How to Control A 4-Digit 7-Segment LED Display Using Phidgets

Phidget user hammerstan set out to create a 5-digit display using a single multiplexed 7-segment LED display and minimal electronic components. Without having an extensive electronics background, he’s found using an IC/chips based method overly complex. So, he wired a 5-digit 7-segment LED directly to a Phidget LED 64 board, via 5 PNP transistors. The same technique can be used to program 2, 3 or 4 displays (single displays don’t need the transistor and can be wired directly to the Phidget board)

Using his transistor method significantly reduces the number of Phidget slots for a 5-digit display versus traditional method of wiring a collection of single digit LEDs, each digit using up to 8 Phidget slots.

What You Need:

How to Control a 7-Segment  Multiplexed Display with Phidgets

Phidget LED 64 board (with power supply and USB cable)
a common anode multiplexed 7-segment LED (up to 5-digits)
• 5x PNP transistors (for instance, Fairchild PN 3638A)
• Example Code

[Circuit Diagram Image]

How To:

The Phidget LED 64 has two common anodes: One for the even numbered connector slots (ie 0 to 62), and one for the odd numbered connector slots (1 to 63). For these instructions, we’ll be using the anode rail for the even numbered slots.

1. Connect cathodes A to G and the decimal point (DP) on the 7-segment multiplexed LED display to 8 black wires (cathodes) connected into the even sockets on the Phidget board. So, it will be connected like so:

LED Display Cathode – Phidget Connector Slot (black wire only)

  • A – 0
  • B – 2
  • C – 4
  • D – 6
  • E – 8
  • F – 10
  • G – 12
  • DP – 14

How to Control a 7-Segment  Multiplexed Display with Phidgets

The red (anode) Phidget wire is not used. Turning any of these Phidget slots on will control each of the segments for the active digit (the active digit is controlled via step 3 below).

2. Connect the common anodes for digit 1 on the 7 segment display to the collector of a transistor (that is right leg while holding the flat face of the transistor away from you). Repeat for the remaining digits.

How to Control a 7-Segment  Multiplexed Display with Phidgets

3. Using the next available even-numbered Phidget connector slots, wire the black AND red wires from each Phidget slot to the other 2 legs on the transistor. The red wire (common anode) connects to the emitter leg (the left leg while holding the flat face of the transistor away from you). The black wire (cathode) connects to the base (the centre leg).

How to Control a 7-Segment  Multiplexed Display with Phidgets

Now, it should be mapped like so:

LED Display Common Anode – Phidget Connector Slot (black & red wire)

  • Digit 1 – 16
  • Digit 2 – 18
  • Digit 3 – 20
  • Digit 4 – 22
  • Digit 5 – 24

Turning any of these Phidget slots on will activate the respective digit.

How to Control a 7-Segment  Multiplexed Display with Phidgets

4. This set up wires 4 digits and punctuation, or 40 individual LEDs using only 12 Phidget connector slots. The alternative would be to use 40 connector slots. The drawback is that only one digit can be displayed at a time, necessitating a small interval (<30ms) between illuminating each digit – Therefore, the program must cycle through activating Digits 1 – 5 rapidly for the eye to see all 5 digits active. Download and compile the sample program written in C++ and work from that to develop your project.

These instructions were originally written by Phidget user hammerstan, as posted in the Phidgets Phorums.

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Math lover. Engineering communicator. Mad-lib enthusiast. Total nerd.

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