A switch selectable timer?

[vanagonvw]

I am working to come up with a ckt that can give me an accurate, timed pulse up to five minutes in duration. It will have to allow a dipswitch to choose between one, two, three, four, and five minutes.

It is my impression that five minutes is a pretty long time to ask a one shot like a 4538 or a 123 type of device to be stable and repeatable, so I am seeking any clever ways others may have come up with to accomplish this.

I don't think the venerable 555 is up to that task either, based on leakage and stability as I understand it.

I cannot use a controller due to the cost, so I am holding out hope for a ckt idea that will point me in the right direction.

How accurate is probably going to be a function of cost, and is hard to quantify, so I am holding out for the very best I can get, using logic parts, and not a micro of any kind.

Much obliged for any advice or suggestions.

Thanks,

John

 

[XNOX_Rambo]

The ICM7240 might be suitable, as far as functionallity goes. Whether it is
accurate enough I leave for you to decide.

**broken link removed**

/Rambo

 

[IanP]

Have a look at these projects:
https://www.arrl.org/tis/info/pdf/9907038.pdf

 

[VVV]

First of all, "accurate" is a relative term. If you could put a number on it, it would be great.
I think the CMOS version of the 555 should be able to do the trick. I would breadboard it, instead of rejecting it beforehand.

If that is not accurate enough, how about having the 555 run as an astable and divide the frequency with a counter, maybe a dual one? That way the period of the 555 only needs to be a maximum of 30s, which is pretty decent.

And, although you said "no controller", I will mention a PIC12x5xx (or maybe the latest 10Fxx series), using the internal RC oscillator. I think you can get better than 5% accuracy over -40°C to +85°C. One pin will be output and three others connected to your DIP switches. Takes up less board space, too.

 

[sajal1975]

hi,
Have a look at may find useful data
www.masterclock.com/data_files/tm100_um.pdf