Step 1: LED math and driver circuit
Now, let’s see how to make display letters with LEDs. The LED connections for the first character of the signboard (MERRY XMAS) is shown above. 17 LEDs are used in creating 'M'. If you have to drive each LED through a 5V supply, you require 17 series resistors, and the current will sum up to 17×15 = 255 mA. If you add up the current requirements of other LED characters in the signboard, the net current would go up to 2 A, which is quite a bit of current and you probably need a bigger heat sink for your voltage regulator. So I thought of doing it differently that would lower the net current consumption in the project and also save me from soldering to many resistors and transistors. This can be done by using a higher supply voltage.
I used a power supply unit from one of my old printers that provides +32 V, +16V, and ground to its 3 output pins. The +32 V is used to drive the chain of LEDs connected in series, and +16V goes to an LM78L05 IC to derive +5V regulated output for the control circuit. By doing so the net current required to drive all the LEDs in a chain is same as required for a single LED as they are in series, and therefore, only 1 resistor is required per chain.
The picture above shows how I constructed the character 'M'. With +32 V, I can only drive up to 16 red LEDs in series, and the letter ‘M’ in the sign consists of 17 LEDs. So I have to divide it into two chains of 9 and 8 LEDs, as shown here. For the first chain, the value of the series resistor would be,
R1 = (32.0 – 1.95 x 9) V/15 mA = 963 Ω.
I used 1 K for this. Similarly, for the second chain of 8 LEDs, the estimated resistor value is R2 = 1.1 K.
The anode terminal is connected to 32 V supply whereas the cathode is connected to the collector of a NPN transistor (BC547). The transistor acts as a switch with a control signal applied to its base terminal through a resistor. Whenever the control signal is HIGH, the switching transistor is turned on and all the LEDs will glow to display ‘M’.
This whole process is repeated for constructing other LED characters in the signboard. The series resistor values are calculated in exactly the same way by considering the number of LEDs in each of the chains formed. The attached table shows the number of LEDs, number of chains, and the value of series resistor used for constructing each LED character.