accelerometerbluetoothbluetooth low energymicrocontrollermotion
I am hacking together a prototype wrist band, with BLE as the main component.
I'd like the device to detect a bump or a wave. What would be the most energy efficient way to do so? I'd like the charge on the device to last at least 6 months.
Any ideas would be appreciated.
If you need a solution that doesn't require a voltage regulator i.e. operates directly on the Vcc lines to the PIC then I've modified it slightly (and for a good reason): -
The reason it's modified is that the BJT pass transistor might drop a little too much voltage under load if you are just switching an already regulated Vcc to the PIC. I've replaced the pass-transistor with a P channel FET and removed the 2k2 and 33k resistors on your circuit. The 2k2 is needed to protect the BJT pass transistor and the 33k put a max limit on the pass transistor's base current - not needed when using a P channel FET.
I've also added a 10k pull-up on PD3 but this can be an internal pull-up in the PIC. What looks like a suitable P-channel FET is the TP0101 - it's the first one I came across when googling "Low Vgs(theshold) P channel MOSFET". There will be plenty that will do but this one has low on resistance (<1ohm) with just 2.5V drive on the gate and that was my primary aim.
It will also work up to over 10V so could be used prior to a regulator too.
Energy efficiency when "on" will be better that the BJT circuit because it doesn't need base current to keep it on. When powered off, both circuits will be really low on current but maybe the P channel leakage will "waste" 1uA leakage into the load - it's probably a good idea to make sure this 1uA doesn't somehow develop a voltage on the PIC that might cause it to "strangely" power-up - use a 100k load on the output - this will develop 0.1V across the PIC's power pins and kill off the possibility.
It's just a spring in a tube. The spring is anchored at only one end. When shaken, the other end of the spring touches the tube and completes the circuit.
The transistor functions as a simple amplifier so that the full LED current doesn't need to pass through the sensor.
Best Answer
Is it a specific type of motion or any type of motion? for a specific type of motion (gestures, wave etc), an accelerometer and/or a gyro (There are devices which contain both) is the most suitable. For any type of motion, a rolling ball switch or vibration detector has lower power requirement and lower complexity.