I have a problem I can't seem to figure out and wonder if I am fundamentally missing something. I am trying to turn on a Raspberry PI (RPI) 3 periodically to perform a task and then shutdown after the task is complete. A TPL5110 timer from Adafruit is used to set a logic line high when it is time to turn on the RPI. The logic line from the TPL5110 connects to the Base of a darlington transistor TIP120 via a 2.2k resistor . The RPI 5V connects to POWER 5V and the ground connects to the Collector of the TIP120. The Emitter of the TIP120 connects to ground.

The problem I am having is that when the logic line goes high, the RPI turns on but seems severely under-powered. The green LED light on the RPI would flicker, then there will be a few flashes of the screen and the device seems to shutdown. The red LED never turns on. With my TIP120 arrangement, the current limit on the Collector - Emitter should be around 1.7A which should be largely sufficient to run the RPI. Why am I having this problem?

Any thought would be appreciated.

 

Darlington transistors have a high C-E saturation voltage, the RP will never get the proper 5V it needs to run.

Use a MOSFET.

 

Darlington transistors have a high C-E saturation voltage, the RP will never get the proper 5V it needs to run.

Use a MOSFET.

Makes perfect sense now. Thanks a lot. Will use a mosfet.

 

..... a logic-level MOSFET.