How to minimize the number of lines to drive a multiplexed four 7-segment digit display

L.A.A.R. Laboratory (DTL team 2), Faculty of Physics,
University of Sciences and Technology - Oran,

B.P. 1505, El M’nouar, ORAN, ALGERIA, phone: (213) 041.42.15.81

Briefly, in order to drive a multiplexed four 7-segment digits display, we need 7 + 4 = 11 lines (asuming the decimal point is not used) as shown in figure 1. The references [1] [2] [3] may be useful to be read for full and detailed explanations. Previous EDN design ideas [4] [5] [6] show solutions to minimize the number of lines, to be able to use pin-limited microcontrollers. But those solutions require, in general, one (or more) additionnal digital integrated circuit(s), which increase components count, PCB footprint and cost.

This article shows a solution to reduce the number of 11 lines downto 8 lines only, keeping the same number of components count. Figure 2 shows that mixing common-anode and common-cathode type, with PNP and NPN bipolar transistors require 7 + 2 = 9 lines only. Caution, this solution involves the use of three-state I/O lines to light the segments, and the pair of totem-pole transistors must be driven in opposite logic levels.

This last requirement allows more optimization, shown in figure 3, and reduces the final count to 8 lines. Here is a little bit more explanation for this last figure: For instance, in order to activate digit 1 only, and displaying the number 7, you should apply logic level “1” for line L8 (transistors T3 & T2 are ON, while T4 & T1 are OFF), and light the segments a, b & c, with the binary code 111 1000, but this yields to light also segments d to g in digit 4. To avoid this, simply apply outputs L1 = L2 = L3 = “0” while L4 to L7 are reconfigured to input state (i.e. high Z). Note that the transistors must have a low Vce(sat).