Here I am sharing a LED sign board schematic diagram which is simple to build. This can be easily be made on a breadboard. The LEDs are connected in parallel. That is each LED anode is interconnected and then connected to the +ve terminal of the battery. Each LED cathode are also interconnected together and then connected to the -ve terminal of the battery. Here I have used LED with rating of 2.2V forward voltage and 10mA forward current. So I have used a 3.7V rechargeable battery which is connected in parallel to all the LEDs. However I have to mention that there should be a current limiting resistor connected between the +ve of the battery and the anode. This would create equal brightness on all LEDs and ensure that each LED only consumes 10mA because of the current limiting resistor.
The parallel connected LEDs are as shown in the circuit diagram below.
Below are the calculations for a parallel-connected LED signboard with 75 LEDs using the given specifications and a 3.7V 3800mAh Li-Ion battery.
Design Specifications:
- LED Forward Voltage ():
- LED Forward Current (): ()
- Battery Voltage ():
- Number of LEDs ():
- Battery Capacity:
1. Resistor Calculation
Each LED requires a current-limiting resistor. Using Ohm's Law:
Substitute the values:
So, each LED needs a 150Ω resistor.
2. Power Dissipation in Each Resistor
The power dissipated by each resistor is calculated as:
Substitute the values:
Each resistor will dissipate 15mW. Use resistors rated (250mW) for safety.
3. Total Current Draw
The total current drawn by 75 LEDs in parallel is:
Substitute the values:
The circuit will draw 0.75A (750mA) from the battery.
4. Power Draw from the Battery
The total power consumed by the circuit is:
Substitute the values:
The circuit will draw 2.775W from the battery.
5. Battery Life
The battery life can be calculated as:
Substitute the values:
With a 3800mAh battery, the LED signboard will run for approximately 5 hours.
