Re: Circuit Power Consumption

On 26 Jun 2006 10:44:37 -0700, "phaeton" <blahbleh666@xxxxxxxxxxx>
wrote:

Hello!

A few questions if I may. I realize this is probably basic Ohm's Law
stuff, but I want to double check with some of you, if possible.

If it matters, the circuit in question would be this:

- One garden-variety 555 timer circuit that holds its output LOW for
around 6 to 10 hours, then puts it HIGH for about 4 or 5 seconds.

- One garden-variety 555 oscillator circuit that (using a small
switching transistor) gets 'powered on' by the HIGH state of the above
circuit. When this circuit is powered on it pumps a square wave into a
piezo buzzer. When the above circuit's output goes back to LOW, the
transistor is biased off and power is interrupted to this circuit. I
would use 2 separate timers instead of a dual timer just for the reason
of leaving the second 555 powered completely off for the 6-10 hour
duration.

So it's a circuit that will sleep for several hours, rudely but briefly
'buzz', then fall silent again for several hours. I plan on powering
this all with a single 9V battery, with a voltage divider to cut it
down to 5V. For longevity of battery life, should I:

1) Use 755 timer(s) for one or both halves? Will the 755 be able to
drive a piezo buzzer?

---
No. A 555 is a very bad choice for such a long timeout. However,
the second one could be used to drive a piezo buzzer.
---

2) Use very high value resistances in the voltage divider, i.e.
V+-->40MegaOhms-->Vref-->50MegaOhms-->Gnd ?

---
No. There is no need for a voltage divider since CMOS is available
which can run directly off of 9V at _very_ low current.
---

3) Use a different timer or approach altogether? On data sheets, is it
the "Power dissipation" field that indicates how current-hungry devices
are (I realize it's depending on a lot of other factors, though).

---
I'd use something like a 4060 for the timer and a transistor to turn
on a 555 astable to drive the piezo buzzer.
---

4) This circuit will be exposed to outdoor temperature changes. Will
that make a very large impact on its timing intervals?

---
depends on what's being used to generate the timer's clock. What
kind of stability do you need?
---


5) Any estimate as what to expect for battery life? I realize that a
precise answer is hard, but am I looking at hours, days, weeks or
months, perhaps?

---
Neglecting the buzzer for a moment, if the rest of the circuit draws
500 microamps from the battery, all the time, and the battery has a
capacity of 500mAH, that means you can expect about 1000 hours out
of the battery. About 6 weeks.

If the buzzer needs 50mA and it runs for 15 seconds a day, that's
only increasing the load on the battery by about 9 microamps, so the
life would still be about 6 weeks. The trick here would be to get
the timer to run on as little current as possible. I think 100
microamps is doable and that would get the life up to about 10000
hours. About 417 days.


--
John Fields
Professional Circuit Designer
.

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