LM555 Astable 1 Minute period

Discussion in 'The Projects Forum' started by Loonie, Jul 16, 2012.

1. Loonie Thread Starter New Member

Aug 30, 2011
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0
Hello, I am trying to make a digital timer, which shows the hours and minutes
E.G
Each of the 4 7-segment displays are powered using a 4026 IC
00:00
09:11
21:56

I am trying to minimize the number of ICs used on the breadboard. Can I use a LM555 timer to generate an astable (continuous stream of rectangular pulses having a specified frequency)?
I used a LM555 calculator but the values it gave are in the millions (for the resistors), and I heard that for correct results, R1 and R2 should be between 1komh and 10Momh. Could someone give me optimal values for the IC? Or some other suggestion for the circuit?

2. wayneh Expert

Sep 9, 2010
12,379
3,235
If you make the capacitor larger, you should be able to do it with the 555.

But, if you care about accurate timing, you may be disappointed. The frequency will drift around a bit with temperature. A crystal oscillator feeding a divider would give better results.

3. tracecom AAC Fanatic!

Apr 16, 2010
3,878
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Following is a table showing different values for C1 to achieve different output frequencies. Although the calculations in the table are precise, component values vary, thus the output frequencies are only approximate.

Making R2 43K and C1 1000 μF will theoretically produce an output of .0166 Hz, which is 1 pulse per minute.

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Last edited: Jul 16, 2012
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4. takao21203 Distinguished Member

Apr 28, 2012
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463
Get someone to program a 16f54 for you, with 32KHz crystal, and 1 sec. output.

I could do this + I have small carrier PCBs for this chip.

5. SgtWookie Expert

Jul 17, 2007
22,183
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A 555 timer is not suitable for timekeeping.
A microcontroller using a 32.768kHz TCXO (temp compensated xtal oscillator) would be a low parts count, relatively inexpensive and reasonably accurate time base.
If you want to avoid programming a microcontroller, you might go with a CD4060B or MC14060B with the 32.768kHz TCXO.

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6. wayneh Expert

Sep 9, 2010
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Gosh, if cheating is allowed (microcontroller), just go buy the timer and be done with it!

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7. takao21203 Distinguished Member

Apr 28, 2012
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or this (see attach). It really works. However, the related schematics found on the internet DON'T work actually. The problem seems to be the amplitude, if there is oscillation at all.

I don't have the schematics actually. But it's simple 2n3904 circuit.

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8. tracecom AAC Fanatic!

Apr 16, 2010
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Right, or add a DS1307 (or better) clock chip to the μC.

9. DickCappels Moderator

Aug 21, 2008
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Whenever I need a timer in the range of several seconds to days and a microcontroller does not seem like the best solution, I usually use one or more 74HC4060 (oscillator/counter) chips. They are easy to use, can be used as a crystal or RC oscillator and seem to work reliably.

The benefit from using a 4060 over a '555 is that you can use lower value resistors and capacitors (or a crystal for accuracy) and thus avoid the starting reliability and timing uncertainty associated with parasitic leakage.

http://www.nxp.com/search?q=74hc4060&type=keyword&rows=10

Last edited: Jul 16, 2012
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10. Loonie Thread Starter New Member

Aug 30, 2011
17
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Hahaha, thanks for the replies! They are very detailed and require a lot of pondering over for me.

I am sorry I neglected to mention the purpose of the digital timer. It is a stage prop for filming a movie. Therefore, there isn't a need for high accuracy. As long as the counter is able to increment in around 1 minute, it is enough. The rest can be handled using photoshop and video editing software! Right now we do not have the "clock", only 4026 chips to set the time, and then stopping the filming to increment the time. I merely want to implement a function to "start counting" when a button is pressed.

I am using a 12 battery source, and a single breadboard to implement the function. Therefore, I am trying for a cheap solution (add a 555 timer, and some gates for increment).

@tracecom " Making R2 43K and C1 1000 μF will theoretically produce an output of .0166 Hz, which is 1 pulse per minute." May I know the value of the R1 resistor then?

I am worried because the 555 chip is usually used for higher Hz frequencies, and might not be able to handle low frequencies for 1 minute. I am a beginner E.undergraduate and haven't gone up to microcontrollers yet. That's why I am working with basic stuff =\

11. Sensacell Well-Known Member

Jun 19, 2012
1,183
276
Such a long delay with a 555 will be very frustrating.

