I'm in the process of designing a separate/independent safety processor using an Arduino.
Some motivation to do that : have a look here
This would make it easier for most 3D-ers to implement this as they know how to flash Arduinos.
design parameters :
- overtemperature bed
- overtemperature heatsink @ hot end
- if one of these input parameters is triggered, power to heaters will be cut using a normal (mechanical) relay.
- indication status using LED's (green OK-led , red BED error, red hot end error, red smoke error)
- reset button
- separate power supply
- trigger levels can be set in firmware as one is used to for i.e. Marlin.
This is a draft of the HW as I have it in mind.
Led's should be clearly visible. Once activated the safety processor has to be reset manually.
FW should do the following :
- on startup : relay should be off - No power (12V) : relay off
- safety pre check. The process shown by blinking all the leds as it proceeds through overtemp check heaters and signal smoke detector. Only run once at startup. The only place where the relay is activated when all conditions are safe.
- If all ok : green led on, red leds off, relay activated.
- Every LED blinks when executing that test
- start main loop :
* check overtemp heated bed and check sensor for minimum acceptable temperature
* check overtemp @ extruder fan and check sensor for minimum acceptable temperature
* check smoke
If one condition is true : green led off, according red led on, relay off otherwise repeat main loop check.
One could add a piezo beeper too of course.
Practical stuff :
- install a 2nd bed thermistor or use the one from the heater (Keenovo only?)
- put an extra thermistor between the fins of the heatsink between the hot end and the extruder. This one will sense when the fan is failing and the temperature of the heatsink is rising above a critical level causing plastic to melt in that area and eventually if the heatsink is mounted on a plastic support melting the support, causing the hot end to fall down on the printed plastic piece, causing that piece to melt and ignite, and ... Liquid plastic behaves like petrol when burning and most of us have a lot of plastic in and near the printer.
- install the smoke detector somewhere in the upper area of the printer
- Leds should be visible
- once activated, you will need to switch off/on, manually reset ... : only after checking and taking care of the cause
- Use of a mechanical relay as this requires a minimum power and status/condition is visible if the relay has a clear housing. It is reliable and proven technology.