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# variable 5,10,15,20 minutes timer circuit

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#### james

##### New Member
i'm here again. Could you please help me make this circuit 5,10,15,20 -minute timer. i've already searched many sites and i have to make certain modification to suite my need. I used 555 timer and varies R and C values. I like to have others design. thankst

Buy a 4 stage switch, set up the resistor loads per switch mode which set up the timer values of the 555 and go for it?

OR put in a potentiometer and make it variable, but that maybe isn't accurate enough for your needs?

//Albert "thec" Sandberg

555

thec said:
Buy a 4 stage switch, set up the resistor loads per switch mode which set up the timer values of the 555 and go for it?

OR put in a potentiometer and make it variable, but that maybe isn't accurate enough for your needs?

//Albert "thec" Sandberg

You would be pushing your luck trying to get a 20minute delay from a 555. You need to make it drive a counter of some sort. eg, a 4006 or similar.

Ooops, I instantly thought of seconds

Yeah, would do. you could easy combine 555 with a counterchip...

//Albert "thec" Sandberg

circuit Diagram to try

Descriptipn:
A switched timer for intervals of 5 to 30 minutes incremented in 5 minute steps.
Notes:
Simple to build, simple to make, nothing too complicated here. However you must use the CMOS type 555 timer designated the 7555, a normal 555 timer will not work here due to the resistor values. Also a low 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 us defined 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 just short of 300 seconds for each position of S3. C1 and R1 through R6 may be changed for different timing periods. The output current from Pin 3 of the timer, is amplified by Q1 and used to drive a relay.

Parts List:

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)
Circuit by Andy Collinson
Source: zen's circuit directory

I'd have hoped that after 6 years the original poster would have sorted out his problem by now

Its worth checking the date of posts before replying to them.

Welcome to Electrotech by the way

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The OP of this thread last visited this site on .....

2nd November 2002

On the plus side, at least the newbie who bumped this 6 year old thread bumped it with useful information rather than the "Dear Sir, My school project is due tomorrow and I want to design my own nuclear reactor"

The OP of this thread last visited this site on .....

2nd November 2002

On the plus side, at least the newbie who bumped this 6 year old thread bumped it with useful information rather than the "Dear Sir, My school project is due tomorrow and I want to design my own nuclear reactor"

hi,

Aren't those resistors 8.2M x 6 really high for a 555?

Aren't those resistors 8.2M x 6 really high for a 555?

I would guess that the leakage current thru the 33uF would be high enough to keep the cap discharged for ever.!

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