Circuit using inverter and transistors to control LEDs with Arduino

johnfOGR Forum Member SUPPORTING MEMBERDIGITAL SUBSCRIBER

I am interested in comments/suggestions on the circuit below, which I designed to control indicator LEDs for turnouts with an Arduino. My main goals are to 1) minimize the number of Arduino pins required by using 1 pin to control the LEDs for both directions of the turnout, and 2) use transistors to drive the LEDs because the current requirements are too high to use the Arduino to supply LED power directly.

Here is a diagram of the circuit. To simplify things, I am just showing the circuit for 1 turnout. My final board will be for a total of 12 turnouts. When the Arduino pin is HIGH the top 3 LEDs will light, and when the pin is LOW the bottom 3 LEDs will light.

Here are the data sheets for the Inverter (CD74ACT04E) and Transistor (ULN2803A). I'm planning to use 220 ohm resistors. Please let me know if I’ve made any mistakes or if you have any suggestions.

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1/2/194:58 PM

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gunrunnerjohnOGR Forum Member SUPPORTING MEMBERDIGITAL SUBSCRIBER

How does the Arduino know what the switch is doing? Why do you need three LED's for each state?

1/2/198:35 PM

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johnfOGR Forum Member SUPPORTING MEMBERDIGITAL SUBSCRIBER

GRJ,

The Arduino Mega knows what the switch is doing because I am also using it to control the Atlas turnouts. The Arduino receives inputs (through an optocoupler) from either a momentary toggle switch, an AIU or an insulated rail. The Arduino activates a relay for 1/2 a second to throw the switch and also stores the position of the switch in memory.

Right now I am only using 2 LEDs for each state - one on the control panel and one on a small block signal next to the switch. In the future, I plan to add a 3rd LED on an indicator panel at the far end of my layout.

I am using relays to control the LEDs now (one state to NC and the other state to NO) and that works fine. I am really just doing this project to learn more about how some electronic components work and also practice using Diptrace software. I am also planning to add some modular connectors to the board to make wiring easier.

1/2/199:20 PM

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gunrunnerjohnOGR Forum Member SUPPORTING MEMBERDIGITAL SUBSCRIBER

OK, I was curious as this only looked like half a solution.

1/2/199:24 PM

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Professor ChaosOGR Forum Member

What about using a TPIC6B595? I think you can daisy chain them to control an arbitrarily large number of LEDs with only 3 Arduino pins (using the SPI interface). The 'inversion' would be in software, so no need for a separate inverter chip.

They're good for 500 mA each - you will sink rather than source the LEDs.

1/2/1910:28 PM

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johnfOGR Forum Member SUPPORTING MEMBERDIGITAL SUBSCRIBER

Professor Chaos,

Thanks for the suggestion. I'm going to order a few chips and try it out on a breadboard.

One quick question. Can the power source for the shift register be different from the power source for the Arduino?

1/3/198:17 AM

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gunrunnerjohnOGR Forum Member SUPPORTING MEMBERDIGITAL SUBSCRIBER

johnf posted:
One quick question. Can the power source for the shift register be different from the power source for the Arduino?

No reason why not. Obviously, they have to have a common ground.

1/3/199:21 AM

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johnfOGR Forum Member SUPPORTING MEMBERDIGITAL SUBSCRIBER

Just a quick update on my project. I decided to go with the shift registers and they're working great. Here is a picture of the board I put together. It can control 3 sets of 32 led's with just 3 wires from my Arduino. The board in the picture only has outputs for 2 sets of led's installed because I don't need the 3rd one yet.