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I'm fairly new to electronics. I wasn't able to find anything addressing this on the Web but that may be because I don't know the search terms to use.

I'm trying to design a device that contains two 555 timers. When powered on, one timer will be used to generate a repeating beep every 5 seconds and the other will be used to turn the device off after 1 minute. It will be powered by a couple button cells which preferably won't be replaced for the lifetime of the device.

Should the 555s remain powered on (ie voltage applied to Vcc pin) when the device is off? This seems like the obvious choice to save battery, but I'm worried that there will be timing issues if Vcc and the input pulse (to initiate counting) arrive at roughly the same time to the 555. Should I add a delay IC to the input pulse? Or should I use 7555s, which draw less current, and leave them powered on? Are they significantly more expensive? How long could I expect the button cells to last with a 555 vs 7555? schematic (shown with 555s powered at all times)

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  • \$\begingroup\$ What is the source of the input pulse? From your description I was picturing an astable multivibrator (no input). \$\endgroup\$ – Houston Fortney Jun 14 '16 at 1:31
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    \$\begingroup\$ Also, for lowest power and price, I would recommend a micro instead. \$\endgroup\$ – Houston Fortney Jun 14 '16 at 1:32
  • \$\begingroup\$ You will need a FF or RS latch to turn the timers on via a P.B. switch, then when timer 2 times out it sends a pulse to the latch to shut off power. Power is applied to the Vcc pins at all times, so CMOS IC's are needed for long battery life. Or use a 8 pin MPU. \$\endgroup\$ – Sparky256 Jun 14 '16 at 2:30
  • \$\begingroup\$ @Houston Fortney I'm not sure how to answer that so I added my schematic to the OP. Thanks, I'll look into a microcontroller. Is it difficult to have a device with a programmed micro manufactured in largish quantities? \$\endgroup\$ – user113923 Jun 14 '16 at 4:46
  • \$\begingroup\$ @Sparky256 I added my schematic, is that more or less what you're describing? \$\endgroup\$ – user113923 Jun 14 '16 at 4:52
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Or should I use 7555s, which draw less current, and leave them powered on? Are they significantly more expensive? How long could I expect the button cells to last with a 555 vs 7555?

The 7555 from Analog or Maxim, or TI's TLC555 or LMC555 draw significantly less current while idle than the legacy 555 ICs like LM555. The LM555 has ~5mA standby current at 5V, 16mA @ 15V. The newer ones are in the microamp range, like 250 µA (0.25mA) for the TLC555, or 50 µA for the LMC555. Since the standby current difference is an 1 or 2 order of magnitude lower, they will last 10 to 100 times longer.

The tradeoff is that the newer 555s have a smaller voltage range, 10V max for the ALD7555. Then again, they also work at lower voltages, some down to 1.5V like the LMC555 instead of the 4.5V of the LM555. There are more differences that may be important, but not really.

The cost difference is small. ~25 cents at 1k units, instead of 7 cents.

Update:

Based on the schematics and end goal, even CMOS 555 timers won't work well for your application. Coin cells don't have much energy. The common cr2032 provides 250mAh at low draws. Just the 555 timers would drain that in 1000 hours, not including active power draw. Add a AND IC, a Flip Flop, the Buzzer and Motor and Transistor? Under a month, max.

Solution? A microcontroller to replace all the logic. A single microcontroller can handle keep the time (5 second counts up to 1 minute), no need for the flip-flop or the AND ic, or inverter. Everything in code. In deep sleep like the MSP430 can have just 10µA current. It sleeps in Low Power Mode 4 or 4.5, wakes up when the push button is pressed, turns on its clocks, not even only 100µA.

The biggest problem is simply how much current is used by the buzzer and motor. If you power the motor and buzzer off AAA or AAA batteries, and the microcontroller on a coin cell, you could get a year. Assuming you keep the microcontroller current under 30µA average, then it will last 1 year. If you just power everything off 2x AAA (3V 1500mAh) or 2x AA (3V 2800Ah) you will get years (depending in how often it is triggered).

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  • \$\begingroup\$ Do you think I could a tinyAVR (atmel.com/products/microcontrollers/avr/tinyavr.aspx) with one AAA and no button cell? It's OK if the AAA needs to be replaced somewhat frequently. \$\endgroup\$ – user113923 Jun 15 '16 at 1:31
  • \$\begingroup\$ @ElliottShugerman Operating Voltage – 1.8 - 5.5V for ATtiny25V/45V/85V You'd need a boost circuit because a single AAA is 1V-1.6V max. 2 AAA is easier. But your Buzzer also needs 3V or so. \$\endgroup\$ – Passerby Jun 15 '16 at 1:43
  • \$\begingroup\$ I'm trying to avoid 2 AAAs to keep it small but I suppose that is an option. My impression from the page I linked is that the tinyAVR has a built in boost regulator that works with > 0.7V. But dang, there is still the buzzer. Maybe I could power the buzzer with a couple non replaceable button cells? Looks like a 3V buzzer needs only a couple mA. I could make the buzzer draw zero current when not in use, right? Using a transistor or relay? Or could the AVR handle that too? Thank you for all your help! \$\endgroup\$ – user113923 Jun 15 '16 at 2:25
  • \$\begingroup\$ @ElliottShugerman I didn't know about the ATTINY43U. It's like the boost circuit I mentioned, built in. It's also like 4 bucks by itself. Regular ATTiny are like 1 dollar. The Buzzer could be powered directly from the ATTiny gpio. The motor likely from the VCC of the Boost Converter. You will need to get the inductor and diode recommended in the data sheet. \$\endgroup\$ – Passerby Jun 15 '16 at 3:13
  • \$\begingroup\$ Wow, cool. I think the price may be worth it for me. Thanks again, I really appreciate your help. \$\endgroup\$ – user113923 Jun 15 '16 at 3:54

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