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The Soft Power Latch Project
Hobbyist Power-On Circuit ...

The Soft Power Latch is a power-on circuit with
a software controlled power-off function.

The ATtiny microcontoller latches the circuit power on after the push button enables supply to the system. Holding the push button down, or a signal from the master microcontroller turns the power off. This was developed for the AuraBean project and has proven to work well with very little leakage current.

Features
  • Progammable ATtiny45 using AVRISP MKII
  • DMP2305U-7 P-channel MOSFET  Datasheet
  • Maximum supply +5V
  • Low leakage current
  • Software controlled shutdown
  • One press to power on
  • Programmable 2 second Hold to turn off
  • Programmable LED
  • Option for external button
  • Countdown alert signal prior to shutdown
Wiring Example

The example below uses the SOFTLATCH to control power to the TEENSY. The optional push button shown in the circuit works in parallel with the on-board push button on the SOFTLATCH and allows the circuit to be switched on or off.

The TEENSY can disable power by pulling the KILL line LOW. If power is to be removed by the SOFTLATCH due to either timeout or push button action, then the STOPPING input to the TEENSY is activated to give that device warning of impending power loss. This feature suits embedded operating systems like the Raspberry Pi that require the execution of shutdown commands prior to power loss.


DEVELOPMENT MILESTONES
Second Prototype
2nd September 2017
  The second prototype has been assembled and programmed successfully. Improvements allow it to interface with 3.3V logic.
Prototype Built
13th May 2017
  The first prototype has been assembled and programmed successfully.
PCB Designed
16th April 2017
  PCB has been developed based on the tested design of the AuraBean.
Soft Power Latch Works
23rd July 2015
  The soft power latch works on paper. I've drawn the circuit using the Circuit Scribe conductive ink pen on paper and soldered some components in place. A momentary press of the ON button turns the circuit on, whilst the KILL button turns the circuit off.
Power Circuit Simulated
17th July 2015

  I found a soft power latch circuit on EEVblog that is exactly what I am looking for. One momentary press of the power button turns the circuit on, and another button turns it off. The circuit draws no power when off, and allows the facility for a microcontroller to monitor the on button and turn the circuit off itself.

PROJECTS

» Softlatch Project
   Power latch with software power-off function
» INA219 Serial Project
   Serial ammeter for project USB supply
» AuraBean Project
   Probability and sensitivity research tool
» AuraBean II Project
   Web-enabled probability and sensitivity research tool
» Guessology Games
   Random browser-based guessing games


HOW-TO

» Raspberry Pi Programmer
   Programs ATTiny85 microcontrollers
» Pycom WiPy Setup
   Setup and program the WyPi



© 2020 Cedric Joyce