Product: Zebra Light Pen
From: Zebra Systems
78-06 Jamaica Ave.
Woodhaven, NY 11421
212/296-2385
Price: $24.95 + $3 P&H
Light pens resemble simple wands you point at your TV display to perform many functions usually invoked with keystrokes. The pens can sense light and dark screen areas and use the brightness it “sees” to help you choose the information or area you want.
Typical light pen uses include menu selection (just point the pen at your choice) and graphics development (tracing complicated line drawings onscreen). Other uses include game playing (selecting objects in an adventure game), and such esoteric attempts as simple photocell operation (sensing light or dark in a room, for example) and measurement functions (sensing intermittent motion).
Zebra‘s pen consists of a wand about 6″ long with photocell assembly. It plugs into a 2″x2″ open board containing input jacks for the pen and ZX/TS 9V power supply, output plugs to power your computer and send the pen’s signal to the EAR port, and hardware to condition the pen’s output. The hardware includes an LM339 Quad Comparator hooked up in four separate modes: comparator, pulse stretcher, oscillator and output driver. Using a simple LM339 for all these functions deserves mention as an outstanding design achievement.
Documentation seems brief, but complete except for lack of schematics. A cassette with a simple adjustment procedure on one side and a fairly involved “Concentration” game on the other provides you with ready-to-run software and some excellent programming examples. Zebra provides an assembly listing of the light pen’s input routine (17 bytes). You must address the pen as port FE (hex) and plug it into the EAR jack after you LOAD your user program. The computer actually reads only one bit.
I set up my pen without difficulty and found it worked adequately, though slowly for choosing squares in the Concentration game and for menu selection. You must place the pen tip flat against the screen to get a reliable read.
Complaints: The computer’s 9V power feeds through the unit–a problem with homemade keyboards with certain types of internal power supplies. You must disconnect the plug you used to LOAD the program and plug the pen output into your EAR jack–an inconvenience. My adjustment program contained a minor mistake.
Despite these flaws, Zebra’s light pen presents a reasonable buy, especially if you need a moderately reliable light pen or just want to experiment. With large enough blinking blocks, the physically handicapped could use this pen. People unfamiliar with computer keyboards could also use it for applications programs. With its low resolution, I see no valid graphics development for the Zebra.