Power Indicator
Last Updated: 17th May 2009
The power of the HX40A cannot be controlled via NewlyDRAW and has to be set via a knob on the laser cutters control panel. The knob rotates up to 10 times so it's easy to record power settings from 0 to 10 in terms of the number of times you've rotated the knob. The down side it that when you come to the machine at the start of a session, the only way to know where it's set, is to turn the knob all the way to min or max. There's a gauge to show the current draw of the laser tube but this only shows anything while you're actually cutting and jumps about so much that you can't really use it to tell you where the knob is set.
I considered getting a vernier knob however the spindle on that pot is 1/8" diameter and most of the vernier knobs are for 1/4" spindles. I did find one on a website in the USA but those knobs ain't cheap and by the time you've added shipping for the US...
Behind this knob I observed that there was a rheostat with 3 wires. I measured the voltage between the yellow and blue wires and found that it varies from 0.42 in the full off position to 3.49v when full on. Thus it occurred to me that I could make a circuit to measure this and display the number of times that the knob is rotated.
The schematic above is based on one from the LM3914 data sheet. It is still a work in progress and,
I'm not going to give a detailed explanation of how it works, or how to build and install it. If you don't know enough about this stuff to work that out yourself, you really shouldn't be messing about with this. What I will publish here are a few notes that will help those who do to get the job done quicker. If you don't understand it, find a friend who does or leave well alone because:
These kind of adjustments are likely to invalidate any warranties you have and in a worst case scenario, if you get something wrong, you could damage your machine. The information I am publishing here is correct to the best of my knowledge but I accept no liability with regards to it.
Having clarified that, on with the notes:
As stated, the circuit is based on one from the LM3914 data sheet and was modified with the assistance of my friend Chris Gosling (who knows more about this stuff than me).
Set up of the circuit is as follows:
Start with R1, R2 and R4 in mid positions.
Turn the knob on the control panel through 5 turns and adjust R2 to set the brightness of the LEDs.
Turn the knob on the control panel to just 1 turn and adjust R4 so that the 1st LED is just lights.
Turn the knob on the control panel to full and adjust R1 so that the 10th LED just comes on.
The LEDs will now indicate the position of the knob such that.
0 LEDs lit = 0-1 rotations
1 LEDs lit = 1-2 rotations
2 LEDs lit = 2-3 rotations
3 LEDs lit = 3-4 rotations
4 LEDs lit = 4-5 rotations
5 LEDs lit = 5-6 rotations
6 LEDs lit = 6-7 rotations
7 LEDs lit = 7-8 rotations
8 LEDs lit = 8-9 rotations
9 LEDs lit = 9-0 rotations
10 = power is at max
Other things to note about the circuit are that pin 9 of the LM3914 is left floating thus making the chip work in dot mode. Connect it to V+ if you want bar mode.
Chris suggested a couple of 10M resistors for the divider on the input however the highest I had in stock were 1M; and they seem fine.
Chris also suggest that R3 should be 510R but I found that it gave a better range on R4 by reducing it to 470R. This is probably due to tolerances and Chris tells me he did his calculations actually called for 500R.
My first thought with the circuit was that I would tap into the 5v supply on the power board however measuring between the ground on that board and the wires on the pot shows a massive 120v difference between them! Fortunately there is a suitable supply available from the control panel itself.
The signal voltage is of course taken from the blue wire attached to the power pot (see photo above) and a ground for the circuit can be taken from the yellow wire. 5v can be obtained from one side of the switched labelled "HX40A" on the front panel i.e. the one you have to press in conjunction with the "Laser Test" button when test firing the laser.
Current State of Play
I have the circuit built up on a breadboard and sitting on top of the machine. It works fine and is saving me a lot of knob twiddling. I intend to build it properly and install it into the machine however:
The presets don't work as nicely as we anticipated and the initial set up took a little more twiddling than the procedure outlined above. In particular R2 has more of an effect on the high end setting of the scale than it does on the brightness of the LEDs. I am thinking that some further playing about with that part of the circuit might make things simpler.
I found some information about converting these machines over to using Mach 3 software, with a couple of new controllers. Amongst other things this would allow the software to control the laser power and would of course make the power know, and my indicator, redundant. More information the conversion to CNC can be found here.