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I'm new (and novice) in electronics. I came to seek your knowledge to see if someone can help me with a very basic circuit.

It is a 555 chip, which should light a LED (for about 1/2 second) every hour. I managed to make the circuit diagram with a well known software.

My question is that I do not know what values ​​to put the resistors and / or capacitors to achieve this frequency of light or flash. Actually, this triggers a parallel circuit (so I need the trigger every 1 hour). I guess there must be some calculations to obtain these values, but what I've seen online I find very difficult to understand. I am not a student of electronics, only an amateur trying to learn as well as realize a project. Now the circuit works, but not in the desired frequency flash lighting (every hour a little flash).

PS: Attached image of the schematic. Sorry for possible faults in the text, I used Google translator.

PS: I think this is an astable and oscillator circuit... but I'm not sure.

enter image description here

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    \$\begingroup\$ The 555 is not designed for such a long period. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 20 '14 at 22:10
  • \$\begingroup\$ Wow!thanks for the quick response! know .... any other solution? I thing use and Atmega386 whith arduino... but its spensive \$\endgroup\$ – xavi Oct 20 '14 at 22:14
  • \$\begingroup\$ This circuit works? It appears that when Q is low (ground potential), it will be shorting out the 3.3 supply. Also, you need a resistor in series with the LED. \$\endgroup\$ – tcrosley Oct 20 '14 at 22:15
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    \$\begingroup\$ I'd use something like the ATtiny4 for this. Sleep most of the time, have the watchdog interrupts count up until it's time to blink, then on for 0.5s then back off, then back to sleep. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 20 '14 at 22:15
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    \$\begingroup\$ yeahh! Thank you very much for your answer, you've been very kind and altruistic. I currently use Atmega 386 for he is easy to program with the Arduino IDE, but will investigate the ATtiny4 to see if I is not very difficult to handle. Greetings! Ah! like ... I can put this on the +1 for helping me? I can not find the option to rate your response to click +1 \$\endgroup\$ – xavi Oct 20 '14 at 22:21
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As awesome as the 555 is, its drift may be a bit too high for periods this long.

I recommend you look at using a MCU instead, such as the ATtiny4. It comes in SOT23-6 packaging and has an internal RC oscillator which is reasonably accurate.

If you move up to a ATtiny25/45/85 (SOIC-8/DIL-8/QFN-20) then you can program it using the Arduino IDE once you have installed arduino-tiny, plus it can support an external 32.768kHz watch crystal which will improve accuracy by several orders of magnitude.

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  • \$\begingroup\$ Nicee! Very good response and useful, thank you very much. Unfortunately I am so new that even I do not have 15 reputation and can not vote +1. \$\endgroup\$ – xavi Oct 20 '14 at 22:50
  • \$\begingroup\$ Much better for a newbie to use DIP packages, which one can put on a solderless breadboard for prototyping. I don't know about ATtinys, but virtually all PICs from the PIC10 to the PIC32 have some variants that come in DIP packages. \$\endgroup\$ – tcrosley Oct 20 '14 at 23:03
  • \$\begingroup\$ All of the ATtinys except the 4/5/9/10 have DIP packages. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 20 '14 at 23:07
  • \$\begingroup\$ A typical hobby electronics supplier like Sparkfun supply something like an ATTiny85 and a 555 in the same 8-pin DIP package. The ATTiny will be more expensive (e.g. $3 instead of $1) but can be programmed using the Arduino IDE. You can also find much cheaper suppliers. \$\endgroup\$ – RedGrittyBrick Oct 21 '14 at 9:29
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What you ask for is not something a R-C timer like the 555 can do. The problem is that for such long times, the current to charge or discharge the cap is so small that leakage currents dominate.

Making 1/2 second pulse every hour is something a microcontroller can do easily. Even the simplest, the PIC 10F200 can do this. You can set up the timer in it to divide the 1 MHz instruction clock by 250 to get 250 µs ticks. Then you count using variables from there.

You could divide the 250 µs tick by 40 to get a 10 ms tick, then use that as the basis for everything else. You count 50 of them to time how long the LED is left on. Then there would be 359,950 of them for the time between the LED pulses. You can count that high using 3 bytes.

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  • \$\begingroup\$ Nice!! Thank you for response! ...im consider to use a MCU instead! Thanks again and regards! ...i cant vote up, sorry, im soo newbie and novice...jijij \$\endgroup\$ – xavi Oct 20 '14 at 22:53
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    \$\begingroup\$ An alternative to a microcontroller would be to simply use a counter. A 12-bit counter combined with a 555 that was set for 1.76-second period would have its MSB go high after about an hour. \$\endgroup\$ – supercat Oct 20 '14 at 23:17
  • \$\begingroup\$ Though this is not a sensible solution to your underlying problem, you /can/ use an cap-based circuit for periods of the order of an 1hr by use of a constant-current source/drain and a bistable, in 4/5 transistors. Charging at only a few dB over a bound on cap leakage for the whole period makes the charge over the cap grow linearly rather than much faster than you'd like initially when the cap has low Z. You would only do this if you had set yourself some kind of artificial challenge (such as no ICs) or were designing a timer IC. Counters/dividers or uC are the sensible way forward, otherwise. \$\endgroup\$ – Dan Sheppard Oct 21 '14 at 1:58
  • \$\begingroup\$ Thanks for all comunity! ...MCU is more simple for me (beginner mode ON),jejeje \$\endgroup\$ – xavi Oct 22 '14 at 12:05
  • \$\begingroup\$ Thanks for mentioning the root reason this wouldn't work, "The problem is that for such long times, the current to charge or discharge the cap is so small that leakage currents dominate." Helps me to understand better. \$\endgroup\$ – raddevus Jun 8 '17 at 20:19
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The "upgrade" to the 555 series products are a family of programmable counters (these date back to the early years before microcontrollers). My company is one of several makers of ICM7555 and its big brothers ICM7240, ICM7217, ICM7224, ICM7225, ICM7242, ICM7250, ICM7260. Not sure if any of these truly ancient chips would be the best solution for what you're doing, but might be worth taking a look.

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  • \$\begingroup\$ Wow! Thank you very much! ...I taking look about this components of fammily big brothers of 555! Did not know them! Thanks! \$\endgroup\$ – xavi Oct 21 '14 at 7:13
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A CMOS NE555 with low leakage (not electrolytic but metal film ) capacitor and high value resistors could work but moisture and dirt will mean it is not stable.

The old CMOS HEF4060 timer and divider chip could work here,

or make a NE555 oscillator that runs faster and cascade a binary divider to reduce its frequency to one cycle per hour.

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  • \$\begingroup\$ Thank you very much!, but finaly I use a MCU (like Atmega o ATtiny), your answer is interesting for someone can´t use a MCU instead! Lot of thanks comunity! \$\endgroup\$ – xavi Oct 21 '14 at 11:39

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