Need Help with possible issue: Built LED light switch a switch running of AAA's

Discussion in 'The Projects Forum' started by thedarkknight782, Sep 19, 2009.

  1. thedarkknight782

    Thread Starter New Member

    Sep 19, 2009
    I got three AAA batteries, soldered them together in battery back fashion, wired the positive lead up to a switch and connected 4 LEDs to it. I have thin wire going from the batteries to the LEDs and I wrapped them all up in electrical tape. I also wrapped the batteries and the switch in electrical tape. So basically the light has 4 wires with lights on the end that can be bent in any manner necessary.

    The LEDs have a 3.3 forward voltage and I think it said a 25 mA forward current or something (the current they are receiving in the circuit is 25mA). They are wired in parallel. Regardless, the thing works great for a light to see my keyboard in the dark but the electrical tape has this obnoxious smell to it. I started to notice that electrical tape had this sort of smell after I put it on the edge of my desk to make the corners not as sharp (so it wasn't hooked up to a circuit at all). Is that smell normal?

    Are there better solutions than wrapping all my projects up in electrical tape that allow for the same flexibility?


    Also, a side question, how long do you think 3 AAAs will run 4 white LEDs w/ a 3.3v voltage drop each and using 25mA? I didn't have a battery holder so when they die I am going to have to redo the circuit (I needed this done before I had time to get a battery holder).
  2. Audioguru

    New Member

    Dec 20, 2007
    Three new alkaline cells in series make 4.5V which is too high for 3.3V LEDs and will cause them to burn out quickly. A series current-limiting resistor should be used for each LED to reduce the current to 25mA each.
    But LEDs have a range of voltage which might be from 3.0V to 3.6V each.

    You forgot to say which type of battery cells you have. If they are AAA carbon-zinc or "heavy duty" then they might last for a couple of minutes. Alkaline cells can drive 100mA as shown on their datasheet for about 1/2 an hour for the LEDs with resistors to be bright then they slowly dim for 8 hours. If you use the new AAA lithium cells then the LEDs with resistors will be bright for about 12 hours then they quickly dim and turn off.

    If you want the connecting wires to be flexible then solder the parts to flexible insulated stranded wires. Insulate the joints with shrink-tubing.
  3. bertus


    Apr 5, 2008

    To connect the leds to a battery it is good to have a current limiting resistor for each led.
    You say you use 3 X AAA for powering the leds.
    The batteties are 1.5 Volts each so we have a powersupply of 3 X 1.5 = 4.5 Volts.
    You say the elds have a voltage drop of 3.3 Volts,
    and you want a current of 25 mA thtough each led.
    To calculate the required you must calculate the voltage accross the resistor and divide it by the wanted current.
    So (4.5 - 3.3) / 0.025 = 1.2 / 0.025 = 48 Ohms.
    A resistor of 48 Ohms does not exist so we take the first next higher value 56 Ohms.
    You will end up with 4 strings consisting of 1 led and 1 resistor.

    For isolating you could look for heat shrink tubing.

    For the question about battery life of an AAA battery see the attached datasheet from duracell.
    In the datasheet there is a curve for the battery life vesrus current drawn.

  4. thedarkknight782

    Thread Starter New Member

    Sep 19, 2009
    My main question was if there was any way around the annoying electrical tape smell and 2) if it is harmful.

    I already have the circuit wired up. There are 3 AAA batteries so there is 1.2V going to each of the LEDs and 25mA of current going through them.

    This datasheet: talks as if I can get 1200 hours out of a single battery providing 25mA. Since the LEDs are in parallel it would be 100mA. So then it is 900 hours. Multiply that by 3 and get 2700 hrs until they stop getting power. Is that a correct calculation?

