Circuit arrangement for activating a fluorescent display

摘要

If a fluorescent display and a keyboard are intended to be multiplexed in parallel by a PMOS microprocessor, difficulties arise in the case of relatively large displays since they require considerable current, which leads to a voltage drop at the control output of the microprocessor so that, when a keyboard key is pressed, the signal amplitude at the associated microprocessor input is no longer adequate. The invention therefore proposes to provide a transistor (13 in Figure 1) between the cathode of the fluorescent display and the cathode supply voltage, which transistor disconnects the cathode of the fluorescent display from its supply voltage during a short monitoring period for the keyboard. The display thereby requires no current during this period and the full open-circuit (no-load) output voltage of the control output is available for the keyboard. The application indicates two embodiments for activating the transistor. <IMAGE>