I'm a little perplexed...I've just been playing with some 555 timer circuits, pretty neat and pretty simple. I then tried making a cct for. 7 minute monostable timer..for a bathroom heater - one push eq 7 mins run time then off till next push of momentary switch.

So I started making the RC combos for this.. it makes sense to use a large cap, say 470uF etc so as to keep resistance down. But there in lies the problem, to go for a electrolytic to get the larger values you get like 20% tolerance which changes the time quite a bit. If I go for a tantalum which might be 10uF means a huge resistance value, even with a 1% tolerance R that can also lead to a very un precise timer.

So what is one to do? How can you make a 555 timer for longer timing periods - ie 10-30 minutes and overcome these issues.

So putting it in contrast I then look at the economies of scale.. I can buy a PICAXE chip for about $9 which requires bugger all supporting components and is quite accurate and infinately more flexible... Why would one not scrap the 555 idea and just go for a PICAXE circuit(assuming one can program picaxe, as I can)..

Appreciate all ideas on all points!

Thanks..

 

The problem with using 555 for large timings is that the required electrolytic capacitors will have to much of a leakage current (Equivalent to having a resistor in paralell with an ideal capacitor). Because of that, errors will be large, and after a certain value of desired timing, the equivalent resistor will be the same value as the resistance you used, and the capacitor wont charge to the required 2/3 for triggering.

An alternative i can suggest to the pic, is in case you just need a longer timing, use a 555 with a much lower timing (maybe 1min or less) as a clock input for a binary counter. The counter will work as frequency divider. As an example if you use a 1 min timer, and an 8 bits counter, you will get 128mins of timing.

If you need the signal to only remain on for a few seconds, use a resistance between the output of the counter and the reset pin, and a capacitor between this input and ground. This will cause the counter to reset after reaching the timing. Calculate appropriate time constant.

 

Several of the PIC10F and PIC12F series can be purchased for under $0.50/each even in small quantities. Many of those have built-in oscillators, so if you can tolerate a 1% variation in clock speed you don't need much in the way of support circuitry besides a voltage regulator & bypass capacitor.

555 timers still have plenty of uses, and are still being produced by the millions by numerous manufacturers.

Long duration times aren't one of their strong suits.

 

The Picaxe 08M is only $3, and it can be programmed for many timer types. It does need one external component though -- a 10K pulldown.