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Is there a way to figure how long can a 555 run on battery, when reset is grounded, to have no output?

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A CMOS 555 like the TI LMC555 has a supply current of around 300 µA at 9v (the datasheets don't have values specific for 9v, but list a maximum supply current of 400 µA for 12v, so I'm interpolating).

Holding the device in reset adds a negligible amount of current (10 pA) which can be ignored.

A typical 9v battery has a capacity of 550 mAH. Assuming there are no other devices drawing current in the circuit (which I find unlikely), just the 555 would run for 550 mAH / 300 µA = 1833 hours, or a two and a half months. You would want to derate that value, but it is likely the 555 would run for a month.

Note:

If you can run your circuit off of 3v instead of 9v, you will get much better battery life. A Duracell AA cell has a capacity of 2100 mAH compared to the 9v 550 mAH. Putting two of these AA cells in series gives you 3v, and doesn't take much more room than the 9v battery. The LMC555 has a maximum supply current of 200 µA at 3v. 2100mAH / 200 µA = 10500 hours which is over a year! If you add a 1 mA LED, 2100mAH / 1.2 mA = 1750 hours, which is comparable to the 9v battery without the LED (1833 hours).

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  • \$\begingroup\$ If I have an output LED drawing 20mA from the 555(continuously on), then the total current drawn would be 20ma+300uA right? \$\endgroup\$ – cheeky Nov 19 '13 at 6:15
  • \$\begingroup\$ you probably have to make a joule theft circuit if you wants to harvest all the energy inside it. \$\endgroup\$ – Standard Sandun Nov 19 '13 at 6:46
  • \$\begingroup\$ @cheeky Yes, if you need an LED to be on all time, adding an LED to the circuit could increase the current to 20.3 mA, and reduce the lifetime to 500 mAH / 20.3 mA = 24.6 hours, or just over a day (in reality, expect maybe 15 hours). However you can get low current LEDs that run on as little as 1 or 2 ma, which would give you 500 mAH / 1.3 mA = 384 hours or 16 days (perhaps 10 days in real life). \$\endgroup\$ – tcrosley Nov 19 '13 at 6:58
  • \$\begingroup\$ @cheeky -- please see my edited answer about switching from a 9v battery to two AA cells. \$\endgroup\$ – tcrosley Nov 19 '13 at 7:44
  • \$\begingroup\$ @cheeky -- when I found the data for the Duracell AA battery, it also had a figure of 550 ma instead of 500 ma for a 9v battery, so I edited my answer. You can't edit comments except right after posting them, so my calculations in the comment above are a little off. \$\endgroup\$ – tcrosley Nov 19 '13 at 17:40

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