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This question already has an answer here:

I am not an electronics engineer. Just working on a project (hobby) that requires me to build a circuit. I just want a timer that turns on an LED after 1 hour. A button will be used to reset the timer which will again turn on the LED in an hour.

I have already made a circuit using a 555 timer IC. For few seconds(19s) of switching, the circuit works fine. I have used this online calculator with C = 0.1mF, R1 = 10kOhm and R2 = 270kOhm. For 3600s, I would either need C=20mF or a very high resistance and equivalent capacitor.

I have been looking online for components but couldn't find any ceramic capacitor of this value. Electrolytic cap are too big in size and I need my circuit to be really small (circle of dia 3cm). I am stuck as there are too many questions now and I dont have the expertise to answer them and can't find much info online

  1. Should I go for a large capacitor or a large resistance? Or is there another way altogether?
  2. I cam across SMD components that seem to be very small in size. But their voltage rating is like 30V. Will this work with a 5V supply?
  3. Is there any other way to achieve this, keeping the circuit as small as possible.

Thanks

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marked as duplicate by Ignacio Vazquez-Abrams, Bence Kaulics, Daniel Grillo, dim, laptop2d Jul 27 '16 at 19:36

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • \$\begingroup\$ how close to 1 hour are you trying to get and how much tolerance are you willing to accept? Does the "hobby" aspect of your project demand that you use 555s or the timing/size aspect of it is more important? \$\endgroup\$ – Wesley Lee Jul 27 '16 at 10:05
  • \$\begingroup\$ @Ignacio - Thanks for the link. Seems I can look into the options stated there. \$\endgroup\$ – nightcrawler23 Jul 27 '16 at 10:15
  • \$\begingroup\$ @Wesley - I do not have any sort of requirements on the components, just the size. I have worked a few projects on arduino so I may be going for the ATtiny microcontroller. Thanks \$\endgroup\$ – nightcrawler23 Jul 27 '16 at 10:16
  • \$\begingroup\$ use a 4060, designed for long periods, not a 555 which is designed for short periods. \$\endgroup\$ – Neil_UK Jul 27 '16 at 11:34
  • \$\begingroup\$ CD4541 electroschematics.com/8749/0-3-seconds-to-10-hours-timer-relay \$\endgroup\$ – Bruce Abbott Jul 27 '16 at 12:38
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That is a very long time for a 555 circuit... However the only reason I can think of it not working is the self discharge of the capacitor, but if you are using ceramic caps, that should be low enough to be ignored.

To answer your questions in order:

1) Larger resistance is easier to get hold of than larger capacitance, especially if you are size sensitive. A 1mF ceramic capacitor will be HUGE, but a 1meg resistor will be tiny, as the size of resistors is related to the power they are rated for, but the size of capacitors is related to voltage and capacitance value

2) SMD components are very good, very small option. If you are confident with soldering them, I would say go for them if you can, but they are not an easy solder option for a beginner, especially if you don't have a PCB ready. Using leaded components would allow you to prototype it on breadboard, which is always useful. As for the voltage, 30V is way above the 5V you're operating at, so would be fine.

3) There is always another way of achieve this, but it depends on what else you're attempting to do. One alternative would be a very small microcontroller and use it's internal real time clock to have variable time limits etc. No doubt there are lots of other options that other people can come up with.

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  • \$\begingroup\$ Although the link provided by Ignacio solves my main issue, your answer clears my doubts. So Thanks. \$\endgroup\$ – nightcrawler23 Jul 27 '16 at 10:17

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