Re: 555 timer circuit

From: Byron A Jeff (byron_at_cc.gatech.edu)
Date: 12/29/04 

Date: 29 Dec 2004 12:24:24 -0500

In article <1104319351.333874.44970@z14g2000cwz.googlegroups.com>,
 <burtonfamily@dodo.com.au> wrote:
>Hello,I made a simple astable 555 timer circuit the same as the one on
>the 555 web site,but im having trouble working out the values of the
>two resistors and the capacitor so it will turn on for 3 minuts and off
>for 5 minuts.Could someone please point me in the right direction as i
>found the data *** a bit confusing.thankyou in advance

Frankly a 555 is a bad choice for such long delays. The precise timing
depends on the values of the caps and resistors of the circuit and at
200-300 second delays, the tolerances will be off by tens of seconds
every cycle.

A better solution is to count smaller more presice cycles. If you have wall
power available, an AC wall wart, along with a zener diode and a resistor
makes for a nearly dead accurate 60HZ reference that can easily be counted.

My belief is that microcontrollers are the 555 of the 21st century. A
single microcontroller like a PIC 12F629 could easily handle your
requirements with or without the 60 HZ reference.

BAJ

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