### Author Topic: Low power counter  (Read 6221 times)

#### TomWS

• Hero Member
• Posts: 1929
##### Re: Low power counter
« Reply #15 on: August 22, 2016, 08:34:38 AM »
Quote
The range that is being measured is set by a monitor resistor that you can easily change from 1K to 100K giving you another 100:1 variation, although you need to switch the resistor in the process.

So, I've ordered INA169 from Adafruit. What were you thinking with the "monitor resistor"? There are two resistors on the board, the shunt, which is 0.1ohm and the load resistor, which is 10kohm.
Keep the shunt resistor but set it up so that you can change the load resistor.  Adafruit's breakout has a 10K surface mount resistor in this position but also has pads for a standard 1/8 axial lead resistor.  If you want to swap values to extend the range, then you can use an 'n' way rotary switch with the common to the active side of the circuit and your various load resistors wired between the respective switch selector pins and ground.

3 resistors will do it: 1K, 10K, 100K.   Alternative approach is to have 100K permanently wired and then simply switch in a parallel resistor that will yield 10K & 1K with simple on/off switches but these will be harder to find.  1K in parallel with 100K is probably close enough for the accuracy you'll get with this setup.  11K (10K+1K) might be close enough as well...  This second approach has the advantage of always having at least 100K in the load resistor position as you switch.

Tom
« Last Edit: August 22, 2016, 08:36:20 AM by TomWS »

#### yoopee

• NewMember
• Posts: 10
##### Re: Low power counter
« Reply #16 on: August 22, 2016, 09:50:01 AM »
thanks Tom,
the results with 100k load resistor seems okay.

btw, how did you calculate the 1.3333uA average current value?

Quote
Assume 7uA MCU sleeping, 8mA when woken to count the pulse, time awake=1mS, interval between pulses=6seconds, your average current will be 8.3333 uA if this condition was continuous 24/7.  Note that the 'awake' average power is only 1.3333uA in this case.  At 8.3333uA continuous drain, a pair of AAA batteries would give you about 17 years of life if Murphy didn't throw other problems at you (like corrosion, etc).

#### TomWS

• Hero Member
• Posts: 1929
##### Re: Low power counter
« Reply #17 on: August 22, 2016, 11:14:51 AM »
thanks Tom,
the results with 100k load resistor seems okay.

btw, how did you calculate the 1.3333uA average current value?

Quote
Assume 7uA MCU sleeping, 8mA when woken to count the pulse, time awake=1mS, interval between pulses=6seconds, your average current will be 8.3333 uA if this condition was continuous 24/7.  Note that the 'awake' average power is only 1.3333uA in this case.  At 8.3333uA continuous drain, a pair of AAA batteries would give you about 17 years of life if Murphy didn't throw other problems at you (like corrosion, etc).

8mA x duty cycle (0.001S / 6S) = 0.001333mA

8mA and 1mS are reasonable SWAGs unless it takes much longer to take a reading than 1mS (which would have a detrimental effect on duty cycle).  However, this model assumes worst case continuous counting at 6S rate.

Tom