An improved circuit for control of LED luminance

Abstract

The light-emitting diode (LED) is becoming increasingly popular as a luminance source in behavioral research. We describe a simple, inexpensive LED driver circuit that produces luminances almost linearly proportional to an input voltage. Negative feedback provides a stable output. Unlike previous designs, the LED is ground referenced. compared by Al with the output of A3 (= VI), and the difference causes the transistor current to change until the voltage drop across R equals VI. This maintains the relationship 10 = V1/R. Thus, LED current is kept proportional to input voltage, with a constant of proportionality equal to R. Although LED luminance is virtually independent of changes in most elements of the circuit, it can vary with changes in R. For this reason, we recommend using a resistor with low temperature coefficient. Unwanted oscillation in the feedback loop is suppressed by adding the RC circuit across A2. The capacitor reduces the gain of A2 at high frequencies, preventing oscillations from building up. However, a residual oscillatory component of 10 is present. Its peak-to-peak amplitude increases from about 1.0 rnA at low values of 10 to about 2.3 rnA when 10 =20 rnA. The oscillation frequency is 25 kHz, so it can have no effect on visual responses. The oscillation remains when the

The threshold-versus-duration curve is nonmonotonic

“…A large uniform field of moderately high photopic intensity was chosen in order to maximize temporal sensitivity. To produce flashes, a highly linear driver circuit (Beaty & Corwin, 1983) supplied current pulses to a single green light-emitting diode (LED). The subjects viewed the LED centrally through a magnifying eyepiece, which produced a uniform circular field 15 deg in diameter.…”