# 2 color LED short circuit my board

I have a 2-color LED (3 pin out: one common, and one for each color). I have a DC motor running on two AA batteries. Both color LEDs are OK while testing on a battery pack.

Now what I need is the green light on with forward motion of the motor, and the red light on with reverse. When I connect the LED, it causes a short circuit that paralyzes everything. How should I connect those to get the color on motion as specified?

here is the circuit board, and the led... i what the red on clockwise motor rotation, and green on

• Schematic, please. Jul 22, 2013 at 2:12
• sorry, i am not good enough in electronis to do that Jul 22, 2013 at 3:26
• @menardmam But if you were good enough to hook it up, you are good enough to draw a sketch of how it is all hooked up. Without that, we cannot hope to tell you what is wrong. Also, we will need to know the manufacturers part number for the LED and maybe a link to the datasheet.
– user3624
Jul 22, 2013 at 4:22
• @menardmam, even taking a clear photo, showing the relevant parts and how they are connected, should be fine. Just make sure we can see clearly how everything is connected. Jul 22, 2013 at 4:38
• Hope the image and problem is now clear enough ! Jul 22, 2013 at 4:56

So you have a bi-color LED for which both colors are known to work when connected to batteries. You've asked for a way to connect them to a motor such that the colors indicate the rotational direction.

What you haven't specified is how you are powering the motor (same two AA batteries?). You didn't specify how you are reversing the direction of the motor (is it a mechanical switch or transistors?). That's the purpose of a schematic: to illustrate how everything is connected.

You should definitely invest some time and effort into learning how to draw a schematic. It doesn't have to be fancy, and it doesn't even have to be drawn with software. You could draw on paper and take a picture. Its whole purpose is to communicate to other electronics engineers what you have in your circuit. Have you heard the phrase "A picture is worth a thousand words"? It's never been more true than in electronics as far as schematics are concerned.

So let's start with an LED. This is a light-emitting diode, so it uses the schematic symbol for diode plus two arrows to indicate light.

Your 2-color (bipolar) LED is two of these with either the anodes or cathodes tied together (this diagram is common anode):

I'll assume that your motor is designed for 3 volts. Even though you can light the LEDs with this voltage as well, you should use current limiting resistors to prevent the LEDs from burning out. Without knowing specifically what LEDs you are using, we will assume that each color $V_f$ of 2.2V, a common value for red/green bipolar LEDs. We will also assume that they have a standard $I_f$ of 20mA. If you're not familiar with driving LEDs (forward voltage and forward current) you should look at some of the other questions here that address how to power LEDs properly.

With these values, we can select the value of current limiting resistor using Ohm's Law: $R = E / I$.

$$R = \frac{3 - 2.2}{0.020} = \frac{0.8}{0.020} = 40\Omega$$

If you only plan on having one color lit at a time, you can use one resistor on the common pin:

You can use a DPDT (double pole, double throw) switch to change the direction on the motor with no other components:

However, you can't add a bipolar LED easily because it doesn't change color based on polarity, but rather on which cathode (or anode) is connected rather than open.

You could add two separate LEDs in reverse bias to each other easily like this:

To add the bipolar LED, you'll need a 3PDT switch so you can use the third pole to alternate the cathode (or anode) of the LED:

Now, none of this may actually be what you are using or how you intend to do it, but hopefully it gives you something to work from, and shows you the importance of diagrams and schematics. Good luck in your project!

Edit:

As @Wouter pointed out in the comments, you can do this with a DPDT switch:

• My god, quite complex and complete. As a rookie, i have a hard time just understan the layout, but seem to work, will try it. I will have prefer a layout that power the led on the motor output. I am already using a 2 led layout that work fine, but a single led dont work Jul 22, 2013 at 5:45
• I realize this might be a little overwhelming so that's why I started out simple and built on the design in steps. Also this will help you get started with schematics. :) Jul 22, 2013 at 5:48
• @JYelton: why use a third switch pole, you can use the bottom one! (3 pole switches are much harder to get than 2 pole) Jul 22, 2013 at 6:05
• @Wouter Doh! You are absolutely correct. I was originally doing the schematic differently and... I don't know what I was thinking. I'll update it. Jul 22, 2013 at 6:28

You can use the following circuit in parallel with the motor.

simulate this circuit – Schematic created using CircuitLab

Only one LED will light up at a time depending on the voltage polarity applied to the motor.

• i dont have 2 led, i have ONE led that is 2 color (red-green) Jul 22, 2013 at 19:24
• @menardmam Yes, I understand, but your two pin LED is equivalent to two LEDs with one common terminal, you have to figure out if the common terminal is the cathode (negative) or anode (positive) if you have one multimeter with continuity test you can use it to verify this. Jul 22, 2013 at 20:02
• My two color LED have 3 pins ! Jul 22, 2013 at 21:38
• @menardmam On my last comment I mean "your three pin LED". A three pin bi-color LEDs can be common cathode or common anode (in that case the polarity of the diodes on my answer is reversed) the most common ones are common cathode. Jul 22, 2013 at 22:12
• SURE i will do it... i will test it tonight ! thanks ! Jul 23, 2013 at 0:08