“Charlieplexing” as a method of multiplexing LED displays has recently attracted a lot of attention because it allows you, with N I/Olines, to control N×(N–1) LEDs (reference 1 through reference 5). On the other hand, the standard multiplexing technique manages to control far fewer LEDs. Table 1 lists the number of LEDs that you can control using Charlieplexing and standard multiplexing by splitting the available number of N I/O lines into a suitable number of rows and columns. Table 1 also shows the duty cycle of the current that flows through the LEDs when they are on.

Clearly, Charlieplexing allows you to control a much larger number of LEDs with a given number of I/O lines. However, the downside of this technique is the reduced duty cycle of the current that flows through the LEDs; thus, to maintain a given brightness, the peak current through the LEDs must increase proportionately. This current can quickly reach the peak-current limit of the LED. Nonetheless, Charlieplexing is a feasible technique for as many as 10 I/O lines, allowing you to control as many as 90 LEDs. To control an equivalent number of LEDs using the standard multiplexing technique would require 19 I/O lines.

This Design Idea proposes a modification to the Charlieplexing technique that allows you to control twice as many LEDs. Thus, the proposed method, “GuGaplexing,” allows 2×N×(N–1) LEDs using only N I/O lines and a few additional discrete components (Figure 1). To turn on LED D₁ using the Charlieplexing method, set P₁ to logic one and P₂ to logic zero. To turn on LED D₂, set P₁ to logic zero and P₂ to logic one. Figure 2 shows the proposed GuGaplexing scheme with two I/O lines controlling four LEDs. The GuGaplexing technique exploits the fact that each I/O line has three states: one, zero, and high impedance. Thus, with two I/O lines, states 00, 01, 10, and 11 of eight possible states control the LEDs.

Table 2 lists the voltage at the output of the transistor pair for various states of the two I/O lines, P₁ and P₂. The transistor pair comprises a BC547 NPN and a BC557 PNP transistor; matched transistor pairs are recommended. For N I/O lines, the GuGaplexing technique requires N–1 transistor pairs. Table 3 shows the state of the I/O lines P₁ and P₂ and the voltage at node PR₁ to control the four LEDs. The circuit requires that the LED turn-on voltage should be slightly more than V_CC/2. Thus, for red LEDs with a turn-on voltage of approximately 1.8V, a suitable supply voltage is 2.4V. Similarly, for blue or white LEDs, you can use a 5V supply voltage. Modern microcontrollers, especially the AVR series of microcontrollers from Atmel, operate at a wide variety of supply voltages ranging from 1.8 to 5.5V, and this design uses a Tiny13 microcontroller to implement the GuGaplexing technique.

Figure 3 plots the voltage at node PR₁ for various supply-voltage values when the input to the transistor pair is floating. The Spice simulation ensures that the circuit would work properly to provide V_CC/2 at the PR₁ node for wide operating-supply-voltage values when the input is floating.

A 24-LED bar display validates the scheme in a real application (Figure 4). The display is programmable and uses a linear-display scheme for the input analog voltage. The input analog voltage displays in discrete steps on the 24-LED display. Controlling 24 LEDs requires only four I/O lines and three pairs of transistors. The system uses 5-mm, white LEDs in transparent packaging and a 5V supply voltage. The GuGaplexing implementation uses an AVR ATTiny13 microcontroller. The analog input voltage connects to Pin 7 of the ADC input of the Tiny13 microcontroller.

The control program for the ATTiny13 microcontroller is available. The source code is in C and was compiled using the AVRGCC freeware compiler. You can modify the source code to display only one range of input voltage between 0 and 5V. For example, it is possible to have a linear-display range of 1 to 3V or a logarithmic scale for input voltage of 2 to 3V.