Introduction
Skills Needed
Components
Function
ATtiny Logic
Assembly
Testing
Limitations
Further Development
Final Note

Published August 22, 2018 © GPL3+

Safe Shutdown and Auto Bootup for Raspberry Pi

Have someone pulling the plug on your project, or perhaps an unstable or unreliable power supply? Here is the fix!

AdvancedFull instructions provided3.5 hours22,677

Safe Shutdown and Auto Bootup for Raspberry Pi

Things used in this project

Hardware components

Microchip ATtiny85

Resistor 10k ohm

Resistor 1k ohm

5V Step Up Voltage Regulator

P-Ch Mosfet IRF 5210

IC Base

General Purpose Transistor NPN

Super Capacitor 40F

Resistor 221k ohm

PCB

Resistor 47.5k ohm

Resistor 10 ohm 1W

MBR360G – 3A Schottky Barrier Rectifier

1N4007 – High Voltage, High Current Rated Diode

Female Header 20 Position 2 Row (0.1")

Hand tools and fabrication machines

Soldering iron (generic)

Story

Introduction

Whether it's a media center, file server, web server, robot, car PC, or an arcade console, a battery-less UPS for the Raspberry Pi gives it enough juice to shutdown properly and then boot back up when sufficient power is restored.

I've had one to many times have the power "fail" on my Pis. Many times it is a human "fail" by inadvertently pulling the plug.

Now I don't need to worry about that anymore.

The idea behind the safe shutdown and restart UPS is based on the function of a shutdown pin on the Pi.

Before going further, a shutdown script on the Pi must work by pulling a GPIO PIN to LOW. Once that is complete, the hardware can then be looked at. I have not (yet) mentioned how this is done as there are plenty of online tutorials out there.

Skills Needed

Soldering expertise and knowledge of the electronic components listed below.

Programming, “Python” and “Bash” for the Pi, “C” for the ATtiny (and also knowing how to program one).

Components

Supercapacitors 40F* x 2
220uF capacitor
Boost up converter
ATtiny
IC base 8-pin
Perf board or PCB
Headers
Schottky diodes 2A x2
Regular diode 1N4007 x2
P-Ch MOSFET IRF5210 IRF4905 (or equivalent)
NPN transistors 2N2222 or 2N3904 (or equivalent)
Buttons (optional)
Resistors 1/4 W 1Kx2, 3Kx2, 10Kx2, 47Kx2, 220Kx1
Resistor 1W 10 or 20 Ohm

*40Fis dependent on load, 35F also worked fine on a raspberry Pi3.

Function

When main +5V is connected, the capacitors begin charging.

When charge> 3V, the boost converter supplies 5V to the source and gate of the MOSFET and the ATtiny

The ATtiny boots up and monitors the capacitor voltage.

When capacitor voltage >3.9V, the ATtiny powers the PWR transistor grounding the MOSFET's gate and 5V is supplied to the Pi.

If the mains 5V drops and capacitor voltage < 3.8V and MOSFET still on, then the ATtiny powers the shutdown transistor whose collector is connected to the shutdown pin on the Pi, hence pulling it to LOW. A shutdown is triggered on the Pi, approximately 25 seconds later the ATtiny de-powers the PWR transistor, turning off the MOSFET and power is removed from the Pi.

ATtiny waits until mains 5V returns and capacitor voltage >3.9V to power the MOSFET transistor again and restore power to the Pi.

ATtiny Logic

Code Logic of ATtiny

Assembly

Components

Gather components first and test their function. Check for correct resistances and correct function of transistors and MOSFET.

Solder resistors first.

Diodes and 220 uF capacitor next.

Regulator is mounted on 3 male header pins. Solder the base of the pins to the PCB and trim the top after soldering onto the regulator.

Solder the IC base and the button in place.

Solder the capacitors and the header pins. Solder some (red) wire to the positive rail and(black wire) to the negative rail.

Testing

Apply power to the device, adjust the regulator to output (counter clockwise is increase) 5.1V at the VOUT point. Check the VCC at the IC base (see schematic) which should read approx 4.8V (due Vf of the diode). Check that there is 5.1V at the source and gate of the MOSFET.

On the IC base bridge VCC and PB1, and check that there is voltage at the base of the transistor controlling the MOSFET.

Bridge VCC and PB4 and check that there is voltage at the base of the transistor controlling the shutdown Pin.

Check voltage at PB2, this should read about ½ of the capacitor voltage (depending on their charge level).

Solder MOSFET and transistors. Connect a bit of wire from the collector pin of the shutdown transistor to the chosen shutdown pin on the Pi (above case BCM pin18).

On the IC base bridge VCC and PB1 and check that there is 5.1V at the Pi supply pins.

Bridge VCC and PB4 and check that the shutdown pin is pulled to ground.

Limitations

Input voltage (mains supply) max of 5.4V.

The button is a “force boot." It bypasses the capacitor reading and sends a higher voltage to the PB2. Using it will allow the Pi to boot; however, you will lose shutdown protection as the capacitors would not have reached their minimum level.

Further Development

Some prototyping area on the PCB allows for further components.

PB3 is an open PIN on the ATtiny. It can be used for further logic if needed.

You can tweak the voltage at the MOSFET once setup by adjusting the boost converter. See Pi's input voltage limitations.

You can set a GPIO pin go high when the PI completes the shutdown. It is done by editing the /boot/config.txt and adding: dtoverlay=gpio-poweroff,gpiopin=17 At the end of the file (Replace "17" with the BCM pin you would like to use). Use that to trigger any other event on the remaining Attiny pin (for example).

Final Note

I've been using these for a few months now. Please feel free to improve on the design and do comment on how you have done so.

Be careful with supercapacitors, when fully charged they can pack quite a punch, e.g. short circuit.

Code

ATTiny ups logic code

int voltagepin = A1;
int powerswitch = 1;
int mains = 0;
int shutmedown = 4;

void setup() {
  // put your setup code here, to run once:
  //Set Pin In/Out
  pinMode(powerswitch, OUTPUT);
  pinMode(shutmedown, OUTPUT);
  pinMode(voltagepin, INPUT);
  pinMode(mains, INPUT);
  delay(5000);
  bootup();

}

void loop() {
  // put your main code here, to run repeatedly:
  if (!digitalRead(mains) && digitalRead(powerswitch) && analogRead(voltagepin)<390 ){
    digitalWrite(shutmedown,HIGH);
    delay(25000);
    digitalWrite(powerswitch,LOW);
    digitalWrite(shutmedown,LOW); 
    delay(5000);  
  }

  if(!digitalRead(powerswitch) && analogRead(voltagepin)>400 && digitalRead(mains)){
    digitalWrite(powerswitch,HIGH);
  }

  delay(10);

}

void bootup()
{
  //bool flag = false;
  
  while( !(digitalRead(mains) && analogRead(voltagepin)>400) ) //while charging lock in loop till voltage is above ref and mains is on
     
    {
      delay(500);//little wait
    }
  //Once Flag true Power On
  digitalWrite(powerswitch,HIGH);
  delay(10000); //bootup time

Credits

Philip Tabone

3 projects • 14 followers

If I need, it I'll build it.

Safe Shutdown and Auto Bootup for Raspberry Pi