Raspberry Pi Soft Power Controller – The Circuit

Part 1 Control Rasp Pi power with an Arduino-based controller

The RetroPie project enables retro-gaming with a Raspberry Pi. All of the Pi models have enough computing power to emulate the major 8-bit and 16-bit computers of the 80s and 90s. With the Pi 3 I have even been able to play PS1 games with no problem. My current project is to put my Raspberry Pi running RetroPie into an old Super Famicom (SFC), or SNES, case. The catch? I want the original SPST power switch to work. And by work, I mean allow the Raspberry Pi to shutdown properly when the switch goes into the off position.  To accomplish this task, I am building a Raspberry Pi soft power controller.

Raspberry Pi Soft Power Block Diagram

Here’s a block diagram of the power controller. The basic blocks in a Raspberry Pi soft power controller include the LDO, a switching supply for the Pi, an AVR-based microcontroller, and the Raspberry Pi. This post will describe each of these hardware blocks.

One design objective was to draw as little current as possible when off. For my RetroPie, I will not be running on battery. However, I do not like the idea of wasting energy when something is turned “OFF.”

This overview is a multi-post write-up. This first part is on the hardware. In the next post, I will explain the AVR’s firmware. Later, I will come back to the Raspberry Pi side of the project.

Last week’s post was on Project Sharing Sites. I’m using two for this project. Hackaday.io will host the build log while GitHub has all of the design files. And by all of the file I mean the schematics, firmware, laser cutter files (soon), and Raspberry Pi code.

Hackaday.io Project Page  GitHub Repo

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If you have any interest in retro computing or technology, The 8-Bit Guy is one of the best YouTube Channels. His latest video hits a little closer to home. He shows how to Character LCDs work and how to hook them up. He always does a great job with his videos, so I encourage you to check out some of his others.

He’s has some other good videos such as the one on how 8-bit video game graphics worked and, one of my favorites, how disk drives worked.

Share projects on one of these 4 platforms

Contribute back to the open source hardware community

share projects

Sharing is the maker community’s foundation. When you share projects with others, you contribute to the community. In the past, you might just post your project on a personal website. Today there are many options to share projects.

This weekend I “finished” my reflow oven controller, Open Vapors. Believe it or not, five years ago there were not a bajillion similar projects. In fact, I based my design on the only completely open source project I found. It is a reflow oven controller Arduino shield from Rocket Scream.

After completing my controller, I was excited to share the project. Then I started to think about where to post the files. Obviously, here at baldengineer.com is one option. But I wondered. Is there a better place where others could benefit from my work?

This post is a few notes on the platforms used to share projects. At first, these might seem like they all serve the same purpose. From a high level that is true. However, there are small differences that you should consider when you share projects with the open source hardware community.

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Detect short and long button press using millis

Different actions based on how long the user presses a button

detect short long button press using millis

To detect a short and long button press using millis can give your project more functionality without adding more buttons. In this line-by-line example, I show how to react to a user pressing a button for a short period (100ms) or a long period (over 500ms). My example changes the blink rate of an LED on short presses. A long button press turns off the LED.

In the code, I make use of a struct so that a single variable can be used to track multiple parameters. The benefit of this method is that adding multiple buttons is easy. You could create an array of these tyepdef’d variables.

Download At GitHub

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7805 on a SNES Motherboard

When I made the AddOhms Tutorial on Linear Regulators, I made a comment about the 7805. I said it may be one of the most important Integrated Circuits (ICs) ever made. That’s a bold statement. The 555, 805,  or 7400 might all qualify for such a distinction. My feeling about the 7805’s importance is because it is a chip that is still popular today. It is used, or at least was used, in so many applications. And it is the heart of many 5V digital systems.

Including the Nintendo Super Famicom (and I assume the US SuperNES).

This picture is from an SFC I disassembled to repurpose the case. While taking it apart, the 7805 caught my attention because it was attached to a shield as a heat sink. Also, I find it fascinating that it is one of 3 or 4 through-hole components on the entire system. As you can see from the picture, it needs some cleaning. I might post more pictures later.

Best Sorting Resistors Method 

At least, according to me.

sorting resistors method

When you buy a grab bag of components, you might need to tackle sorting resistors. Here’s how I sorted some bags of random resistor assortments last week.


