The 321 is one of several of Don's light modules used for projector control.  This panel is a white solder-masked PCB.

This portion of a catalog describes the 350 and these related modules.

The 321 Cross Fader was used for projector control at the Electric Circus. Tony Martin was associated with the San Francisco Tape Music Center and the Electric Circus in New York. Interview with Tony Martin, Liquid Light Artist and Painter details some of his history there. It's worth the read.

My position at the Electric Circus came about from an invitation to come over and tell them what I was about. There wasn't a need for a big introduction. They already knew what I was able to do with projection equipment from the Fillmore West. Stan and Jerry went out to the Bay Area to look at the light show. I didn't meet them there, but they met my associate Bill Sward, somehow, and he said, "Oh, like an electric circus!" - he may be the origin of that name. 

The projection component was substantial. It wasn't derivative of what Andy Warhol had done previously, at the same location as the Electric Circus. It was new, based upon Mort's conception. The idea of a "circus" was something I liked - the notion of a circus.

The module provides control for two projectors. The projectors, typically Kodak AV-900, had to be modified to allow for a separate mains power for the lamp. The 321 provides outputs for advancing (for'd) and reversing (back) of the projectors. The intensity outputs would control a 311 or 320 for SCR brightness of the projector lamps.

The 321 provides automatic crossfading and forward/reverse control of the projectors. Forward, reverse, or switch could be controlled by front panel buttons or external pulse inputs. The cross-fade time could be set by a front panel control or external CV. The 321 could be set to self-cycle or to operate with the switch, forward, or backward switches.

Construction

The 321 is constructed on a single PCB with front panel wiring. This rear of the PCB panel is tinned so is very reflective and doesn't photograph very well.

All the PCB traces are on the rear. The circuit consists of multiple multi-vibrators to retain the state of the projectors and the intensity. You can see the two lamp/CdS cells covered in heat shrink.

This module uses 15V only.

Operation

This scope image shows the projector 1 controls of intensity (yellow) and forward (cyan) and projector 2 intensity (magenta) and forward (green). The 321 is set to self-cycle from projector 2 to 1 with a slightly longer period than the fade time. You can see the intensity of projector 1 increase while the intensity of projector 2 decreases. When projector 2 is nearly off, it is advanced to the next slide. Then you can see the opposite cycle, with projector 1 decreasing and projector 2 increasing and projector 1 advancing when off. The increase and decrease have slightly different durations as these are controlled by two incandescent lamp/CdS cells. The lamps have just a slightly different intensity at the same current. Adjustment of the alignment of these DIY cells would probably correct this.

This scope image shows using the forward switch to advance from projector 1 to projector 2. Note that projector 2 was on the correct slide so no advance pulse is needed while projector 1 is advanced at the end of the cycle.

This scope image shows using the forward switch to advance from projector 2 to projector 1. Note that projector 1 was on the correct slide so no advance pulse is needed while projector 2 is advanced at the end of the cycle.

This scope image shows the CV control of the fade time. The lamp intensity is controlled by a very simple Sziklai Darlington circuit. The Sziklai Darlington uses a NPN/PNP pair to decrease the forward bias to 0.6V. Note that the current is exponential so the fade time is very non-linear.

This scope image just magnifies the CV control so you can better see the active control portion of the CV input.

back