Transistor to make a timer controlled charge for a device.

Hello everyone.

I was wondering on how to control charging of a device via a timed arduino.
I have a car USB charger that converts 12V to 5V , which is ok, but i want to put an ESP8288 in the middle to control the time that a mobile hotspot is charging. For reducing the power consumption i would like to avoid putting a relay, which would be easy.

I would like to use a BC547 transistor between the + from the 5v USB converter output and the mini usb input usb port on the Hotspot.

Something like this:

The GPIO from the ESP is 3,3V , R1 value is 220Ohm and even that i did not drew it, GND from the ESP is shared with the USB converter negative output because this converter is also powering the ESP.
I really don't know how much current does the Huawei E5577 hotspot draws from the 12v->5v converter.

I need the opinion from someone more experienced than me if i can do it this way or will i be putting anything in danger. Specially my car, i wish to use this on my car.

Thank you all!

The main problem with using a transistor in this way is the voltage drop that will occur. You are going to lose about 0.6V across the transistor which may leave insufficient voltage for your USB device to charge.

Have you measured the current draw of your hotspot? Most devices with rechargeable batteries contain a battery management chip that will shut down the charger once the battery is fully charged. You can get a USB inline power meter (such as this one) for a few dollars which will tell you if excess power consumption is a problem.

Hello rw950431!
Thanks for your help!

Most devices with rechargeable batteries contain a battery management chip that will shut down the charger once the battery is fully charged.

I really don't know on this case, it points out always as charging. Your idea about the USB inline power meter is great, did not thought of that. But maybe i could make a better use of a 12v inline power meter, because my main worry is to drain the car battery and not the hotspot itself.

The main problem with using a transistor in this way is the voltage drop that will occur. You are going to lose about 0.6V across the transistor which may leave insufficient voltage for your USB device to charge.

Did not know this happened. If only i could understand datasheets.....
I figured out how could i use a relay for it, i think, but anyways beside the 0.6v drop i could safely use what i drew in the diagram?

Thank you a lot.