555 countdown timer design question

[Radio Fox]

Re: 555 countdown timer design question

But this won't have any negative, possibly destructive effect either, right ?

Yes, you're right, precision is not critical up to the millisecond (though it would be nice to have, yet impossible with such imprecise components). A tantalum cap would require a much larger resistor, possibly in the order of mega ohms, since its value is by default lower....really, what WOULD be the maximum available tantalum cap ? On second though, you could perhaps parallel them...

There's no reason why keeping the button pressed should have any detrimental effects. It will just keep the output high until the switch is released.

100uF 16V (or higher voltage) tantalums are commonly available. I'm not sure what you mean when you say "...since its value is by default lower...".

[stj]

Re: 555 countdown timer design question

look at a chip called 74ls74,
it can be configured in interesting ways.

[Dannyx]

Re: 555 countdown timer design question

Found THIS which would be a very quick, albeit "dirty" solution to my problem. I just find it a bit cumbersome to use a relay for this purpose....I'm thinking of something less "bulky" and more "electronic" like a transistor.

[Dannyx]

Re: 555 countdown timer design question

set and reset with the same button, or different ones?

I believe I stated that quite clearly Same button. The "trigger" signal thus needs to be toggled somehow between the trigger and reset pins.

[stj]

Re: 555 countdown timer design question

set and reset with the same button, or different ones?

[Dannyx]

Re: 555 countdown timer design question

However, if you press & hold the switch for longer than 60 seconds (or hold the switch down once the timer has started for a period that will take it over 60 seconds since the first press) then the output will remain high until the switch is released.

But this won't have any negative, possibly destructive effect either, right ?

Yes, you're right, precision is not critical up to the millisecond (though it would be nice to have, yet impossible with such imprecise components). A tantalum cap would require a much larger resistor, possibly in the order of mega ohms, since its value is by default lower....really, what WOULD be the maximum available tantalum cap ? On second though, you could perhaps parallel them.

I'm now struggling to implement a feature to set AND reset the timer with the same button. You press the button once, the countdown starts, then if you want to cancel it before the time is up, you press the same button again and it resets it. The thing I'm actually using is in the picture (I removed the relays for hacking purposes). The relays have a common + and are switched on/off by that driver IC which is essentially a series of transistors that pull the other pin of the relays to GND. In our case, it'd pull the trigger pin to GND which is perfect (the action is momentary, so it doesn't latch the pin to GND), but I now need to figure out a way to switch that "pull to GND" from the TRIGGER to the RESET pin once the circuit is on. I've been looking at Dave's vid here which is close to what I want, except I don't have physical switch, but rather a transistor which can only pull to GND (can't switch a positive signal), so I'm trying to wrap my head around this

Yes, I know a micro would make this a breeze, but I like it analog and overly-complicated...."I like it ANALog"...HMMMM

[Radio Fox]

Re: 555 countdown timer design question

You could consider using CD4060 which has built-in oscillator (for R-C or crystals) and 14-stage flip-flop divider taps. Easy to go from seconds to hours with it, lots of circuits on the web.

For long time delays, I find even a CMOS 555 is not so great. The timing capacitor for 1 minute, I think around 100uF and 560k.
Leakage current on 100uF electro makes the timer "moody", it is never consistent because the leakage current is around 1/10 what the charging resistor is providing and ends up part of the timing - but never shown in datasheets and formulas.

555 simulator

I use Attiny85 with Arduino IDE for small time delay projects becuase it is more flexible, but very hard to program the MCU.

I suppose it depends on the application as to how accurate you want the timer to be. I assume in Dannyx's case where it's needed to keep a light on for around a minute, precision timing isn't required.

A tantalum capacitor could be used for the timing capacitor. This would have a lot less leakage than an aluminium electrolytic capacitor.

[Radio Fox]

Re: 555 countdown timer design question

Another question (breaking this thing down bit by bit): what happens if you ground the trigger pin TWICE or ground it before a countdown cycle is up ? Nothing should happen, as far as I can tell, though my simulator doesn't seem to like it for some reason - could be a bug or it could really signal something....

That's right. If you ground the trigger pin again, before the timing cycle has finished, it will have no effect. e.g. if you press the button momentarily 30 seconds into your 1 minute timer, it will still end 1 minute after the first press of the button & not 90 seconds.

However, if you press & hold the switch for longer than 60 seconds (or hold the switch down once the timer has started for a period that will take it over 60 seconds since the first press) then the output will remain high until the switch is released.

