EspNowSkip

Project EspNowSkip
Music skip button based on ESP-Now protocol
Status	In progress
Contact	bertrik, jelly
Last Update	2018-07-04

Introduction

This page is about using the ESP8266/ESP32 ESP-Now protocol in the music skip button.

A music skip button has some tricky requirements:

• you want it to react reasonably quickly, with little latency between pressing the button and skipping to the next song
• the skip button should be mobile, so it is battery powered and has to be conservative with current

The reason to explore the ESP-Now route, is because the existing skip button implementations have some serious disadvantages:

• we used to have a wireless infrastructure based on NRF24L01+ modules, this was never really reliable and it got more complicated with incompatible fakes appearing on the market
• we have a WiFi-based skip button, but it's online continuously, drawing way too much power to make it mobile (so it's tethered to a 5V charger)

The ESP-Now protocol is connection-less, you don't need to set up a session/get an ip-address to communicate between nodes. This saves time and battery power, exactly what we need.

The current ESP-Now skip button prototype needs only about 200 ms after wakeup to send a message and return to deep sleep again.

The plan:

• the skip button itself has been built and works
• create a receiver that consists of an ESP32 (e.g. a SHA badge):
  • runs the "revspace-espnow" soft AP and receives ESP Now messages
  • connects to the space WiFi and forwards the skip to the music player (and towards MQTT)

Alternatives:

• use two ESP8266, one to receive ESP Now, another one to communicate with the space WiFi: doesn't appear to be necessary when using an ESP32
• use a raspberry pi, orange pi or similar: which one? existing one like projector? adds more dependencies/complications/discussions
• use an ESP32 with wired ethernet: complicates things, needs more hardware, is a separate project by itself

Hardware

schematic

The hardware is based on ESP8266 (featuring the ESP-Now mode), in particular a Wemos D1 mini board, they are cheap, small and easy to find. A Wemos D1 mini board has an onboard regulator with typically much lower quiescent current than the similar NodeMCU. Another candidate would be an ESP32, it also features the ESP-Now mode and is present on the SHA2017 badge, which is already a kind of mobile platform.

The proposed way of wiring it up, is to make the actual physical button just power up the board. A (super-)cap parallel over the supply wires makes it possible to run the skip program to completion from just a single short press. In case a ESP-Now re-initialisation (which takes a bit more time) is needed, you can just keep the button pressed for a bit longer to complete the initialization process. This way, the SKIP program starts executing as soon as you press the button and there is no quiescent current drain on the battery.

The receiver/master is connected to a raspberry pi or similar, with the EspNowSkip receiving packets from the air and sending them over serial to the raspberry pi. The raspberry pi can then do its magic to make the music skip (e.g. GET/PUT a specific URL of the playback software, send an MQTT message, etc.)

Capacitor calculation

The formula for calculating the required capacitance for the (super)cap over the power supply lines:

 C = I . dT / dV

where C = capacity, I = average current, dT = how long the current is drawn, dV = how much voltage drop we can tolerate. Suppose I = 100mA, dT = 0.3 sec, dV = 1V, then the required minimum capacity is approximately 30 mF, or 30,000 uF

Battery life

Suppose we power the entire circuit with a 18650 battery of 2000mAh capacity.

The deep sleep current of the Wemos D1 mini has been measured at about 0.16 mA. So in deep-sleep, the battery should last > 10000 hours. Battery self discharge current is estimated at 20% per year. For a 2000 mAh battery, that is equivalent to about 400 mAh/8765h = 45 uA, about 1/3rd of the deep sleep current of a wemos d1 mini.

Suppose a skip takes 500 ms consuming a current of 150 mA on average. That's about 0.02 mAh per skip, so the battery should last > 10000 skips.

Regarding the use of a capacitor on the power lines: If we put a (super/ultra-)capacitor parallel over the supply to the wemos d1, we can reasonably assume that the charge in this capacitor is lost after a skip. Suppose we use a 0.1F supercap charged to 4V, that represents a charge of Q = C.V = 0.4 coulomb, equivalent to about 0.11 mAh. This is already much larger more than the 0.02 mAh calculated earlier, though still allowing over 9000 skips from a 1000 mAh SHA badge battery. It also scales with the capacitor value, so we should be careful choosing a capacitor value that is not excessively large.

