Help in Timer circuit

[Katherine1]

Hi everyone.. I wanted to come up with a circuit that acts as a reminder. The general concept is that when I turn ON the switch, the timer will be triggered. After selecting the time interval, timer starts to coundown and a buzzer will beep to remind me that my time is up. I have to turn OFF the switch or else the buzzer will remain beeping.

I intend to use a 555 timer, piezo buzzer,9V battery and microswitch.
The time interval is set to 15mins, 30 mins and 60 mins (3 different buttons)

My problems are,
1. Is 555 timer able to run up to 60 mins?
2. How should I connect the resistors and capacitors in order to obtain the time I wanted?
3. Is there any similar diagrams that I can refer to?

Thank you for any helps and sorry for any mistakes because I am new to electronics and circuit.

 

[Harald Kapp]

Welcome on the forum.

No 2 first: as per the standard monoflop application (datasheet or Google)
No 3 second: as per the standard monoflop application (datasheet or Google)
No 1 last: In theory yes, but this long time will be very imprecise. You'll need big capacitors and resistors and the circuit will not be very stable. Don't do it.

For such long time intervalls a counter and a clock oscillator are to be preferred. You could use the CD4060 CMOS IC. It has an integrated oscillator and 14 bit counter. Figure 12 in the datasheet gives an example for a simple RC oscillator, figure 13 shows how to use a quartz. Setting different times will require additional decoding logic for the outputs.

Today, a modern solution would employ a simple microcontroller (e.g. Attiny, PIC...) where you can set the time easily in software.

 

[PStevenson]

a modern solution would be to use software in ucontrollers however
it's not real electronics and you won't learn very much so I would stick with either the CD4060 or the 555 timer

if you want to go to my stripboard site you will find some cool projects there also www.paulinthelab.com

 

[Katherine1]

thx Harald Kapp & PStevenson for ur opinion.. I did think of using PIC microcontroller and RTC. But the problem is, it hv to be programmed. Since I'm still new to electronics, I wish to start with something simple.

you said that big capacitors and resistors used for long time interval will not be stable, does this meant the leakage?

If 60 mins is too long, hw bout i just set 10, 15, 20 & 30 mins?

i think i shld be using monostable 555 right?

i calculated the time interval using resistor & capacitor with this formula. T= 1.1*R*C

However, i cant obtain the exact mins.. why? for eg: 1.1*1.5M ohm * 470 microF=12.9 mins ( nt 15 mins)

 

[BobK]

So change either the resitor or capacitor. Pick one and you can calculate the other.

T = 1.1 * R * C

Pick 470uF, then solve for R. T is 15 * 60 = 900 s

900 = 1.1 * R * 470 / 1000000

900 / (1.1 * 470 / 100000) = R = 1.74MOhms

That said, the leakage is going to be high enough that I think this will underestimate the time if it even works at all.

Bob

 

[Katherine1]

hmm...sorry I don't really understand. does that meant no matter wat resistor and capacitor i used, the time will nt be accurate ( less than the exact time)?

 

[(*steve*)]

With such high values for the resistor and the capacitor the time will not be accurate.

The actual time will be somewhere between what you calculate and infinity.

Imagine the capacitor as a wooden bucket and the resistor as a restriction on a pipe.

As you increase the restriction (value of the resistor) the flow into the bucket gets less and less. At some point the flow gets smaller than the evaporation from the bucket and the leakage through it. The whole process is made even worse because you are waiting until the bucket gets 2/3 full and your method of measurement causes a little water to leak from the bucket.

You're at the point where the amount of water flowing into the bucket may not be enough to counteract all those losses. Clearly those losses change the time it takes to fill the bucket, but they also might prevent it filling at all.

The following will help explain the problem and suggest some answers to it:

https://www.electronicspoint.com/long-duration-timers-notes-beginners-t244516.html

 

[Harald Kapp]

How about this kit?

The too have recognized the failure of using a 555 for long times and now use an oscillator/divider based circuit.

 

[TedA]

Katherine1,

I think you can make your original plan work well enough. All of the problems mentioned are valid, but your requirement is not for precise timing, I think. I suppose that you intend to make one or two units for experimentation and personal use. The answer would be different for a design for mass production.

For the longer time periods, the calculated times will not be exactly accurate. Leakage currents will become more and more important. You will want to include a trimmer resistor in the timing resistance. I would suggest having on hand a few inexpensive trimmers and an assortment of fixed resistors to use while experimenting. A limited number of fixed resistor values can be stretched by connecting them in series and parallel to get the value you want.

Using something like the 555, you will have to adjust each time to the desired value. A crystal controlled digital solution could be designed to have each time as accurate as you like with no adjustments, but that is a different project!

To help with the leakage problem, you might use a CMOS version of the 555 timer, such as an LMC555.

http://www.ti.com/lit/ds/symlink/lmc555.pdf

For the longer times, tantalum capacitors often have better leakage specifications than aluminum electrolytic. Or, at least, you can look for a low leakage aluminum part to use. Most of the major capacitor manufacturers offer these. For instance, see the Nichicon KL series. Usually, capacitors have much better leakage than the specified maximum. At worst, you might have to try several different caps to find a good one.

You did not mention where you are located. Suggestions about parts to use might be better tuned to your requirements if we knew where you are. For instance, in the USA, Mouser Electronics have both CMOS timers and low leakage caps in stock. You might also mention what test equipment you have to use while debugging. We can try to avoid suggesting things you cannot do with the resources at hand.

You should figure out how to post a schematic for your project, while you are working on your design.

A schematic drafted on your computer is always nice, but not necessary. A pencil sketch can be scanned, or even photographed. ( If you have difficulty, start another thread on posting schematics.)