Then method I use for sorting resistors achieves these objectives:

  1. Fewer Bins. It doesn’t take long to create a large matrix of resistor values. My resistor sorting method is relatively compact.
  2. Quick to find. When I’m building up a circuit, I don’t want to spend time sorting through a pile. Once I know the value I need, I find a single package and then look for a single color band.
  3. Works with 4-band and 5-band resistors. Let me be upfront: I *hate* 5-band resistor color codes. While the 5th ring is supposed to be slightly offset, or wider, or a different type of color; it doesn’t matter. It’s nearly impossible to tell read a 5-band resistor color code when they are in a pile. However, using my method for sorting resistors, it doesn’t matter if I’m looking at a 4-band or 5-band resistor. I can immediately identify the resistor value.

Based on #3 alone, you might be wondering what is the fantastic method (and how much will it cost to get it!) Here’s the basics of my method for sorting resistors. (For FREE!)

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Arduino Data Logger: Serial Monitor Alternatives

Forget the serial monitor, here's 5 other ways to do Arduino data logging

arduino data logger options

The Arduino serial monitor is usable when you want to watch data from an Arduino. However, it does not have a built-in method for saving the data. Here are some ideas if you want to build an Arduino data logger with or without a PC.

Important note on Arduino Data Logger examples

With all of these examples, please remember that whenever you open the Arduino’s serial port, the board will reset. So if your log file shows “Initializing SD card…” with a few data lines in between, it is because there is a reset happening.

Initializing SD card…initialized.
Temp: 34, Time: 03:24:44
Temp: 33, Time: 03:24:45
Temp: 34, Time: 03:24:46

Initializing SD card…initialized.
Temp: 34, Time: 03:24:50
Temp: 34, Time: 03:24:51
Temp: 33, Time: 03:24:52
Temp: 34, Time: 03:24:53

In that code you can see data logging started and then restarted. What happened is that after programming, the board starts logging. Then when you open the Serial Monitor, the data logger restarts.

To solve this issue, either disable auto-reset, add a 3-4 second delay at the start of setup(), wait for a character to be received, or wait for a button press. That will give you time to open the Serial Monitor.

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Sending simple serial commands to an Arduino

Don't send words when characters will do

sending simple serial commands arduino

Sending simple serial commands to an Arduino is the easiest way to communicate between an Arduino and a computer. The computer could be a PC, a Raspberry Pi, or any device that communicates with serial.

By sending and “decoding” a single character it is easy to add a simple debug menu or even serial menu. Plus, it is easy to extend.

Single Character vs. Full Words

The mistake I see many people make is that they try to send full-text strings as serial commands. For example, to turn on a LED, I have seen (silly) commands like “RED LED ON” or “RED LED OFF.” While you could use something like strcmp(), as I showed on the Multiple MQTT Topics example, that tends to be overkill for most serial commands.

Humans like words, computers like binary. Just send one character over serial.

switch (variable) {
  case ‘a’:
	// A Stuff

  case ‘b’:
  case ‘c’:
	// B and C Stuff

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Multiple MQTT Topics with Arduino PubSubClient

Adding a few more subscriptions is pretty easy.

multiple mqtt topics

In my Arduino MQTT Examples, I kept things simple by only subscribing to a single topic. One of the strengths of MQTT is that a device can subscribe (or publish) to multiple topics. The broker will sort things out. Even though my first example only showed one, it is straight forward to get the Arduino PubSubClient library to subscribe to Multiple MQTT topics.

The quick answer is that you need to look at the MQTT response to find out which topic sent the payload.

tl;dr version

If you’re looking for a quick answer, here’s the magic code we’ll add to the callback() function.

void callback(char* topic, byte* payload, unsigned int length) {
if (strcmp(topic,"pir1Status")==0)
  // whatever you want for this topic

Keep reading for a more detailed explanation of how to Subscribe to Multiple MQTT topics with Arduino’s PubSubClient. Obviously, this code will work on Arduino boards with a TCP/IP interface and, of course, the ESP8266 based boards.

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It is commonly known that ceramic capacitors change capacitance with applied voltage. What isn’t always as well known is how strong this effect can be and why it occurs. At KEMET we’ve put together a technical video that answers that question.

What is Ask An FAE?

Ask An FAE is a new video series we launched at my day job, KEMET. An FAE is a field application engineer. These engineers are very common in the electronics industry. Companies like KEMET, where I work, have FAEs who meet with customers to answer technical (and very detailed) questions about how to use their products. In UBM’s Mind of an Engineer survey, FAEs were ranked as one of the top information sources for design engineers.

At KEMET we decide to use FAEs to answer the questions. While I’m not an FAE today, I was in the past and happy to kick off the series with our CEO.

Check out KEMET’s Ask An FAE