universal display

7.84 elektor july/august 1980 90 The LED display described here can be used in a variety of applications ranging from tachometers to audio modulation meters. It consists of two rows of twelve LEDs and is controlled by means of three logic signals and one analogue signal. The three logic signals determine which row of LEDs is enable and whether a thermometer type scale or a spot scale (a single LED illuminated) is used. The analogue input signal is continually compared to a number of reference voltages derived from the supply. When the input signal level is higher than a particular reference voltage the corresponding opamp output will go high. One of the LEDs connected to the opamp will then light provided T14 or T15 is conducting. It is important to ensure that these two transistors do not conduct at the same time as then excessive current would be drawn from the opamps. If, for instance, the input voltage is higher than the reference level at pin 9 of K6, the outputs of K1 ... K6...