Sparkleswitch

The Noisebridge Laser cutter is an expensive piece of machinery that has some non-negligible running cost and requires some safety-training before use.

We require to authenticate with an RFID showing previous laser training in order to switch the laser on. This project contains the relevant electronics and software to build this switch.

We use a generic EStop/On/Off switch box (Aliexpress, Amazon) as basic user interface, coupled with an additional RFID reader.

Requirements

Have a switch that provides on/off and emergency off buttons as well as an RFID reader.
The safety relevant triggering of the emergency button must shut off power reliably by simple electromechanical means with no software involved to work even if the computer has issues. After an Emergency-Off, it is required to reset the EStop button (manual twisting) and re-activating via RFID.
Have a display to show the currently active user, potentially laser time accounting, uplifting idle-animations etc.
It is common to pass the custody of the running laser cutter to the next user, so make it simple to switch the active user without power-cycling the laser.
Connection between Pi and emergency off button is separate, so it should be simple to wire up.

Design considerations

As controlling computer, we use a Raspberry Pi: it is cheap, can run a LED display, and has enough additional GPIO pins to read button state and trigger relays.

Since we need to switch line voltage with the Pi, we use a relay. The basic switch mechanism is a simple relay circuit acting as a Set/Reset Flip-Flop that fails safe on power loss. Two control relays for set/reset are triggered by the Raspberry Pi; the main switching 4-pole relay is part of the flip-flop and also switching the mains phases for the machine.

The mains relay uses one of its poles to act as self-sustaining relay. A momentary switch of the 'on' trigger relay will switch on the machine, and a momentary interrupt from the 'off' trigger will switch off the machine:

The Emergency-Off button is cutting power to the relay circuit, thus acting as master reset (thereby meeting the no-software-involved safety requirements). Set (= switch power on) and Reset (= switch power off) are triggered by the Raspberry Pi.

Inputs and outputs are

in Emergency Off button. Directly wired in the circuit acting as physical 'reset' for the SR-FlipFlop. Also wired to a GPIO as input to inform the application about the state.
in On button. Wired to a GPIO input as a soft-button for the application.
in Off button. Wired to a GPIO input as a soft-button for the application.
in a sense input to detect if the mains-relay coil voltage is on. Mostly for fault detection via software.
out Trigger on relay. Controlled by GPIO to trigger set of SR-FlipFlop.
out Trigger off relay. Controlled by GPIO to trigger reset of SR-FlipFlop.
serial in/out RFID reader (connected to UART on Pi).

Since we use the on/off buttons to interact with the application as soft-buttons (e.g. to trigger 'pending off'), they are not physically wired into the circuit but used as input to the computer (this is fine as they are not in the safety critical path).

There is a little space in the switch box which will house the trigger relays and possibly the RFID reader. The relay switching of the line voltage for the laser cutter is mounted near the outlet for the machine, connected with three wires (see above:GND, Sustain, Trigger ON) remotely from the switch box.

As trigger relays, we use off-the-shelf and cheap relays.

State machine

The software on the Pi implements the following state machine; it uses inputs and outputs mentioned above. Inputs are on the transition edges; external actions triggered by the application are represented as hexagons. Since we often have the situation that we want to pass the running laser to the next user without having to switch it entirely off and on again, there is a provision to change the user while the machine is running:

Components

There are three physical parts to the system:

RGB LED matrix screen, that has a Raspberry Pi connected to it to display the user interface.
The switch box is separate from that and mounted under the display.
The mains relay that is close to the outlet of the laser-cutter, and remotely controlled with a 12V control voltage.

The connection between the switch-box and the Pi is a simple flat cable with an IDC-12 connector on both ends.

Desc	Pin	Pin	Desc
+5V	1	2	+5V (low current; for trigger relays and RFID)
Sense EStop `in`	3	4	`in` sense relay voltage (e.g. for failure detection)
Off-Button `in`	5	6	`out` Relay Trigger Off
On-Button `in`	7	8	`out` Relay Trigger On
GND	9	10	GND
UART RX on Pi	11	12	UART TX on Pi (3.3V TTL)

There is a PCB on the Raspberry Pi to wire up the LED panels and provide the connector to the switch-box, and a PCB in the switch-box as a breakout board.

Pi Sparkle HAT

The Rasbperry Pi connects to the Pi Sparkle HAT, that provides connections for two chains of HUB75 RGB LED matrix (using the rpi-rgb-matrix project) and the Sparkleswitch-specific IDC-12 connector to connect to the switch box.

Inputs (from switches and sense-lines all operating at 12V) are insulated with opto couplers. The relay trigger outputs are 5V TTL (but non-insulated as the receiving end uses opto-couplers already).

Switch Box

A small breakout board in the switch-box provides a way to connect the switches, the Trigger-Relays (housed inside the switch-box) and the RFID reader (attached to switch box). It also provides a screw terminal to wire up to the mains-relay whose wires are also terminating in the switch box.