on for a certain time determined by the resistors and capacitors),

astable timing (essentially generating an on and off signal), bistable timing (controlling two lights; one

on and one stage, and one on at another stage), and with some clever 'hacking' it can be used toencode an analog signal into a PCM (pulse-code modulation) signal, often used for devices such as

remote controls and toy aircraft.

A quick overview: There are the pins 4 and 8 which should be powered. When you use pins, you count

from the bottom left, which is indicated by the small indented circle in the chip. Pin 1 should be

connected to the negative end of the battery or power supply. The chip can run from 2 volts to up to

14 (some chips support 18) volts but for something like an LED, 9 volts should be good (a different

voltage would mean you'd need a different resistor for the LED). Pin 5, optionally, should be

connected through a small capacitor (100nF or near) to the negative end of the battery; the capacitor

should not be electrolytic. This is optional as it just helps settle the value on one of the internal

comparators and makes the timing more accurate.

Pin 7 should be connected through a 10k resistor to positive. Pin 6 should be connected to pin 7. At

the end of this series, you should connect a capacitor, which determines the timing amount (as well as

the resistor). The second capacitor should be about 100 to 220 uF, which will give you a range of 1.1

seconds to 2.42 seconds. You can of course adjust to any value suited. Pin 2 is the trigger. A 10k

resistor should be wired though a push-to-make switch, connected to the negative end of the battery.

Then, the positive end of the push-to-make should be wired into pin 2. Pin 3, the last pin, is the

output. Wire a 330 ohm resistor and a green LED into this and you are set. Wire the end of the LED to

ground (the negative end). Press the button, and it should work, timing for the value of the capacitor.

The formula for the time is T=1.1(CR) where C is the timing capacitors' value (C being in farads, and

it will be very small), and R being the resistor connected, in ohms. This will be the larger value. The

1.1 multiplication is just because of how the timer works.

The whole circuit can be build on a breadboard/prototyping board. Or, if you have the skills, you can

build it on a printed circuit board.

It should be noted however, this timer is not 100% accurate. Because of component tolerances, you

should allow for as much as 17.5% inaccuracy in both directions. If you need more accuracy, you

need to get into the very complicated world of quartz crystals, real-time clocks and the like; and even

they suffer from 'clock drift' (being incorrect by up to 30 seconds a month depending on temperature).

In short, if you need accurate timing, you need an atomic clock!

Read more:http://wiki.answers.com/Q/How_do_you_build_a_simple_timer_circuit#ixzz2FwY1mGyD

5 to 30 Minute TimerDescriptipn:

A switched timer for intervals of 5 to 30 minutes incremented in 5 minute steps.

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Notes:Simple to build, simple to make, nothing too complicated here. However you must use the CMOS type555 timer designated the 7555, a normal 555 timer will not work here due to the resistor values. Also alow leakage type capacitor must be used for C1, and I would strongly suggest a Tantalum Bead type.Switch 3 adds an extra resistor in series to the timing chain with each rotation, the timing period usdefined as :-

Timing = 1.1 C1 x R1

Note that R1 has a value of 8.2M with S3 at position "a" and 49.2M at position "f". This equates to justshort of 300 seconds for each position of S3. C1 and R1 through R6 may be changed for different timingperiods. The output current from Pin 3 of the timer, is amplified by Q1 and used to drive a relay.

Parts

Relay 9 volt coil with c/o contact (1)S1 On/Off (1)S2 Start (1)S3 Range (1)IC1 7555 (1)B1 9V (1)C1 33uF CAP (1)Q1 BC109C NPN (1)D1 1N4004 DIODE (1)

C2 100n CAP (1)R6,R5,R4,R3,R2,R1 8.2M RESISTOR (6)R8 100k RESISTOR (1)R7 4.7k RESISTOR (1)