Monday, 27 February 2012

Wireless sensor node - watch battery (8)

Back in part 4 I made some improvements and did some sums and convinced myself that I could run my TX node on 3x AA batteries for more than a couple of years. Here are the sums I used.

Entering the following figures into this website:
  • Battery capacity: 1700mAh
  • Sleep current consumption: 0.006mA
  • Wake current consumption: 8.5mA
  • Duration of wakeup: 2000ms
  • Number of wakeups per hour: 12 (once every 5 minutes)
The result: 2.6 years.

It occurred to me that the wakeup time of two seconds is pretty long. I used this time to allow for 3 transmissions with high power sleeps between them. However, if I use low power sleeps between retransmits I can be running at high power for a much shorter time and hence get a much better battery life. The results are pretty impressive.
  • Battery capacity: 1700mAh
  • Sleep current consumption: 0.006mA
  • Wake current consumption: 8.5mA
  • Duration of wakeup: 100ms
  • Number of wakeups per hour: 36 (once every 5 minutes * 3 transmissions of each reading)
The result: 11.38 years! It seems pretty unlikely that my batteries would actually last for a decade but this long battery life means that I could get away with a much smaller battery. A single CR2032 watch battery is very cheap, 3V and usually has a capacity of 225mAh.
  • Battery capacity: 225mAh
  • Sleep current consumption: 0.006mA
  • Wake current consumption: 8.5mA
  • Duration of wakeup: 100ms
  • Number of wakeups per hour: 36 (once every 5 minutes * 3 transmissions of each reading)
The result: 1.51 years! That's an ideal length of time - I think I'll be using CR2032 batteries with my wireless TX node.

5 comments:

  1. Hi,
    Thank you for your great posts!
    How do you switch the transmitter on and off? Do you use a transistor?

    ReplyDelete
  2. I just connect the VCC pin of the transmitter to one of the pins of the ATtiny85 and toggle this pin to control when I want the transmitter to be on/off.

    ReplyDelete
  3. Hi,
    This is not about the sensor. Seemed you know the ATtiny85 quite well. According to its datasheets, ATtiny85 has only 2 PWM pins. Is it possible to have one more PWM pin?
    Thanks.
    Man

    ReplyDelete
    Replies
    1. I'm not sure - you should refer to the data sheet.

      Delete
  4. http://bit.ly/1ZBcVED

    mortal combat X

    ReplyDelete