Model 158 Sewing Machine Controller: AC Interface Circuitry

That polycarbonate slab holds most of the pieces in place, with the rest on the prototype board to the left of the monster heatsink:

Model 158 Controller - Interior Overview
Model 158 Controller – Interior Overview

That bulky wire harness got bent out of the way for the photo; normally, it’s jammed down beside the ATX power supply and over the blower.

The AC Interface circuitry looks like this:

AC Power Interface
AC Power Interface

The relay on the top disconnects the AC line from the circuitry when the clamshell case opens.

The key hardware spreads neatly across the middle: the optoisolator, a 2955 PNP power transistor in a TO-220 case on a heatsink as a current amplifier, and the ET227 controlling the motor current. The gain of that mess depends strongly on the transistor temperatures, so there’s not much point in calibrating it. More on that later.

Down at the bottom of the schematic is the slit toroid and knockoff SS49(E) Hall effect sensor that senses the actual motor current.

A closer look at that board:

HV Interface board - detail
HV Interface board – detail

The board in the bottom left corner of the overview picture holds the Arduino Pro Mini that runs the whole show (so far, anyway), along with various & sundry analog circuitry that I’ll write up in a bit.

Conspicuous by their absence:

  • Motor speed sensing
  • Shaft position sensing
  • Power to the LED strip lights
  • Permanent mount for the pedal cable socket

Now I can make measurements without killing myself…