Hi,
I want an oscillator with a square waveform duty cycle 9 mins low (approx), and 500ms high (approx).
It is important that the low duration doesn’t exceed 10 minutes, so 9 minutes is just a generous error margin.
It appears the only difference between t1 and t2 in the diagram is achieved by R2.
Is it feasible to expect the desired outcome with the same capacitor C1 responsible for both durations?
It occurs to me that I could replace R2 with a very high value resistor to increase t2,
but can I keep t1 very low at the same time? The capacitor can’t be so large that t1 blows out.
Hello there,
You should know right off the bat that this is not a good way to do it even if it does work. So get off the bat and on the ball and find another way to do it or one day you'll be stuck out in left field, and if i can think of any more baseball metaphors i'll get back here later :-)
But really RC timing with larger caps is a little problematic both in initial set points and long term age factors. There are other ways.
One way in particular that doesnt involve a micro controller is to use a timer like the 555 with reasonable cap and resistor values and a digital divider chip that divides the 555 frequency down to a much lower frequency. You can go pretty far with this idea because then the 555 will be operating normally and digital dividers are very very very repeatable in their divide ratios, and in fact may be flawless for the most part. This technique is actually used in very important applications where a change in timing could blow out very expensive equipment. In fact in those cases even an LM339 can be used as the base frequency oscillator with the dividers following.
If you really MUST go with a 555, then perhaps you can try multiple 555's. If you have one 555 trigger the next 555 you get a longer delay time using lower value caps and more reasonable resistor values. If you use more than two 555's you can probably go pretty far with this but you have to have the room for the chips on the board. Dont know if you are into SMD or not but the 555's also come in a dual package type so you get two 555's on one chip, and two chips means you have four 555's to work with. In fact, i would bet there is a better solution using LM339 comparator chips with a little thought. I might give that some thought myself it may be interesting.
I've done delays that go into the days and one i remember best is a special kind of battery charger that had to charge the cells once every 12 hours. The timing was around 11.75 hours off, then 15 minutes on. I just have to say it was pretty darn easy to do that using a very very cheap microcontroller chip at the time made by MicroChip. That takes a bit of learning to do if you never programmed a microcontroller before, but today there is no excuse because of products like the Arduino. The Arduino microcontroller is used by hobby folks all over the world and with good reason. There is a host of information on the web and sample programs that are so easy to change you'd wonder why you never used one before.
With the Arduino IDE you dont have to deal with the chip itself directly, dont even have to know how to solder. You can get jumpers that you plug into the Ard board and run a program that came with the IDE. From there a minor mod to the program and you can get delays that probably last longer than the age of the universe (ha ha). Seriously though you can get hours, days, months, years, you name it. You just need a 5v supply or even a 9v DC wall wart.
The cheapest ones are the Nano and they are small too. The entire board is something like 1 inch by 3 inches and all the parts are on board already. If you need an output transistor that's really all you need to add except for maybe a resistor or two. If you dont know how to mod the program im sure someone here would surely help you and it would not be much to change either, maybe a few lines of code which is very human readable too. This kind of solution should get you a sure home run.
Ok no more baseball metaphors for now, good luck with it and if you can please come back and let us know how you made out with it, whichever way you decided to do it.