[petehall347]

Re: 555 countdown timer design question

is there an app for it ?

[redwire]

Re: 555 countdown timer design question

You could consider using CD4060 which has built-in oscillator (for R-C or crystals) and 14-stage flip-flop divider taps. Easy to go from seconds to hours with it, lots of circuits on the web.

For long time delays, I find even a CMOS 555 is not so great. The timing capacitor for 1 minute, I think around 100uF and 560k.
Leakage current on 100uF electro makes the timer "moody", it is never consistent because the leakage current is around 1/10 what the charging resistor is providing and ends up part of the timing - but never shown in datasheets and formulas.

555 simulator

I use Attiny85 with Arduino IDE for small time delay projects becuase it is more flexible, but very hard to program the MCU.

[Dannyx]

Re: 555 countdown timer design question

Another question (breaking this thing down bit by bit): what happens if you ground the trigger pin TWICE or ground it before a countdown cycle is up ? Nothing should happen, as far as I can tell, though my simulator doesn't seem to like it for some reason - could be a bug or it could really signal something....

[Dannyx]

Re: 555 countdown timer design question

The output pin is either high or low, it doesn't float

NAILED IT - that's exactly the word I was thinking about: "floating" Cheers.

[Radio Fox]

Re: 555 countdown timer design question

I noticed another thing: shouldn't there also be a pulldown resistor on the base of that BC transistor to keep it off when the output of the 555 is low ? Or is the 555 already doing that internally ?

If the output pin of the timer is already low, then an external pull-down resistor is not needed.

The output pin is either high or low, it doesn't float, so the transistor is switched on or off.

Although the output pin (when in a high state) never actually goes as high as the supply, & (when in a low state) never goes as low as 0V, the low state voltage is still below 0.6V, so will not switch the transistor on.

[Dannyx]

Re: 555 countdown timer design question

I noticed another thing: shouldn't there also be a pulldown resistor on the base of that BC transistor to keep it off when the output of the 555 is low ? Or is the 555 already doing that internally ?

[stj]

Re: 555 countdown timer design question

the reason i asked, you can get a microcontroller module with built-in wifi for a few $ and just add a 5v psu and a solid-state relay.

then control the light from a fone.

[Dannyx]

Re: 555 countdown timer design question

Hmmm...here's a good opportunity to learn about something new: Sync counters...haven't worked with those TBH. I believe they were used in clock radios ? Would THIS count as one of those ? The supply voltage is way to small for my application - would need to keep it in the 12v range to keep the part count at a minimum. It would certainly overcomplicate the project, but I like a good project, plus if it's worth doing, it's worth overdoing

[Dannyx]

Re: 555 countdown timer design question

what's the end product?
is this related to the remote controlled corridor illumination thing?

Yes it is. For this reason, it doesn't have to be uber-precise (just in the ballpark as they say). We're not building a time server here - just switching on/off a lamp. It certainly helps if it IS precise while still keeping a minimum of parts and complexity....though it's probably not possible

[stj]

Re: 555 countdown timer design question

what's the end product?
is this related to the remote controlled corridor illumination thing?

[Longbow]

Re: 555 countdown timer design question

Here is the way I have approached this design in the (distant) past with good results. You can, of course, play with 555 analog timing, capacitor and resistor values and all the rest. With the same amount of design and construction time, you will get a much better and repeatable result using discreet logic chips. The parts are cheap, and you can easily re-adjust the circuit for different timings, repeat/no repeat, etc.

Do this with a couple of 4-bit synchronous up/down counters. Assuming the unit is for turning a table lamp on and off, use the mains signal to derive a perfect 1 second clock pulse by dividing the line frequency by 60/50. (use your power transformer) The transformer can also supply the power for the chips. The counters must take a preset - 60 or 50 - and then count down to 0 synchronously. The Zero state is decoded simply using a couple of diodes. When the Zero state is detected, it will operate an opto-triac, which will isolate your circuit from the mains. The delay can be made to repeat indefinitely by using the Zero detection to reset the chips. Or, the counter can easily be made to delay manually, or act on an external signal.

I really don't think you need a SPICE model for this design. Good Luck.

[budm]

Re: 555 countdown timer design question

The 555 has all the stuff you need to accomplish what you want to do with OP Amp with very minimum parts requirement.
Look at the internal circuit of 555: It has comparators, it has flip-flop, it has Output (Source or Sink) for driving the load, etc.