If the capacitor is leaky (1000 uf electrolytic = leaky) it might not oscillate at all- and you will be waiting... and waiting... and have no idea what is going on.

At the minimum I would make the 555 run at a higher frequency and divide it down with a CD4040 binary counter.

This way you can really see it working- and even add a pot to tweak the frequency.

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12. Loonie Thread Starter New Member

Aug 30, 2011
17
0
Tch, yeah I was hoping someone have tried to run a 555 with a low frequency.
So with a 4040 or even a 4020 (14 stage ripple counter), I can set the period as 4 seconds 60/14 = 4~ s = 0.25 Hz. This will work right?

13. WBahn Moderator

Mar 31, 2012
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Oooohh! Oooohh!! Here! Here!

My first "contract" was for my employer (Taco Bell) where I worked as a side-job while a co-op student at NIST (it was NBS in those days). The manager wanted a "hospitality timer" that would buzz for every 15 minutes telling the employees that it was time to do a lobby/bathroom sweep (literally, if necessary).

As someone that had had only had the physics department's analog electronics class at that point, I knew about 555 timers but not about digital counter chips and certainly not about microcontrollers. When the only tool you have is a hammer, everything looks suspiciously like a nail; so I designed a circuit around a CMOS 555 chip and it worked and they used it for a couple of years before an employee intentionally destroyed it (and I never did get paid the \$50 I was promised).

While you might not need it for a 1-minute timer, I pretty much had to use the CMOS 555 because to get 15 minutes I was running into the problem of capacitor leakage stalling out the charging given the high valued resistor I was having to use.

If this were for a "real" clock, I would be steering you strongly away from the 555 route. But after seeing what you want it for, I actually strongly suggest that you DO go this route. Not because it is the best (or even necessarily the "easiest") route, but because it IS a route that is well matched to your set of engineering tools at this time and it will be a very good experience for you to design and build a circuit that YOU can fully understand and that you can struggle with and figure out why it isn't doing what you want and figure out how to get it doing what you need.

Prior to my co-op outing I did some similar work for the school theatre group as well. I designed a triac-based dimmer so that they could easily control the intensity of some on-stage lights and also a telephone ringing circuit to make an old mechanical phone ring. Both of those pushed my pretty hard given my knowledge at the time and I learned a number of valuable lessons, including putting anti-kickback diodes across transistors that are switching even small relay coils.

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14. tracecom AAC Fanatic!

Apr 16, 2010
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910 Ω (as per the table previously posted) should work. Here is a schematic of an adjustable astable. Note that you do not need all the different values of C1; you only need 1000μF plus changing R2 to 43K.

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15. Loonie Thread Starter New Member

Aug 30, 2011
17
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@WBahn
I know that feel! All I asked for was \$20 but they treated me to a dinner and gave me a movie-tie-in t-shirt instead.

I guess my plan will be to use a LM555 IC using tracecom's values, and if that fails, switch to a LMC555 chip/add a ripple counter. Thanks for the help guys

Apr 30, 2011
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For TV show insert shots of a bomb timer, I've seen them use the guts out of a cheap digital alarm clock, record a segment of normal run then reverse and speed up the playback. Magically, minutes become seconds and it counts down instead of up.

17. WBahn Moderator

Mar 31, 2012
18,085
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No! I don't believe it! That can't be how they do it, because cheap digital alarm clocks don't beep!

And it must be a cheap military digital alarm clock, otherwise it would count down to noon!

18. #12 Expert

Nov 30, 2010
16,673
7,320
Just a comment: I've done the math for capacitor leakage on a slow 555 and, according to the math, you can't do that. Aluminum electrolytics leak too much current. Fortunately, capacitor leakage is usually better than the specs, which describe the worst case leakage. Try it. If you can get it to work, fine, you win. If not, the CD4020 is a great helper. You're on the right track.

When your circuit accuracy depends on having a capacitor that is better than the guaranteed leakage ratings, you are depending on the leakage rate for your accuracy, and it is poor, and it changes over time. For your purpose, "poor" is good enough. If it's actually unworkable, clock it down with a CD4020 or similar. I did a 30 minute oscillator that way, and I wasn't even pushing it.

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