I discovered a couple of weeks ago that the 2.4 MOSFET is grounded (I was writing an article for Ariel Yahni’s Blog on different ways of triggering the Laser and had one of thode “doc brown” on the toilet moments)
The K40 PSU has the positive side of it’s optical isolator tied to +5v internally, so all you have to do to fire the laser is pull it to ground. The current to do this is very low since the +5v has already satisfied the Optoisolator LEDs forward drop requirement. (I did it with a wet finger once just to see how sensitive it really is - it fires)
If you hook to the - pin of PW0. (it’s the drain of Q8), Q8 pulls it to ground, firing the laser. The gate of Q8 is already tied to pin 2.4 internally.
The downside of this is the lack of a positive shutdown. There’s nothing preventing the pin from going low and firing the laser. That’s just not safe.
It’s works, but it’s not considered a good engineering practice beacause of this.
With safety such a serious issue, and the fact that the Smoothieboard leaves the line uncontrolled during it’s boot period, I’m not comfortable telling people to do something you’re taught in engineering school isn’t a safe design practice. Leaving a logic input floating is a “Never do this” example that’s even shown in the TI TTL Data Book.
You can improve the safety level by using a nice low value pullup resistor on the line. (the FET can handle a bit of current no problem)
With a good quality level shifter coming in at under 2 bucks, and the few wires needed to run it, I think it’s a safer option.
While the FET hookup works, the potential for an unexpected discharge exists and considering the low quality of the K40 PSU and controls, I just can’t feel ok with recommending a circuit that adds the the risk of someone getting hit by a laser or shocked by a power supply they thought was off. It’s just not worth the 2 or 3 bucks and 5 minutes of wiring you save to do it right.
If you use this method, be aware, it’s akin to leaving out the neutral safety switch on your family car. While it might not happen right away, sooner or later somebody who doesn’t even know what a neutral safety switch is will start the car without pushing in the clutch and run into or over something.
It’s just not worth the risk.
In interest of full disclosure, I do sell products that compete with this arrangement, but have never witheld information or assistance from anyone ‘rolling their own’, and in fact rather enjoy helping people get their lasers running. I’ve been a hobbist since before I was 12 and have been building homemade stuff since. (I was granted a General License at 13) and worked at a TV repair shop by 14.
I truly support the do-it yourselfers, but don’t want someone to assemble a hazardous device because they saw it done on the internet.
I recently started making a plug in level shifter which uses an optiisolator and MOSFET to link the Smoothieboard to the K40PSU. It’s self-powered, offers 4500v of isolation, and is plug and play.
This device (and my new ACR Mark II) use positive logic, pulling the K40 PSU to +5V anytime the signal is not positively controlled (Like during the boot period of the Smoothieboard)
It requires that you remove the ! from the Pin designator in the Config file, but it’s worth it.
The design requires power to fire, so is fail-safe, unlike negative fire systems (including th stock K40 arrangement)
I’ll send anyone who asks for it a drawing on my shifter and a copy of the EZ~Config V6.0 which has the change implimented. Just send me your email.
My level shifters are also available as a kit. (they’ve been delayed slightly because I screwed up the board design by working when I was too tired to pay attention)
Please be careful out there, I never want to come onto our forum to find someone has been blinded or worse.
Scott