Using AC With LEDs (Part 2) - and Make This Handy Counter Light.





Introduction: Using AC With LEDs (Part 2) - and Make This Handy Counter Light.

In Using AC with LEDs (Part 1) we looked at a simple way to run LEDs with a transformer connected to AC Mains.

Here, we will look at getting our LEDs to work without a transformer and build a simple light that is integrated into an expansion bar.

WARNING: For countries with 110v mains, we'd be working with voltages of 150 volts! For Europe and other countries, we are talking about 300 volts or more! At these levels, electricity is lethal! Do not continue unless you are comfortable with working with high voltages and are aware of the precautions to take!

AC supplies are quoted in rms (root-mean-square) values. The PEAK voltage is sqrt(2) * Vrms, which is about 1.4 * Vrms

Step 1: Some Background

The simple and obvious way to get hundreds of volts down to a level to operate a LED at 20mA is to put a resistor in series with the LED. To find out what values we are talking about, we'll use the peak value of 110v, which works out to be 150v for our example (it'll be double for Europeans and Ozzies)

150 / 20mA = 7500-ohms (we should subtract the voltage of the LED from 150v first, but the difference is minor)

7500-ohms? Not too bad... But then let's consider the power rating of this resistor, using the Power rule: P = (V2) / R, we get:

150 * 150 / 7500 = 3 watts, and that's a pretty hefty resistor. Brits with 240v mains will need a 17000-ohm resistor rated for almost 7-watts. And these will be running HOT!

Fortunately, by substituting a capacitor for the resistor, we can get the same reduction on voltage without the (or as much) heat. Capacitors delay the phase angle of AC which we can use to oppose itself, much like receding waves on the shore cancelling out some of the force of incoming ones.

Step 2: Our Circuit

Using the resistor value from before, we can calculate the value of the capacitor. Since we are already using a 1K resistor, the reactance, X (a fancy term for resistance with capacitors) can be 1000 less than what we need.

C = 1 / (2 * pi * f * X) where f is the mains frequency

which is .4uF for 110v 60Hz, and .2uF for 240v 50Hz.

Instead of watts like resistors, capacitors are rated by volts, we have to make sure we get caps rated for AT LEAST 250-volts (States) and 450-volts for 200-volt countries.

WARNING: Capacitors with insufficient voltage ratings may explode!

This very simple design will drive 2 - 16 LEDs without any changes. Just put the same number of LEDs in each branch, and make sure you hook them up with opposing current flow.

Step 3: A Non-lethal Installation

Actually, you can test out the circuit without risking your life. It's flexible enough to work as a telephone ring-indicator.

Use a .4uF (.33 to .5uF) capacitor and attach your device to the 2 leads of your telephone junction box (usually the red and green wires), and it will flash when you get a call.

NOTE: this ONLY works on home phone circuits - PBX and Central phone systems are totally incompatible.

Step 4: Making the Socket Expansion Light

Now that we have the basics out of the way, this is what you need for the project.


Socket Expansion - check to make sure it has a screw-in back. I got mine (a 'Noma') at Target (Radio Shack seem to have a similar one as well). Obviously you have to get one that is suited for the power system in your country.

Capacitor - (US, 110v 60Hz) any value from .33uF to .47uF 250-volts MINIMUM!
(Others 200-240v 50Hz) .15uF to .22uF 400-volts MINIMUM!

Resistor - 1000-ohm (1K) 1/2W. I did not have a 1/2W resistor, so I took 3 x 3300-ohm 1/2W resistors and wired them in parallel to get a 1100-ohm 3/4W resistor

LEDs - 14 pieces of high-brightness, 20mA 5mm (T-3) White

Heat shrink tubing

Step 5: Preparing the LEDs

I made this test rig with 2 (charged) NiCd batteries. Even though it can only supply 2.5v, it will power the LED at a low level, which lets me see the quality of the light. I also confirmed that the + lead is the longer one.

Rank them in brightness and place the brighter ones in the middle.

