A few articles have recently appeared describing novel ways to increase the number of LEDs a microprocessor can drive with a limited number of pins (Reference 1). The standard multiplexing technique made popular by multidigit seven-segment displays has, in pin-scarce designs, given way to “Charlieplexing.”

Charlie Allen devised this technique while working at Maxim, which has since introduced ICs using the technique (Reference 2). Allen used the high-impedance third input state available to most microprocessors for turning off LEDs in a matrix; the LEDs’ respective microprocessor pins’ high or low states individually turn on these LEDs. Using this method, you can drive nine seven-segment LED displays using only nine microprocessor pins rather than the usual 17. For N pins, you can individually address N×(N–1) LEDs using Charlieplexing.

One of the gripes people often level at Charlieplexing regards its poor duty cycle. A previous Design Idea compares the standard multiplexing method with Charlieplexing (Reference 3). Using Charlieplexing, the maximum duty cycle for a 20-LED display is only 5%. The poor duty-cycle figure is due not to the method, however, but rather to the driving capability of the microprocessor and the parasitic-leakage paths. A single pin cannot usually sink the current a number of LEDs require to effectively light up, so these designs often require one source pin and one sink pin to light only one LED at any time. However, adding a transistor or two resistors allows you to circumvent these issues.

If you rearrange the LEDs in the familiar cross-point array and add a transistor to each column to carry the common current, you’ll see the duty cycle of the Charlieplexing method does not differ much from standard multiplexing (Figure 1). For a 20-LED, five-column matrix, each LED remains on for 20% of the time compared with 25% for standard multiplexing, but now using only five pins instead of nine (Table 1).

One of the drawbacks of adding the transistor and resistors to each column is that you need additional components to achieve a reasonable LED brightness when a large number of LEDs is involved. This approach, however, is a better alternative to using a costly IC and no worse than standard multiplexing or “Gugaplexing,” which also requires additional transistors and resistors. From a cost and benefits point of view, consider that, by the time you get to 90 LEDs, the PCB (printed-circuit-board) real estate and cost of the additional 10 transistor/resistor sets pale in comparison to the display itself.

Examining the circuit in detail, you’ll notice that it has five microprocessor pins, P₁ through P₅, available, for a total of N×(N–1)=20 LEDs. When P₃, for example, is high, the emitter of Q₃ is at approximately 4.4V, and you can turn off D₁₃, D₂₃, D₄₃, or D₅₃ if you make P₁, P₂, P₃, or P₅ low. Any pin that you set to input, or high impedance, alternatively turns off the corresponding LED. When P₁ and P₄ are low, P₃ is high, and P₂ and P₅ are in high-impedance states. With P₃ high, transistor Q₃ biases on, all the other transistor bases are either low, which ensures that no current will flow, or high-impedance, which supplies no current into the base to allow the transistor to conduct. All the diodes in the third column can turn on, but only D₁₃ and D₄₃ have a path directly to ground through P₁ and P₄, which are low and through the 100Ω current-limiting resistors.

D₂₃ and D₅₃ connect to the high-impedance input pins and can conduct only through the 100Ω resistors attempting to turn on Q₂ and Q₅. Because of their forward-voltage drop—typically, 2.2V—the emitters of Q₂ and Q₅ will be less than 1.6V, as the following equation shows: 5V_CC–0.6V (Q₃)–2.2V (D₂₃ or D₅₃)–0.6V (Q₃ or Q₅)–I_LED×100Ω<1.6V, where I_LED is the current of the LEDs. This scenario does not allow any LED in Column 2 or Column 5 to light up to any level that would have an undesirable effect.