LIGHT PEN MAKES MENU CHOICE DIFFICULT

Among Add-On Electronics hardware is a light pen for the Spectrum, a device which, when pointed at the TV screen, can indicate to the computer where the pen is pointing. Included in the software provided with the pen are 16 routines to allow you to draw pictures using the facility.

The light pen is in two parts, the pen and, second, a small box containing the electronics into which it is plugged. The Spectrum power supply is then plugged into the side of the box and another lead is plugged into the Spectrum power socket. Once the software tape has been loaded the other lead from the box is plugged into the Spectrum EAR socket and you can use the pen to run the program.

A television set works by moving an electron gun across the screen and where the electrons fall the phosphor on the screen will glow. To make up a picture the gun starts pointing at the top left of the screen and then moves down in a zig-zag fashion making, on a modern TV, 625 sweeps across the screen. It does that 25 times per second. If you know when the gun starts scanning the screen you can time how long it takes to reach any given point.

Inside the light pen is a BPW 148 photo-transistor with one leg cut off. When light falls on it a current passes through it. The software enables the computer to measure the time between the electron gun starting a new scan and the light pen passing a current. The time interval shows at what point on the screen the pen is being pointed.

Obviously the timing is critical and here the software fails to do its job. It was almost impossible to use the program provided as, when picking from the menu, the pen would be pointed at the chosen square, but the software assumed it was being pointed up to 2in. to the left, choosing another menu selection.

If it had been consistent, that problem could have been overcome but the error varied according to the area of screen selected and by a random element. The instructions provided with the pen give a short Basic program to allow you to adjust your set to the pen. That did not solve the problem. The error may be due to the television set used and may not occur on all sets. If possible, borrow a pen from a friend and try it before buying.

It is interesting to note that the instructions mention there being a fine control on the light pen box when there is none, either externally or internally, on the printed circuit board.

The only other light pen with that control is one made by DKTronics, which shares the same address as Add-On Electronics. To confuse the issue further the software cassette with the pen bears a copyright notice from a now defunct company, Kayde, and the picture on the light pen box is the same as that used in Kayde advertising.

Both the other light pens sold for £10 less than the Add-On Electronics version.

At £30 the pen can be obtained from Add-On Electronics, Units 2, 3 & 4, Shire Hill Industrial Estate, Saffron Walden, Essex CB11 3AQ.