In practical classes, and in some kinds of research, there is a need for a device that can display a target moving in a controlled manner, to evoke eye movements or other directed responses, without tying up an entire computer to do so. One obvious approach is to use a laser and voltage-driven mirror, but such systems are expensive and insufficiently robust to withstand much student use, and intrinsically limited in frequency response. What is needed is a compact device with no moving parts that can be controlled by an analogue signal, which in turn can be derived from a physiological recording system such as EPIC, or from a DAC connected to a USB or printer port.Whilst the new technologies using luminescent plastic appear promising, they are still expensive, the response-times can be long and the driving voltages needed to run them are difficult to generate.

Here we present a device using the very bright LEDs that are now available in micro-miniature, surface-mount form without individual diffusers or lenses (Infineon LSR976, 632 nm, 27mcd typical at the 8.5 mA used, 160° viewing angle). 128 such devices are arranged in a row, and individually driven with pulse-density modulation for control of brightness. Although the LEDs are close-packed (1.5 mm pitch), if the selector outputs were simply used to activate single LEDs the movement would appear jerky at slow scanning rates. For this reason, a high-frequency signal of small amplitude is added to the driving voltage, its waveform being such as to generate an approximately gaussian blur across a small number of the LEDs. The resultant voltage is digitised to 7 bits, the result then being sent to a 1/128 selector driving the LEDs. When the display is viewed through a mild diffuser, what is seen is a small spot of light, admittedly lacking sharp edges, but appearing to move smoothly even at the lowest velocities. The frequency response is absolutely flat in the range required (10 Hz at most), with complete linearity in terms of transverse distance.