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5. Power Supply

5-1. Power Consumption

The major power consumer of this device is:

  • Raspberry Pi Pico W: The output voltage of the RPI PicoW is 3.3V. With the power just on, the idling power consumption is from 50mA to 100mA. But if it undertakes power-consuming task like Bluetooth LE, it rises up to 60mA to 110mA. The figure varies from one test to another. [Source]“Raspberry Pi PicoW Power Consumption (mA) and How to Reduce It”, “PicoW power consumption”.

  • Piezo Electric Buzzer: The model available at hand is Uxcell DC 1-30V 90dB consumes maximum 3.0mA.

As I use the RPI PicoW as a controller, the external power supply should be 3.3V.

5-2. Decision on Supply Voltage

I made the board for the Final Project in Week16 System Integration. After the circuit simulation with Tinkercad, I decided to place four 3V coin batteries. With them together, 12V is created and then with the voltage regulator in between, it’s reduced to supply 3.3V to the RPI PicoW board.

However, immediately after I cut the PCB board for this project, I started wondering myself why it’s 12V, not 6V (with just two coin batteries). The reason I first thought of 12V was that I had expected that the higher the supply voltage, the louder the piezo buzzer would be. But once I connected the power supply circuit to the RPI PicoW board, again I wondered why it should be 12V if it’s not directly supplied to the buzzer.

It goes well enough with 12V coin batteries. While I decided to proceed to the final presentation with the existing device, there still remain a few questions:

  • What about 6V with only two batteries? (That will reduce the size of the box case. According to the Tinkercad circuit simulation, it could produce the same level of volume.)

  • Is there any way to directly supply voltage to the buzzer? What will happen to the buzzer? If this gives me more loudness, how will the alternative circuit design look like?

  • How do the panic buzzers being sold in the dollar shop make that panic siren?

5-3. Unsolved Issues on Power Consumption

Also, in the later stage, I faced another problem. I once tried to check how soon the batteries will expire if I keep switching on the Alarm Device. I started measuring the voltage at 7:30pm on June 6, and then measured from time to time. Here is the result:

June 6.     19:30       8.30V
            21:00       2.56V
June 7      7:30        2.57V
            15:00       2.55V
            20:30       2.54V

In fact, the PicoW board stopped advertising on the evening of June 6 after it dipped down below 3.3V.

I found that the voltage went away so fast, much faster than I had expected. I still need to do the second test with fresh 3V batteries. In general, Bluetooth Low Energy (BLE) does not cause much power consumption. But if we maintain the BLE connection with our mobile phone, the batteries will expire in less than 24 hours.

According to the MicroPython libraries, there is a program code for the deep sleep mode. It generally brings the board to deep sleep and then the power consumption could be minimized. However, once it goes to deep sleep, the BLE itself will be disconnected and that’s not what I wanted. Therefore, I decided not to explore the possibility to include the program code for deep sleep.

I am still looking for better options to reduce the power consumption.

Last update: September 5, 2024