Recently I bought an LED that cycles through three colours, red green blue I believe. However when I use the above circuit It just seems to light up with a red light. It does not cycle through the nice red/green/blue cycle. The guy in the electronic store said it was 3 volt led. I tried a lesser Rvalue 69 and then a higher 120, but all to no avail. It Just glows red. The led is clear and is one of those larger ones. You know there are the normal size that are commonly used and then there are the next commonly larger size. This is that larger size. I have put a battery on it and it seems to work ok just alone. Can anyone help me on this.
Maybe these fancy LEDs with little chips ?? in them to make them rotate the three colours dont lend themselves to be used in this way??? Any advice.
Likely, the AC 50% duty cycle keeps resetting the chip inside the LED. Try a small half-wave or full-wave rectifier to provide DC instead. You might need to increase the R value, though.
I expect that the chip in that LED is resetting between AC cycles when the LED is not getting any power. It will need rectification and some filtering of the 120VAC.
I have another concern: Does its current consumption drop when it changes color or between any PWM pulses of any fading that some of these LEDs have between colors? If so, then the voltage reaching the LED through the resistor can spike to excessive levels and I suspect that could blow the chip. A zener diode, I would guess 5.1V, in parallel with the LED can protect the LED from that.
Thanks Don, Rectification seems to be what someone else said too. The Zener diode is a good idea too. however am I wrong in thinking that the higher the R the higher the wattage of the resister 1/2 or more should be used? Regards
Actually, the lower the resistor value the more current will flow and power dissipated in the resistor will increase as its value is decreased.
For a worst case, assume that the load voltage will be small and nearly the full rectified/filtered 120VAC (169 volts DC) will be across the resistor. That will dissipate about .285 watt into a 100K resistor. This is assuming that you have a rectifier and filter capacitor and the resistor is downstream from the filter capacitor.
If the resistor is 100K and you rectify and filter 120VAC, I would recommend a 1/2 watt resistor.
Keep in mind that this will pass only about 1.6 milliamps, which is fairly likely to be quite low for a color-changing LED.
Allowing more current with reasonable parts and reasonable heat dissipation gets into the more complicated matter of having a capacitor rather than a resistor limiting the average current, plus a resistor to limit peak switch-on current into this whole thing, and the capacitor having an actual AC rating since many capacitors do not do well with long term application of AC merely having peak voltage below the capacitor's DC voltage rating.
It is starting to appear to me that the simplest way to power a low voltage color-changing LED from 120VAC is to use a "wall wart" with low voltage DC output, a 7805 or similar 5 volt regulator, and a dropping resistor that drops about 1.5 volts (5 volts minus typical LED voltage) at 20 or 30 mA, which is basically 47 or 68 ohms. However, I expect 100 to 220 ohms to work OK with tolerable reduction of light output and 33 ohms to overheat the LED only a little if at all.
Meanwhile, there are some color-changing LED "nightlight bulbs" (and complete nightlights with such removable candelabra-base bulbs) on the market. I somewhat remember that Target is where I bought the two that I have, although I do not guarantee that you will find those at your nearest Target.
That product is reviewed by Craig Johnson ("LED Museum" / "Punishment Zone") at:
In addition to the others' suggestions, I would test the LED with a 9V battery and appropriate resistor. First convince yourself that the LED works correctly with a simple DC source before playing around with rectifiers and such.
After I wired it up as in my first diagram I tested the led to see if it still works after connecting to the 120. I tested it with a batery and it works fine. So my circuit has not burned the led it just doenst change colours as it is suppose to.
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So you're not talking about a LED but about an IC that contains three LEDs and some other electronics. This type of circuits tend to have type number and specifications described in a data sheet. It should help if you could tell something about it.
Even if you can't tell something about the part, it apparently works with a battery. What battery and what series resistor did you use? Do you have a multimeter so you can tell something about voltages acros battery, resistor and LED? If you don't have such an instrument you'd better buy one that's to say if you want to do something with electronics. A simple one costs around ?10,--. Good enough for a beginner.
The circuit below may work but I can't be sure due to lack of information. Beware! Circuits directly connected to the mains are always dangerous.
perhaps the LED needs continuous DC. not pulsed like that circuit gives, stick a 10uF capacitor in parallel with the led and another diode in series with the pair.
Unfortunately the store I bought the part at was one of those re package stores, eg it came in a plain little plastic bag. The store guy said that it was 3 volts, unfortunetely I dont know how many ma. He showed it to me using a flat watch battery about the size of a dime. I have tested it with my diode/continuity setting on my multimeter and it works fine. Last night I took a little rectifier and put on bread board put, house current on it 117v AC and then tested the output on it with my multimeter on dc. It shows about 105 volts. I tried the led as wired in my first diagram on my first post and got no light at all. I then tried the same thing with a normal led and it worked fine.
Not sure why it works ok with normal led but this one it does not work with. By the way Im sure your right about the 3 colour led its probably more IC than LED. In fact when I look at the individual light component in the led lens they are flat not the normal sort of stork looking thing with a long leg.
I will try the diagram you have posted and get back to you . I might also go to another store and see if I can get another different 3 coloured light.
I put together your circuit and sure enough it did the trick. Thank you. The led is really quite something. The program is not just coloured lights that flash one colour and move to the next its seems to run a little program that must be 15 to 20 seconds long before it recycles over again. It circulates the colours and then pauses and sort of pauses through all the colours. This may be all a mater of routine for all of you but its amazing how much they can pack into one of these. Thank you again.
I am going to try your ciruit to see what it does to this led. However I discovered I dont have any 10uf Capacitors so I have to go to the store. By the way Im guessing that I would use a capacitor that is NOT polerized. Am I right ??
Your schematic works too. The sequence is a little different however. When using the bridge rectifyer the lights seem a little slower. With your way of doing it the lights blink quicker with more of a burst and less of a pause. Im not sure why this is, whether the voltage is less or more with yours. I will have to experiment and with the variables and see what differences it makes.
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