Step 6: Preparing the Socket

Disassemle the Expansion Bar. Note the small isolated areas on the top and bottom edge of the unit which we can install our parts. Decide the end you wish to work on - whether you want the light to shine UP or DOWN.

Mark out 2 rows of 7 points, 3/8" apart on masking tape. Center it on the end you decided on and start 14 holes with a 1/16" bit. Expand, using a 3/64" bit. Smooth the holes SLIGHTLY - it should hold the LEDs snug.

Step 7: Installing the LEDs

Make a mark on one end of the bottom row, and another mark on the opposite end of the top row. This will tell you the side the positive (longer wire) end of the LED should go.

Bend the LEDs in an "L" shape (short wire on the appropriate side) and snug them into the bottom row.

Spread the wires about 30o to cross it's neighbor's. Solder lightly to keep in place, but DO NOT TRIM ENDS.

We will be assembling the LEDs in a lattice - see the second image. Except for the end units, each LED should have a lead touching 3 other LEDs.

It helps by pre-forming the LEDs for the top row so it can clear the "X" junction. (See image)

When you are finished, CAREFULLY reposition the leads to make room behind for the resitor(s) and capacitor. Make sure the solder joints are solid and no wire is shorting out.

Step 8: Finishing Touches

Solder zip wires from the power-carrying plugs to the capacitor and the resistor. Protect with heat shrink tubing and attach each to one end of the LED chain.

Space IS tight, so only use as much wire as is needed.

The long green bundle under the cap is the resistor assembly.

The cap is linked to the other end of the LEDs with the red wire.

The big arrow is to make sure I don't start operating on the wrong end of the patient!

Make sure everything is tight and reassemble.

Step 9: TA-DA! my dark, dank workspace has light!

Again, high-speed photography shows the two rows of light alternating.

Continued in Part 3.

Also, check out some other LED projects at my website!



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    Good point - if the wiring is exposed, a 4.7k 1/2w resistor across the capacitor will reduce chances of electrical shock, but since the circuit was intended to be hidden away in an enclosure, I did not include that.

    hi, i build this and it works on 220ac 50hz. quite a bit of flickering, but point is it works. one comment, you might want to add a discharge resistor in parralel to the cap. els e that cap will shock you when you unplug the device

    I understand the resistor in series with the capacitor is to protect the capacitor, but how do you determine that it needs to be a 1/2W? Could I use other resistors in series with the LED branches to change their brightness, and if so, would they need to be the same Watt rating as the main resistor? Thanks.

    Forgive me if this is a noob question, but, why the lattice connections with the LEDs? Why not just two straight row connections? Thanks.


    The "lattice" arrangement is such that each pair of (top-and-bottom) LEDs protect each other from over-voltage.

    Thanks for the reply. Sorry to keep bothering you, but just to clarify: without the lattice connections would there definitely be over voltage or is it just there in the slim chance that there "could" be over voltage?


    Since we'll be running this thing 24/7, the over voltage is just about guaranteed. Never play games with safety!

    Thanks for educating me on that. Based on this Instructable and some additional research in creating a required resistor array, I have developed a schematic for a small project. If you don't care, could you look it over to make sure everything's right and safe? This will be encompassed in the bottom shell of a spent CFL bulb so as to be able to screw it into a 120V light socket. Thanks.

    LED Bulb Schematic.jpg

    Ok I have read most all the comments, and I am no closer to my particular situation. Please clarify your formula just a little for me. I am not dealing with 110v a/c, I am using a 36 volt a/c leg and a 16 volt a/c leg. So I should be able to use a smaller resistor, and higher capcitor correct. I used every thing you did but just used a 1n4007 as the second diode, they are just beieng used as panel lights on a power supply I am building. Thanks for the help. Just help me with the formula.


    For low voltages (under 50v) use this formula:

    R = (Vi - Vled) / 0.2
    R is the resistance to put in series, (You do not need the 1K or the capacitor!)
    Vi is the input (source) voltage (for RECTIFIED a/c, multiply this by 1.7)
    Vled is the total V of the leds you are using (use 2.4 for each Red; 3.3 for White or Blue)
    and round UP to the nearest standard value

    Hope this helps!