Software

Source code

Source code can be found here https://github.com/bertrik/EspNowSkip

It contains the following programs:

• skipbutton.ino: an Arduino program that is the button part of the system (slave) which sends the skip message
• skipreceiver.ino: an Arduino program that is the central part of the system (master) which receives the skip message
• skipforwarder.ino: an Arduino program that accepts a skip message over serial from another ESP and forwards it to the squeeze-player and MQTT
• forwardskip.py: a Python script that takes a topic/payload over the serial port from a skipreceiver and forwards it to MQTT

EspNowSkip protocol

It works like this:

• there are two roles:
  • a single central "master" node
  • one or more "slave" skip-buttons
• when the skip button is pressed, the ESP8266 resets and starts running, reads the last known ESP-Now channel and MAC address from EEPROM and sends the "SKIP" message
  • if the skip button doesn't receive an ACK from the central node, it starts a kind of discovery to re-establish the channel and MAC address of the central node
• the central node is configured as an AP with a specific name (e.g. "espnow-revspace")
• the discovery procedure tries to find the central node by doing an AP scan, the AP name is the only thing that needs to be agreed upon in advance between master and slave code
  • in the list of the APs, the slave node finds the SSID belonging to the central node, then it also knows the wifi channel and MAC address of the central node
  • the slave node stores both wifi channel and MAC address in (emulated) EEPROM and goes back to sleep

The data we send is an MQTT-like topic + message, for example

 revspace/button/skip now

The master node can simply parse this into two parts (split on the first space) and bridge this to the Revspace MQTT infrastructure.

Skip protocol

Classic HTTP GET

Our audio system skips to the next song in the playlist using an HTTP GET with the following path:

 /Classic/status_header.html?p0=...&p1=...&p2=...&player=...

where

 p0 is "playlist"
 p1 is "jump"
 p2 is "%2B1", or "+1" escaped
 player is "d8%3Ad3%3A85%3A17%3A30%3A9c", or "d8:d3:85:17:30:9c" escaped, which is the unique id of the particular player instance

Example:

 http://jukebox:9000/Classic/status_header.html?p0=playlist&p1=jump&p2=%2B1&player=b8%3A27%3Aeb%3Aba%3Abc%3Ad5

JSON Protocol

To skip, send a HTTP POST to:

 http://jukebox.space.revspace.nl:9000/jsonrpc.js

with content

 {"id":1,"method":"slim.request","params":["be:e0:e6:04:46:38",["button","jump_fwd"]]}

Expect a result like:

 {"result":{},"params":["be:e0:e6:04:46:38",["button","jump_fwd"]],"method":"slim.request","id":1}
 

References

• Read-the-docs about the low-level IDF ESP-Now API
• Harringay MakerSpace ESP-Now
• WifiEspNow project on github
• Our current WiFi-based skip button

Future

Done:

• make sure ESP8266 ESP-now implementation is interoperable with ESP32 ESP-now implementation indeed it is!

To do in the immediate future:

• finalize the ESP32 receiver code, use the JSON-RPC interface: more REST-like (POST iso GET), better result codes
• install the software on an ESP32 platform, like a SHA badge, connect it up with a cable, etc.
• add a supercap to the skip button, to allow a short press (e.g. 100 ms) to also be registered -> I have various types in my Samla (e.g. 0.1F 5.5V).

More distant future:

• use ESP-Now also for other kinds of hacker space infrastructure where low-power consumption is important and/or a power wire is inconvenient: outside temperature for example.
• add more buttons besides just skipping.
• send data back to the skip button, for example we could send a "now playing" message or album art back to a SHA2017 badge and have it shown on its e-ink display.
• use encryption/authentication on the ESP-Now link. I think the protocol already allows this, using a simple shared secret.