Sensor powering
HobbyIoT NET - The Technology behind
Introduction
The End device Type 1 THR XLP is built around a 3.3V version of Arduino Pro Mini. The current design idea relies on direct powering from a single 3.6V Lithium battery cell. All the peripherals including MRF24J40MA radio, DHT11 and a Reed sensor can be powered either by the battery rail or by dedicated Arduino output pins. Following are some design facts and considerations and also the current state of the sensor design.
Further investigations are going to be conducted this spring for even better sensor battery life!
You are welcome to join the project!
Powering Arduino Pro Mini
The Arduino Pro Mini Board is directly powered from the 3.6V Lithium battery cell. To minimize the power consumption the internal regulator should be disconnected or removed (depends on the certain board modification).
Powering peripherals
Powering peripherals from the battery rail is suitable for those ones who have relatively low sleep/standby current. Powering from an Arduino pin helps to totally switch off the unit when not needed but requires specific interface lines set up and also initial configuration when the peripheral is powered on again. Also the fact that the logical '1' is about 0.7 - 0.8 V lower than the battery rail has to be taken into account.
The DHT11 is a very hungry sensor. Even in non-active mode it consumes 60 uA. So it is really better to be powered off when not in use. The easiest way to do so is just to connect it to a dedicated Arduino pin configured as output. The required pull up should also be connected to that pin instead of the battery rail. The problem in this scenario is the operation voltage range of the DHT11 - according to the documentation it will not work under 3.0V. Measurement times of the DHT11 are also critical for the overall power balance. Further investigations on that powering scenario are required.
The MRF24J40MA is consuming about 28 mA in TX/RX periods. In the current implementation its power rail is successfully switched on and off. The SPI interface pins are reconfigured in a way that no current sinks from them during power off. On the other hand, according to documentation, the sleep current of the MRF24J40MA is as low as 2 uA. Putting the module to sleep is a fast process and could be much more power efficient compared to completely powering off the module and then powering it on again and configuring parameters. Further investigations will lead us to a decision of the right sleep algorithm.
The Reed sensor is just a magnetic contact. It is connected to an interrupt input of the Arduino Pro Mini board and will wake it up once the state of the sensor is changed. For lowest power consumption a resistor about 1M is used for pull up and no internal pull up is configured.
Powering calculations
Sample power calculations are shown in the table below. The battery life is mainly dependent on the operation time of the MRF24JMA. One is welcome to make a copy of the table and do variations.
Average current consumptions in sleep mode:
Arduino Pro Mini - 4 uA; MRF24J40MA - 2 uA; DHT11 - 60 uA; Reed pull up - 4 uA
Average current consumptions in active mode:
Arduino Pro Mini - 4 mA; MRF24J40MA - 24mA, DHT11 - 300 uA; Reed pull up - 4 uA
