LED Board Rev 1

Owned by Ethan Abraham
Last updated: 2024-05-21 by Daniel Puratich
4 min read

  • Requirements

    • Accepts 12V for LED power and GPIO input (5V)

    • Does simple switching to turn ON and OFF LEDs based on GPIO input

    • Requires clean connecting interface for multiple LED strips

    • Handles up to 2A

    • 4-6 outputs

  • LED strip details

    • Four inputs: +12V, and three ground connections for R, G and B channels

    • Two inputs are mislabeled, need to verify which

  • Details of use case

    • Only R and G channels need to be illuminated

 

  • Design notes

    • N-channel MOSFETs as switching elements

      • DMG2302UKQ-7 has appropriate ratings, and is already in part library

    • Source terminals connected to power ground

    • Drain terminals connected to R and G channels

    • Gate terminals connected to control signal input

    • +12V connected directly to LED strip

  • Connector notes

    • Need 2x signal inputs - can use 3-pin 2.54mm headers to receive signal from microcontroller

      • Leave Vcc pin disconnected (no need for it), only connect GND and signal

      • Would be good to have clear way to indicate correct connector polarity

    • Need 4-6 output connectors

      • Connectors need 3-4 pins

      • Suitable for up to 2A

      • Ability to solder a connector directly to LED strip (and use cable with mating connectors on either end) so LED strips can be easily replaced without replacing the cable or needing direct access to the board

    • 12V power input - XT60 should be appropriate. Need to check whether we use female or male connector for receiving power (would assume female but good to confirm)

Block Diagram

Drone Integration Diagram

 

Power Architecture

  • Spec LED’s that take 12V and total less than 2 amps

    • Possibly use the PDB 12V which can supply 3A

      • currently only see the VTX which draws 250 mA

    • Could also use another BEC is required which can supply upto 12V, 5A

 

How to Achieve Strobing in LED’s

Pinout of the 555 Timer

Pin

Function

1 (Ground)

Used to ground the IC.

2 (Trigger)

It’s an active low trigger pin so when the voltage on this pin drops below 1/3 of the supplied voltage, the output is set to the high state.

3 (Output)

Digital output alternating between (close to) 0V and the supply voltage.

4 (Reset)

It’s an active low trigger pin responsible for resetting the timing cycle, so if it’s grounded the cycle gets reset. Therefore, it must be connected to the supplied voltage.

5 (Control Voltage)

This pin is used to control the timing cycle, however it’s usually just connected to the power supply through an external capacitor to level out power fluctuations.

6 (Threshold)

While the trigger pin is responsible for starting the cycle, the threshold pin ends it. When the voltage here reaches 2/3 of the supplied voltage, the cycle ends, and the output goes to its low state.

7 (Discharge)

This pin typically discharges to an external capacitor and resistor which is responsible for controlling the interval.

8 (Power Supply)

Used to supply power to the IC.

How the IC Works Internally

The core of the 555 timer is essentially two comparators comparing the current voltage with the threshold and the trigger voltages. The outputs of the two comparators are then connected to a flip flop circuit which is responsible for the IC’s output.

555 Timer as an Oscillator

The 555 timer is configured in its astable mode and will continuously switch between an on and off state. The duration of the cycle can be selected by choice of resistor values. This configuration connects the trigger and threshold pins so it’s never stable and constantly flipping between the two modes at set intervals controlled by resistor values.

Circuit Diagram Depicting the IC in Astable Mode  

 

Additionally, the equations to select t1 and t2 are as follows:

t1=0.693*(R1+R2)*C1

t2=0.693*R2*C1

Where t1 is the on time, and t2 is the off time.

How this Configuration can be Used to Control Strobing

This method is commonly used for strobing LEDs due to its simple and well researched implementation. The 555 timer in this configuration switches between a high and low DC output where the duration of intervals can be easily selected depending on resistor values which will correspond to the LED’s on and off states. Due to this, strobing periods can easily be selected to meet the proper regulatory requirements. However, an important note for this implementation is that the duration of the ON period cannot exceed the duration of the OFF period.

Sources Used

https://www.electronics-tutorials.ws/waveforms/555_oscillator.html

https://www.circuitbasics.com/555-timer-basics-astable-mode/

https://www.learningaboutelectronics.com/Articles/555-timer-pinout.php

https://www.theengineeringknowledge.com/555-timer-as-oscillator/

Revision 1 of the PCB

The need for a pull up resistor

A pull up resistor is not necessary for the reset. This decision was reached due to the chip having an internal pull up of forty kiloohms

The need for an external clock

An external clock is not necessary for this design. This selection was made due to the following description provided in the ATiny’s datasheet describing its internal clock: “ …up to 1 MIPS/MHz performance with high efficiency”. Given how this exceeds the projects required strobe time significantly, it was elected that the internal clock is sufficient for this projects needs.

Revision 1.1 of the PCB

The need for a pull up resistor

The need for a pull up resistor has since been reevaluated as revision one demonstrated that the internal pull-up was too weak.

 

Replacing the current connectors