I'm planning on using a momentary button to power an Arduino with a 18650 lipo battery through an Adafruit Powerboost 1000c. I already have a circuit working but it is simply controlling power to the Arduino. It relies on the Powerboost outputting power at all times. This is obviously wasteful for a project that relies on battery power.
I would like to utilise the enable pin of the Powerboost by tying it to ground when not in use through a BC327 PNP transistor to disable the output. The enable pin is held to VBAT by design therefore enabling output at all times.Once powered, the Arduino will hold the base of the transistor HIGH, therefore disconnecting the enable pin from ground and enabling output. Once shutdown conditions are met, the Arduino will set the base LOW to tie the enable pin to ground again shutting down the circuit.
Could someone please check my schematic to make sure it will work.
And beause of the setup with two resistors button-pin will never read high because it will rise to 0,7V + 0,7V = 1,4v max. Move (and reduce to one) the resistor directly to the transistor base. Also add a pull down resistor.
And you can probably do just without the transistor if you just flip the logic of the Arduino and switch as well
Other thing to note, most converters will only shut down the boosting when disabled but NOT turn off the output. So the when disabled the output voltage will just drop to battery voltage. Might give you some trouble...
And because of the setup with two resistors button-pin will never read high because it will rise to 0,7V + 0,7V = 1,4v max. Move (and reduce to one) the resistor directly to the transistor base. Also add a pull down resistor.
This confuses me. Where does the 0.7V come from? Vbat will be between 3.6V - 4.2V depending on the charge state of the battery and the on/off pin will be either 0V or 5V ie on or off. Also, if i move the resistor directly on to the transistors base then there will be no current protection on the buttonpin.
The pulldown resistor is a good idea! I hadnt thought of that.
The Powerboost does indeed completely shut off the output when disabled.
The more i look at it the more i don't understand why i need to flip the transistor. The enable pin is tied HIGH to Vbat by design so will keep the powerboost enabled. The whole reason for using a PNP transistor is that it will keep the enable pin LOW when the circuit is not powered. Given i need to add a pulldown resistor. Once the circuit is powered using the pushbutton, the Arduino will turn on and the ON/OFF pin will be turned HIGH in the code. Therefore turning the transistor off and enabling the Powerboost.
Do i have that right or have i completely missed the mark?
0,7V is roughly the (rule of thumb) voltage between base and emitter. And because fo the same reasons, it's the (rule of thumb) voltage over a diode.
Missed you use a PNP, but that will no work. Use a NPN. Nothing pulls the base of the PNP low now so it will not activate. And even if it did, the enable line would end up 0,7V higher (because of VBE) then the base.
I would just bin the transistor and do:
Forgot to draw the button line, you connect it between the button and the diode. Left pull down is only there to pull down the button line.
Thanks again for your suggestions Septillion. Looks like ill be breaking out the breadboard again when i get home next week.
Here is a link to the device......
It does indeed turn the output off. The product website and datasheet for the booster ic both state that the output is completely turned off with enable pin LOW. Ive also tested it with a multimeter.
So the fact that the board already comes with a pullup resistor on the enable line holding it high wont be a problem? Essentially we would be pulling it up and down at the same time.
Or use a pull resistor which is at least a factor lower then the onboard pull up.