K2579 Stop Start timer

Hi. Please understand my electronics knowledge is 40 years old and not used for most of that time. Having recently retired I’m trying to get back up to speed and interest and I’ve purchased a K2579 kit to assemble and use in my workshop. I want to modify the time delay to achieve selection of 4 specific periods ie. 4mins;8mins;16mins and 32mins and to achieve this I want to add increasing capacitor values via make & break switches.
Excuse me if this is niaive, but would I need double pole switches to make or break both sides of capacitor. It would be easy to switch +side and use a common contact for the -neg but is there a problem with feedback to the wrong side of the other caps ?

A single pole rotary switch could do the job, but keep in mind that caps have huge tolerances, so it will not be easy to determine their value.
Maybe it is a better idea to have one fixed cap and select the resistor value. Instead of fixed resistors, you could use trimmers, which can be accurately adjusted for each setting.

Hi Vel 417
Thanks for the post. I like the sound of the idea of using a single cap and variable resistors (potentiometers ?) Trouble is I’m not confident of identifying the right components - can I pick your brains please ?

C2 will need to be increased and you will have to provide 4x RV1 set to a different value and selected with a rotary switch.
Sorry, no values, please experiment.

Hi Vel 417

I think I’ve got a problem !

I’ve now built the kit exactly as the instructions and only with kit components but I cannot get the timer delay greater than 5 minutes with R1 adjusted to max - should be about 15 minutes according to user manual.

I replaced the 100uf Cap with a 470uf Cap and delay time extended to 30 minutes with R1 adjusted to max. I expected that if 100uf would achieve 15 minutes then 470uf would be more than enough to achieve my required maximum @ 40 minutes.

Any ideas ??

As cap tolerances can be quite big, it is possible that their is a big difference between theoretical time and actual time.
Basically, there are better ways to achieve long delays, as the cap values get really high.


OIC, so the statement on your literature that says ’ set the desired delay with the trimmer… easily lengthened up to 60 mins’ isn’t actually true !
How I wish you’d told me that before I’d invested so much time and money in the project. Thanks

You can time up to 60 minutes with this kit, that is not the problem.

This is a very basic circuit, mainly geared towards education or proof of concept circuits.
Please keep in mind that this classic kit was designed in 1988.
Meanwhile, better designs with microcontrollers have become available.
Of course, they are more expensive.

No, not true.

You cannot time up to 60 minutes without increasing the Cap value significantly which leads to huge inconsistencies because of wide tolerance of Cap!
Yes I understand it is only a basic circuit and I aimed it at education - educating me ! However, not achieving my objective, which we discussed before I started, has only confirmed that this circuit does not work beyond starting and stopping and pretty well without any control at all.
You mention better designs with microcontrollers - do you produce/market any of these as kits ?

We have three different timers with microcontrollers in our range.
Unfortunately, none of them allow addition of a simple rotary switch to select different delays.
Anyway, the references are: