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Low Power Consuming Reed Switch Timer Circuit

Nathan Abourbih

Mar 31, 2015
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Mar 31, 2015
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Hi,

I have not worked with electronics in a while and am having a hard time working out what terms to use when searching Google. I am hoping that if I can describe what I am trying to do, someone might be able to point me in the right direction to get me started.

I would like to build a timer circuit that:
- Is as simple as possible.
- Consumes as little power as possible when the timer is not active.
- Is powered by two AA batteries.
- Has a single reed switch that is used to activate the timer.
- Provides voltage to an existing unrelated circuit for a period of about 30 seconds (does not need to be exact).

Function:
When the reed switch changes state (open to closed or closed to open), the timer will be activated and power will be provided.

Examples:
1) Reed switch goes from open to closed. The timer is activated and power is provided for about 30 seconds.
2) Reed switch goes from closed to open. The timer is activated and power is provided for about 30 seconds.
3) The timer is already active and the reed switch state changes (open to closed or closed to open). The timer is reset to 30 seconds (preferred) or the timer completes its existing 30 seconds.

When thinking about this, my mind goes to logic gates and 555s, but those memories are from years back when I was in high school and I am sure there might be something better that uses less power these days. I would prefer to use basic components for this task instead of using a programmable microprocessor (I assuming basic components would consume less power than a programmable microprocessor).

The goal is to power this circuit with two AA batteries for as many months / years as possible (obviously this depends on how often the timer is activated). When the timer is not active, the power consumption should be as low as possible.

If anyone has any ideas or can help point me in the right direction I would be very happy for the help.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
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Jan 21, 2010
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25,510
30 seconds is a long RC delay, and depending on the accuracy may be difficult to achieve.

A circuit based on a CMOS oscillator and counter/divider would be best (but also more complex).

Triggering on either transition is also a little tricky.

It is best to look at both of these issues separately.

The first problem can possibly be researched by googling for " CMOS oscillator timer". I'm on a mobile device right now so I won't do it for you right now :)
 

Gryd3

Jun 25, 2014
4,098
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Jun 25, 2014
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4,098
I know you stated that you want to avoid using a microcontroller, but if you can't find the desired outcome you are looking for, it may not hurt to read this : http://ww1.microchip.com/downloads/en/DeviceDoc/01146B_chapter 2.pdf
There are PIC microcontrollers that can be put into various low power states which would allow very very little current draw. This document claims many many years on a single AA cell. (I doubt this number, but you can come to your own conclusions based on the current values)

How often do you expect the read switch to toggle?
Will the Timer be providing power as an output from a separate power supply?
(If so, is it possible to use rechargeable cells?)
 
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