Max sink current for Laser-Fire pin?

Its a 2018-era laser mini board running Cluster firmware.

Hi Ray, asked this on twitter, but this might be a better way to reach you. Hope all is well in these weird times.

What’s the maximum sink current that the laser-fire pin can handle? I want to add a laser-fire indicator that will draw an additional 12-15ma. While I intend to put the indicator on the test-fire line, my PSU seems to cross connect them, so pulling low on laser-fire will also pull the test-fire pin low… but I don’t want to overload the laser-fire pin when that pin’s doing the pulling-low.


Keep in mind the “laser fire” pins are going to be PWMed, so your indicator might not work the way you expect it to (unless it’s an LED or something, and even then it might not light at low duty cycles).

Good point. I’m just replicating the LED on my stock panel, which probably suffers from the same issue. Just mine new indicator draws 3 times more current than the stock LED.

I could get more complicated but I guess, but that’s for the next iteration of the panel.

Hi Jake,

Thanks for your concern. I am alive, busy, and I’ve been told that the sanity ship sailed a long time ago, so what’s a little more chaos, right? Hope you are well also.

On the Mini, if you are using pin 2.5 for laser fire, that’s through a fairly hefty Mosfet whose capability is measured in tens of amps, not milliamps, so you’re fine there (these are 2.5 and 2.7 on the bottom left, I believe). The small FET 3 and 4 in the upper right should be fine for at least 100mA, surely.

What you want is laser_module_ttl_pin and define one of those other FET pins to drive it, possibly 2.7 since that’s a big FET. Don’t use 2.5 because that is what is driving the laser fire PWM.

I am not actively on social media and do not check it regularly - so official channels like this forum are the proper way to make support inquiries, where my staff and I can help.

The right way to do the indicator off a PWM source is to feed an op-amp buffer, low pass filter, and comparator to fire the LED. I have a schematic of what I think might work in my head. but i’d need to play with it a bit.

The other way, if you don’t care about the brightness/duty cycle limitation, is to build a gate driver and use a mosfet as a switch controlled by the ‘alternate 5v pwm’ line to drive the LED from the power supply’s 24V rail to avoid loading the 5v bus.

Or just use the firmware indicator switch as I described above :slight_smile:

(We were both typing replies at the same time)

yeah, mapping an unused pin to that function is a way easier solution. I forgot we were dealing with a mini here and not the regular laserboard :slight_smile:

Ray, sounds like things are “situation normal” for this pandemic, same as here. At least it’s given me some time to tinker with things again and I can finish this K40 upgrade which has been sitting for so long.

Thanks for the suggestions. Yeah, I’m using the default laser_module_pin=2.5 on my Laser Mini.

My idea was just to put the indicator on K- (or K+, whichever one you pull low to test-fire, I can’t recall) On my LPSU, this also gets pulled low by the laser-fire pin. This way, the indicator would light whether pressing test-fire or when the controller pulls laser-fire low with sufficient duty-cycle. This basically emulates the current LED on the PowerLED control panel.

Seems like 2.5 can handle it, so I might do this for a “minimum viable product”. Later, I can loop it through the controller later or perhaps a little circuit like loather is thinking. Or maybe I’ll look into the smoothieware “switch” module and handle the test fire and indicator both through the controller.

Thanks again.

Thanks again. Everything works as expected. Green LED lights when the interlock circuit is completed and the red light lights when laser_fire from the button or the C3D Mini.

I think I’m going to switch the 5v supply for the the LEDs from to the C3D Mini’s 5v regulator rather then the LPS 5v supply, as I notice voltage dips (0.1-0.2v) depending the whether the green LED is lit.

Just need to print a button cap for the back button.

Yeah, the power rails on the LPSU are notoriously weak, especially the 5V rail. Are you using a current limiting resistor on that LED?

They seem to be current limited internally. The red draws about 8ma and the green about 12ma without any external current limiting.

Sorry to bump this thread, but I wanted to share something I learned.

As I mentioned, when the P- pin is grounded by my button, the green LED lights up on my panel. At the same time, the voltage drops to the potentiometer.

I assigned this dip to the draw of the green LED. However, I decided to power the high-side of the LED from the C3D Mini, to not tax the internal 5v regulator on the LPS.

What I learned:

  1. This didn’t make a difference. I still saw the voltage dips. In fact, just the act of grounding the P- with no LEDs or anything still resulted in a voltage dip of 0.1-0.2v on the LPS power supply. I guess whatever P- is enabling inside the LPS draws enough to cause a voltage dip.

  2. I noticed a blip on the current meter during start up and shutdown. I’m not sure the mechanism, but my guess is that the C3D Mini’s supply has a different startup/shutdown voltage curve than the LPS and somehow causes both a fire and a P- interlock signal to be asserted during startup and shutdown.

So I went back to a using the LPS 5v to power my LEDs, voltmeter, and the little temperature gauge. Using the 5v supply on the C3D Mini didn’t fix my voltage dip problem anyway, and added a weird, unsafe condition to startup and shutdown.

I could investigate both behaviors more, but I don’t really care to spend any more time.

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