Using Relays to Power off then Cut power to Pi Zero

Question

I have a raspberry pi zero that I am running off a battery for remote logging reasons.

There is a low battery shut off module I have connected to the battery in case the battery gets below a certain voltage (as this is a AGM battery i have it set to 12.3 to extend batterys life).

My original idea was to wire a 5v usb car adapter directly to battery, and have the low battery cutoff module turn on a relay that would ground a gpio pin telling the pi to shutdown. (this should all work theoretically. I already have some 12v relays and also some 5v relays.

now the pi would be off, but as it consumes power even when off, I want to cut the power to it. so i was thinking putting a relay before the 12 to 5v power adapter, and when the pi is powered off, that would somehow trigger the relay. But after much research it seems like this might not work. And i may need to put some resistors in there?

i drew a schematic of what i was thinking.

any help would be awesome, thanks!

I have done some soldering, I just do not understand circuitry and resistors and i have not found anywhere someone has done this exact thing.

thanks

Answer 1

If I understand your objectives correctly, it seems that your idea is generally sound. But, the devil is in the details as they say.

Reviewing your diagram:

• It appears that the purpose of the Low Battery Cutoff Switch and the N.O. 12 V Relay is to signal the RPi (via GPIO input) that it should execute a shutdown soon. However, 12.3V seems too high for a low-voltage cutoff on a 12V AGM battery. Please edit your question and/or the diagram to clarify this.

• It also appears that the purpose of N.O. 5V Relay is to allow the RPi to signal the relay via GPIO to remove battery power from the 12V-5V converter, and thus from the RPi. But that's a bit of a "Catch 22" because the RPi cannot set a GPIO output if it has no power!

If your objective is to remove power from the RPi when the battery reaches a Low Voltage or Low State of Charge, there is a reasonably straightforward way to do that. It will require adding a Device Tree Overlay to the RPi's boot configuration file, and a new piece of hardware known as a latching relay. Here's how it works:

1. Add the Device Tree Overlay:

Modify the /boot/config.txt file on your RPi to add one specific overlay :

$ sudo nano /boot/config.txt

Once the file is open in the editor, add this one line to the bottom of the file:

  dtoverlay=gpio-poweroff

Save the file & exit the editor.

You should know that there are several options in the gpio-poweroff overlay. You can find a brief description of them hidden away in /boot/overlays/README, or on RPi's firmware GitHub repo. But as we'll be using the defaults for this overlay, that's all that's needed for changing the boot configuration.

You should now be wondering, "What does this do, exactly?".

To answer that question, shutdown your RPi (sudo halt or sudo poweroff), remove the USB power plug, and then connect an oscilloscope to GPIO 26 - the default GPIO for the gpio-poweroff dtoverlay. Once the oscilloscope is connected, re-boot the RPi by re-connecting the USB power plug. Once the RPi has booted, the dtoverlay you just added is now in effect for future shutdowns. we'll verify that with the oscilloscope measurement:

Login/SSH to your RPi again. Then, issue the following command:

$ sudo shutdown -h now

Shortly after issuing this command, GPIO 26 will be be cycled as shown below in the screen-capture image from my oscilloscope:

This output on GPIO 26 enables us to RESET the contacts in a latching relay, thereby providing a straightforward solution that will completely remove power from the RPi, and avoiding any further drain on the battery. Here's the hardware portion of this solution:

2. Add the Latching Relay:

A latching relay operates in much the same way as a light switch you flip to turn lights on or off. When you flip it up, the lights come on, and they stay on until you flip the switch down. Once the switch is in the desired position, no energy is required to keep it in that position. It is the same with the latching relay, except instead of "flipping the switch" to turn our room lights on or off, we will control the latching relay with the pulse created by the gpio-poweroff overlay. Review the schematic:

^{simulate this circuit – Schematic created using CircuitLab}

The relay shown in the schematic is available through distribution; Panasonic P/N DSP1-L2-DC12V-F from this distributor for approx US $5.50. From the spec sheet:

Here's a brief overview of what's going on in the schematic:

• The relay contacts at pins 9 & 12 are N.C., and so under "normal" conditions, the 12V battery is connected to the dc-dc converter, and the RPi has power on its USB power input.

• When your "Low Voltage Alarm" is triggered, the RPi must get that input via a GPIO pin. Your code must detect the state change on this GPIO pin, and act upon it. One of these actions (and there may be others) will be to issue the shutdown command we covered earlier to the system.

• The shutdown command will cause the system to commence a safe and proper halt to all processor activity, avoiding the risk of corrupting the file system.

• GPIO 26 will now transition to a "HIGH" (3.3V) state, turning on the transistor Q1, thereby opening the N.C. relay contacts and disconnecting the 12V Battery. This will also remove power from the RPi.

• In other words, the "light switch" has now been turned off, and it will stay off until pin 16 of the relay is grounded - returning the contacts to their "normal" position. The schematic shows a momentary pushbutton switch for this purpose, but it could be automated of course. For example, a signal that the 12V Battery has been restored, and/or other conditions you wish to check.

Conclusion:

That concludes this answer. There may further questions re implementation details for your project, but since you did not ask them, they weren't answered here. If you do have questions re other aspects of this project (e.g. the software) please post those in a new question. Otherwise, the comment section will serve for clarification on what's covered here.

---

Addenda:

1. An alternative part number for the latching relay shown in the schematic above is the TE RT424F12; MPN: 5-1393243-4. The spec sheet is available here.

Answer 2

you need something like a dead man switch. I cannot draw schematic, but i would use two relay system. One relay as you said that can pull pin low to shutdown, another (3.3V) that connects power to Rpi throuth COM-NO contact. You need to bypass this COM-NO with pushbutton. As soon as RPi boots, put a script that will turn on 3.3V Relay with some GPIO, after that you can release the button (or put a switch there and turn it off after boot).