I'm looking to build a timer (it was suggested I look to use a 555) to turn on and off a load (which I will use an external relay driven by the timer). The on time will equal that of the off time. The time will be approximately 13 minutes.

I've done a bit of research on creating timers using the 555 chip and all the designs I found were to achieve cycle times in under 1 second. So, when I started to modify the values of the R & C to achieve 13 minutes I started getting some pretty large numbers. Then I found a software program called 555 Timer Pro. When I used to check my calculations the program warned me that the high resistor requirements needed to achieve t=13 minutes would result in too little current to drive pin 6.

So, now I'm out of my area of knowledge.

Can anyone suggest a design that would meet my needs here?

Thank you in advance.
Frank

 

555 won't cut it. A small microcontroller could do it.

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Any additional direction would be very helpful. I really don't know anything about "micro-controllers".

Thanks for the guidance.
Frank

 

Have the 555 drive an n stage counter. If the 555 puts out 1 Hz and 13 minutes is 780 seconds, a 10 stage [2^10 = 1024] counter should do it.

The other way is to cascade CMOS 555 chips.

 

Use a CD4060 which is an oscillator and a divider or a CD4541 oscillator timer.

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How accurate does this need to be?

If you use cheap 20% tollerance ceramic capacitors for the oscillator, the time could vary between 10:24 and 15:48 minites.

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555 could not be used for a t=13 minutes for sure
u can use a 8051 microcontroller with the program using wait states for 13 minutes

 

I think Willbe's idea is a sound one. A 555 driving a CD4040 with the divide by 9 output would give equal times of 6.5 minutes if the 555 is set for astable period of about 0.76 seconds. The 4040 would probably drive a solid state relay to turn power off and on. With the 555 a regulated power supply would not be needed as the 555 period would not change with a unregulated power supply where the 4060, or 4541 both as oscillators and dividers would change frequency with varing supply voltage.

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Thanks!

 

I agree but I am sure that is no surprise to anyone here

For under a $1 you can get a PIC that can be programmed to do exactly what you want and with fewer parts.

Once more I forget where (and I can not find it) a site that used to sell small pics to replaces timers and various other simple chips/apps. They ceased their commercial effort and offered the code to the public for free download.

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... and for another 49$ you can purchase a programmer.

Hans

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Maybe... but a LVP would cost next to zero. I have wanted to play with the low voltage programming but with a number of programmers on hand have not yet found the motivation. You would have to get a chip the supported LVP.

Or if you have a friend with a programmer you could use a boot loader.

IMHO either you want to program or not. Cost is not a real factor.

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