Meanwhile Google will turn up a zillion circuits. You might start with the TI and NXP datasheets and application notes for both bipolar and CMOS versions. ( NXP because it is the successor company to the 555's original source.)

So good luck with your project. Keep us posted with your progress.

Ted

 

[KrisBlueNZ]

EXCELLENT advice, TedA!

Katherine1, it would be useful to have some idea of what these time delays are to be used for. The range, the specific values, and the accuracy requirements are not clear. They determine whether a simple solution is suitable or not. Anything involving crystal control and a CD4060 divider etc isn't worthwhile if you don't need it.

Good luck from me as well

 

[Katherine1]

tq v much

 

[Katherine1]

oh, this circuit will be included in one of my school project. I'm sorry but I live in Malaysia.

Actually I wanted something like in
http://www.eleccircuit.com/the-basic-delay-timer-circuits-using-ic-555-base/

I want to test on this circuit for a month trial. this circuit will act as a reminder to people to do a certain task after the time set had lapses. if they fail to do so, the buzzer will keep on beeping until the switch is off..

 

[(*steve*)]

You probably want something which employs a divider so you can have an oscillator running at a reasonable frequency.

Here is an example. Here is another. And here is yet another.

These are practical examples of using a divider to extend a timer over what can be achieved with a 555. The tutorial on long duration timers (https://www.electronicspoint.com/long-duration-timers-notes-beginners-t244516.html) explains this as a solution for the problem which occurs when you try to use a long duration RC circuit.

 

[TedA]

Katherine1,

It sounds as if the timer needs to work pretty well, and that this may be more important than I imagined it was.

There is no doubt that the digital timer circuits will provide more reliable performance, for the longer times that you need.

The ”CMOS Pocketable Timekeeper" at http://electronicsproject.org/cmos-pocketable-timekeeper/, that Steve found for you, looks attractive because the piezo buzzer can be mostly off, even after the time-out period. This will save the battery. It would be simple to make the LED "pilot light" also be mostly off. Anyway, one of the circuits using a CD4060 IC should be a good choice.

That said, if your teacher suggested that you should use a 555 circuit, some careful discussion might help you determine if something else better might be OK. You must decide if this is an important consideration. Even out in industry, it is sometimes better to do it the bosses way, if that is at all possible.

If you already have the parts for the 555 circuit, it is still worth a try. But if you do not, perhaps a better bet would be to consider one of the digital circuits.

People on the forum can provide help in modifying the circuit to do what you want.

Someone on the forum may know where to get parts in Malaysia. Perhaps you can also source components from Singapore. Of the USA distributors, Mouser may not be a good bet, but I know that Avnet, at least, have (had?) a Singapore warehouse.

And do you have at least a multimeter to use while you are working on this? Or do you know someone nearby who can help with test equipment and expertise?

Ted

 

[Katherine1]

You probably want something which employs a divider so you can have an oscillator running at a reasonable frequency.

Here is an example. Here is another. And here is yet another.

These are practical examples of using a divider to extend a timer over what can be achieved with a 555. The tutorial on long duration timers (https://www.electronicspoint.com/long-duration-timers-notes-beginners-t244516.html) explains this as a solution for the problem which occurs when you try to use a long duration RC circuit.

thx, does that meant i cant chg the time interval but to follow the time as provided in the cmos pocket timekeeper? & whr shld I connect the buzzer to?

SOrry to ask such ques since this is my first circuit project.

 

[(*steve*)]

Take a look at this circuit.

You can connect a buzzer in place of the relay (or power the buzzer from the relay).

I think it does what you want. After you press reset, there is a variable delay until the buzzer goes off. It keeps buzzing until you press the reset button (and then the process starts again).

 

[TedA]

Katherine1,

I think most of the digital circuits shown in the linked pages switch the buzzer circuit to different digital counter outputs to set different delay times. This leads to times that progress by powers of 2: 1, 2, 4, 8, 16.

It would also be possible to switch different resistor and capacitor values into the oscillator circuit. These could be preset for the particular times you want. This would work on the same principal as the way the 555 circuit times were selected.

An alternative way to set the delay time would be with a continuously adjustable dial.

The schematic for the "CMOS Pocketable Timekeeper" labels the buzzer "PZ1". ( And I don't like the way the buzzer is driven in this circuit. It would be better to make the CD40106 astable multivibrator provide an asymmetrical output waveform, then connect the beeper directly to the CD40106).

Ted

 

[Katherine1]

hmm.. aft much thinking..I think I have to chg my plan to using a microP. but the problem is to code the program.

 

[TedA]

Katherine1,

The microcontroller approach can result in a very simple hardware design. This is how you would want to design the device for mass production.

However, depending on your current programming skills, it might make the project much bigger for you, possibly taking too long to be completed.

The skills for programming microcontrollers are very valuable, and worth acquiring, but can take some time to learn.

In my experience, the software part of a hardware + software project always takes much more time and work than estimated.

So if you have not done much programming already, I would hesitate to recommend that you take this approach, particularly if there is a deadline for completion. ( If you have someone who can help you, who has the equipment and skills already, it might work out OK.)

So perhaps the 555 or 4060-based approach might be more practical.

Ted

 

[Katherine1]

thx Ted. I know I'm still lacking to complete such a project using a microcontroller. The 555 is easier for me to do, but time accuracy is another big problem. I cant find any solid reason to convince my lect that 555 can do a better job than microcontroller.

So, for now I will try to do both ( one with 555 and another one with PIC)

One question here, is lcd display a must to be included in the circuit if using PIC?