Guidance on humidity alarm project

Hi all,

I want to do a project for monitoring the humidity and sending an alarm if the humidity is over a certain threshold. The alarm should be sent as short message to my cell phone. The whole thing should be battery powered and run autonomously for several months. Alarms should occur at most every month. Btw. I am an experienced software guy with some basic knowledge in electronics. I'd prefer solutions that do not require extensive soldering etc.

The first part is to sense the humidity. So far I've learned that I can measure the humidity using the DHT11 sensor. I do not expect the humidity to rise and fall very quickly, sensing the value every 10 minutes should be sufficient. So I am very concerned about sleep modes, battery saving tricks etc.

  • Any recommendations about the power supply (I read that 9V blocks are not that fine and I should go for AA-packs instead)
  • Should I go for the Arduino Uno R3 or another model?
  • I read that LEDs can be battery-eaters. Any way to disable them in software or do I have to solder them out?

The second part is related to use a GSM link to send the short message. I know that they wont run all the time on battery power. So I am seeking for a solution to turn on the cellular modem on demand, connecting to the cellular network and send the short message.

  • Any recommendations on GSM shields and their power requirements
  • Will the GSM shield be able to run with the AA battery pack?
  • I also thought of a cheaper solution in using an old mobile phone that might be enabled via an external signal (interrupt), runs on its on batteries and can be used as serial modem after it has booted up. What do you think?

Thanks for all advises in advance, Ingo

It all seems feasible, but this part makes me think you're going to need a big battery:

IK81: The whole thing should be battery powered and run autonomously for several months.

You'll want to use a clone designed for low power consumption, rather than one of the standard Arduinos. You'll also need to provide a way to keep the GSM modem powered off except when you're actually using it. Finally, you'll want to keep the whole thing powered down as much of the time as possible and just use a RTC or watchdog interrupts to wake it periodically to take your readings.