Measuring symbol driving circuit for a digital multimeter

摘要

A measuring symbol driving circuit drives the measurement symbols on a liquid crystal display in a digital multimeter. This measuring symbol driving circuit includes a rotary measuring range changeover switch which selects the respective measuring symbols to be displayed. A control signal generating circuit receives the backplane pulse signal supplied to the common electrode of the liquid crystal display, and phase inverts this signal through the use of an exclusive OR gate in order to develop a control signal supplied to the measuring symbol selected by the measuring range changeover switch. Further, the driving circuit includes a circuit for monitoring the presence of a short circuit between the input jack and common electrode of the digital multimeter to selectively display a short circuit indicator symbol. This input comparing circuit receives the backplane pulse signal at one terminal of an exclusive OR gate and supplies the other terminal of an exclusive OR gate the output of a comparator which is logically high when a short circuit is detected in order to reverse the phase of the backplane signal to produce a control signal for application to the display symbol, and supply the logical low signal to the input of the exclusive OR gate to apply a signal equivalent to the backplane pulse signal to the control terminal of the short circuit symbol to maintain this symbol in a non-displayed state.