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  • Im_int entered IoT Water Alarm in the Wireless Contest contest 11 hours ago
  • Im_int commented on Im_int's instructable NiMH Battery Over-discharge Protection Circuit2 months ago
    NiMH Battery Over-discharge Protection Circuit

    The cut-off voltage for 3 NiMH batteries in series should be around 3.3 V. The built-in voltage reference on attiny85 is only 2.56 V, so the battery voltage would have to be decreased in order to be measured against the internal reference. The easiest way to do it is with a simple voltage divider (https://learn.sparkfun.com/tutorials/voltage-divid...) which is just two resistors in series between the battery voltage input and the ground: [Vbat terminal] —/\/\/\/\/-(R5)— [attiny85 sensing point] —/\/\/\/\/\/-(R6)— [GND terminal]Here's what the underside would look like: https://imgur.com/4mR6iV4Aiming for R5+R6 ~10 kOhm (so the parasitic current through the divider stays under 0.5 mA) we could use R5 = 3.3 kOhm and R6 = 6.8 kOhm which give a scaling factor of 0.67...

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    The cut-off voltage for 3 NiMH batteries in series should be around 3.3 V. The built-in voltage reference on attiny85 is only 2.56 V, so the battery voltage would have to be decreased in order to be measured against the internal reference. The easiest way to do it is with a simple voltage divider (https://learn.sparkfun.com/tutorials/voltage-divid...) which is just two resistors in series between the battery voltage input and the ground: [Vbat terminal] —/\/\/\/\/-(R5)— [attiny85 sensing point] —/\/\/\/\/\/-(R6)— [GND terminal]Here's what the underside would look like: https://imgur.com/4mR6iV4Aiming for R5+R6 ~10 kOhm (so the parasitic current through the divider stays under 0.5 mA) we could use R5 = 3.3 kOhm and R6 = 6.8 kOhm which give a scaling factor of 0.673 at the sensing point. 3.3 V cut-off then becomes 3.3*0.673 = 2.22 V. The cut-off threshold in the code ("const uint16_t threshold = 800;") would need to be changed to 2.22/2.56*1024 = 888.

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  • Im_int's instructable NiMH battery over-discharge protection circuit's weekly stats: 9 months ago
    • NiMH battery over-discharge protection circuit
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  • Im_int followed Im_int10 months ago
      • NiMH battery over-discharge protection circuit
  • Im_int commented on Im_int's instructable NiMH battery over-discharge protection circuit10 months ago
    NiMH battery over-discharge protection circuit

    That's a very good idea! The protection circuit on its own draws ~60 uA, and with one of those 3.3V boost converters from ebay it is at least twice as much. If for some reason it is impossible to replace the battery immediately (e.g. it's powering a remote sensor), a reduction in current draw could be very helpful.

    robojeron, thank you very much for your kind words!

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