SLLA603 March 2023
While the PIR sensor itself consumes very little power, the required motor poses a challenge. The measured power consumption of the system with the chosen motor comes out to be 0.036 mAh per cycle. A cycle is defined as the mirror rotating in one direction to the extent of the field of view and then rotating back. This provides the PIR with two opportunities to detect the feigned motion. The 1.5-Ah battery discussed in the Part Selection section therefore allows for a total of 41,667 cycles under ideal conditions.
There are two simple ways to significantly reduce power draw: use a less powerful motor or reduce the rate of sweeps of the device. Due to the relatively light weight of the mirror and flat operating conditions (that is, the motor does not have to rotate the mirror against gravity), a low torque motor is the best choice. For example, the SER0050 is the high-torque counter part of the chosen motor, the SER0053. This motor has a 40% increase in rate torque, but at the expense of double the current draw at the same voltage.
The other solution to consider is how often the device is scanning for occupancy. While a battery has a fixed number of cycles for which it can power the device, optimal spacing for these cycles can significantly increase the operating period of a charge. The first thing to consider is when to actually start scanning for occupancy. Since the PIR operates based off of motion, it does not make sense to start scanning for occupancy until somebody has been detected moving into a room. This can be done by simply positioning the mirror at a desired angle, turning off the motor, and leaving the PIR sensor to detect motion as normal. Once motion has been detected, the motor can be activated and can continue sweeping until occupancy is no longer detected.
Once somebody has been detected entering a room, the frequency that the device scans for occupancy can be adjusted based on the desired accuracy and battery life. Using the previously-stated number of 41,667 cycles for a 1.5-Ah battery, with one scan per minute, a battery life of about 28.5 days when running 24 hours per day, 7 days a week can be expected. However, this is considered maximum use, since the device is unlikely to need to be scanning a room for occupancy at this frequency. A more typical use is scanning for occupancy for about 8 hours a day. This results in an expected battery life 86.5 days. As expected, using larger batteries significantly increases the lifespan of a charge, but is also more expensive. Alternatively, the device can be wired up to a line, negating the need for recharging, but at the cost of a trickier installation process. Since this data was captured using a basic prototype model, battery life can be extended further by using a custom design. This can include using a low power motor or a lighter mirror.