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How to monitor a reed switch connected to a battery

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ulrikp

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Monitoring a reed switch

I have a battery driven device that is normally off and can be turned on by either a (mechanical) reed switch or an RTC connected to a mosfet gate. The two switches work on the same battery. The system wakes up once a day and does a job. I would like to have a way of monitoring if the reed switch closes while the system is on (ie. while the RTC controlled switch is closed).

Of course, I can put a second reed switch on there and connect it to an I/O pin on the microcontroller, but I'm looking for a smoother way. I use an ATMEGA 328 microcontroller. Hope someone has a good idea.

Best regards,
Ulrik

 

Hali,
I think is more "intelligent" to put a small magnet sensor to your Read relay, called Hall element.
You can select some nowadays, their are "digital"(too) -> the output has low or even high if magnet on is :)_ exactly what your PIC likes.
You can check Honeywell`s Hall sensors, i.e. SS443A, at Farnell or Schuricht for € 2-3.- ...
You need an open drain(collector?) resistor & a 100nF capacitor (both,SMD?, direct at the sensor) & 3.3...5V, also only 3 wires + 3 components.
K.
 

Your hall sensor has several good properties for my project. However, power saving is a major issue, and a hall sensor uses too much power (eg. SS443A uses up to 7.5 mA). Thanks though :)
 

The traditional method is to have one extra set of contacts on the relay. This one can be connected between ground and a pull up resistor.
 

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    ulrikp

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Hi,
you see, it was only an "i.e."., an idea_one possibility..
I remember of currents so at 1-2 mA for "actuell" Honeywell, but you can have it as regular product...
K.
ps:
We designed it in an industrial project some time ago, & I saw in past time the advertisings for lower power versions_ I think in summer 2009... possibly a Cherry product?
Otherwise, you can try it with 3.3V or less :), i.e. MLX90217L...

Added after 22 minutes:

You can measure _of course_ a coil voltage on the control switch(between GND & relay)= zero if it will be switched/activated:)_ it is a real low power houskeeping.
 

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    ulrikp

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