Electronics Workbench

AC Interface Board: Line Voltage Interlock

The relay at the top connects the AC hot line to the rest of the circuitry, with a feeble red LED to show when it’s live:

AC Power Interface
AC Power Interface

The driver lives on the Low Voltage Interface board:

LV Power Interface - AC Relay driver
LV Power Interface – AC Relay driver

The GX270’s front-panel hard drive LED now serves to indicate when the AC power goes live.

I’d originally intended to turn the AC on when the Arduino gains control, but after seeing those pictures, I think it’ll remain disabled unless there’s a call for motor motion.

The interlock switch closes when the case opens, grounding the transistor base and disconnecting the AC power.

Of course, you can cheat by simply unplugging the switch, so it’s not failsafe. If you want failsafe, you need a normally closed switch in series with the collector; that’s not what Dell used as a chassis intrusion switch. That’s my story and I’m sticking with it.

8 thoughts on “AC Interface Board: Line Voltage Interlock”

  1. I told Julie about the accident you mentioned, and after wincing, she had a suggestion. She uses the worklight as a manual pilot light–if power is on, that light’s on. Maybe an indicator for when the motor is enabled would be useful, especially if you have some fancy cycles like needle up or needle down…

    1. an indicator for when the motor is enabled

      Right now, Mary’s machine and all the lights come from a power strip, so if she can see, the machine’s ready to run. Which is, by and large, the entire time she’s sitting there.

      We’ve seen machines with state indicators on the arm just over the needle and, with a bit of effort, I could put LEDs in the endcap along with the needle floodlight. But, of course, everybody ignores indicators that appear all the time; an indicator showing that the motor is running doesn’t provide any useful information.

      Turning the floodlight off when the machine stops is not an option! [grin]

      Just watching Mary’s fingers while she’s sewing puckers me right up: to make those shapes, she must get really, really close to that needle! An industrial light curtain interlock definitely won’t work for quilters…

      1. I’ve learned a certain amount of paranoia respect for power equipment, and the sewing machines spook me a bit. Something to do with a couple of chainsaw “events” over the years. [wince] Julie quilts a fair amount, patchwork mostly, and without serious trauma, so it’s all good so far. She’s going to try freeform now, so it’ll be interesting.

        I still have all my fingers, and the scars have faded…

        1. chainsaw “events”

          A friend gave me his little chainsaw and I used it a few times. When it didn’t start any more, I put it on the back of a shelf and there it remains to this day.

          If the branch is too big for the bow saw, it doesn’t get cut.

          1. The branches get the little chainsaw. [grin] I use firewood in the shop, and I’d rather use saws powered by hydrocarbons than by carbohydrates. (A big tree for me runs 30-36″ in the trunk. Small, 6 to 12″). Most of what gets cut is medium to big.

  2. The linked photo makes me think there’s a market in adapting this gadget to sewing machines…

    1. We obviously need a pyrotechnic needle stopper: touch the needle and blam the machine’s arm flies away!

      I suspect that gadget appears in high-end saws destined for industrial settings, where liability spreads its tentacles. In a home shop, you don’t cut your fingers off enough to justify the expense. [sigh]

      I take exaggerated precautions every time I fire up the radial arm saw and, so far, so good…

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