    On the other hand it looks like I would get about 50 hrs out of a single AAA at 25mA and 8hrs with 100mA. So if I have 3 batteries then it would be.. 24hrs? Isn't there a way to wire LEDs in parallel and use only 25mA of power? Something about a matrix-like circuit where there is a parallel circuit inside of a series circuit? Say I have one wire connected to 4 wires with LEDs on them from negative. Then I have another single wire hooked up to the other end of the LED wires connected to positive. Has anyone heard of this before? Regardless, it would have been much easier if I had gotten a battery holder, put 4 rechargeable AAs in it and put some higher ohm resistors on the LEDs. Now I am gonig to have to tear the circuit apart when the batteries die.
    Last edited: Sep 19, 2009
  5. SgtWookie


    Jul 17, 2007
    Well, you said that you had the LEDs wired in parallel.
    That means they all have the same voltage across them.
    If you wired the three AAA batteries in series, the "battery pack" will measure 4.5v from - to +.

    Since you didn't use current limiting resistors for each LED, they are probably receiving excess current. At any rate, your batteries have an internal resistance that is low when they are new. As the batteries become exhausted, their internal resistance increases.

    As a wild guess, I'd say your LEDs were getting around 50 some mA current each when you first connected the batteries to them. That's a total circuit current of 200 some mA, and at that rate of discharge (see chart in PDF) they have about a 700mAh capacity. So, 700mAh divided by 200mA = 3.5 hours until the batteries will reach 0.8v.

    However, 3 x 0.8v = 2.4v, and your LEDs probably need at least 3v to output any light at all.

    According to the "Constant Current Performance" chart, you'll get about 2 hours before the batteries reach 3v.

    It's more complicated than that, though. The LEDs will be quite bright for the first hour or 1.5 hours. Then they'll slowly get dimmer. They may continue to glow dimly for a long time. That's because as the voltage across the LEDs decreases, the current decreases.
  6. Wendy


    Mar 24, 2008
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
    Strange, I sniffed two tapes, one had a slight iodoform smell, the other has no smell, both Chineese , same mfg. CSA. Try changing brands.
    There is the right way,and then there's my way of connecting batteries & LED's. When bright LED's were first available I made 10 conversions using 3 N cells in series, and 1 LED, several using 3 AA's and 1 LED; all still work, but have fallen out of favor with introduction of multi LEDs and cheap. Would advise only paralleling LED's of tested same V drop
  8. thedarkknight782

    Thread Starter New Member

    Sep 19, 2009
    Each LED has it's own resistor at whatever is the closest ohm resistor to 50 and they are wired in parallel. So, 4.5v-3.3v = 1.2v. 1.2v /50 = 0.024 amps. Is there something I don't understand about Ohm's law? Do things work differently with parallel circuits? Tying in to those questions, why do you think there is 50mA per LED?

    So you don't think the electrical tape smell is anything to be concerned over? It isn't so bad it is unbearable but every so often I do smell it from a couple feet away. I noticed it with a roll of tape we had had for a while and I noticed the same type of smell with a brand new roll of electrical tape from radioshack.

    I suppose next time I should hook the circuit up to a battery holder so I can use rechargeable batteries for these some or projects. On a somewhat unrelated note, does anyone know how that wireless electricity stuff that is becoming more common works (seen things about wirelessly charging electronics devices somehow as fairly common in 2010 or 2011).
    Last edited: Sep 21, 2009
  9. SgtWookie


    Jul 17, 2007
    Well, I'm not sure where your confusion lies. :confused:

    Your power supply is three 1.5v batteries in series, for a total of 4.5v (when fresh).
    Let's just say you wanted to connect just ONE LED to the 4.5v supply.
    And you say the LEDs are rated for a Vf (forward voltage) of 3.3 at 25mA.
    So, 4.5v - 3.3v = 1.2v that the current limiting resistor needs to drop.
    1.2v / 25mA = 48 Ohms. Now is decision time, because the closest standard resistors to 48 Ohms are 47 Ohms and 51 Ohms.
    Let's see what happens with 47 Ohms.
    Since I = E/R, and E=1.2v and R=47, I=25.532mA. That's not a lot over 25mA (about 2%), and your tiny AAA batteries are sure to have at least 1 Ohm internal resistance. So you could get away with 47 Ohms, or use the 51 Ohms.

    Then you could add more LEDs with their resistors in parallel across the battery supply, keeping in mind that higher current use with batteries causes them to become exhausted much more quickly than they would with a low current drain over a long period of time. This is because more power is wasted across the batterys' own internal resistance.

    Because of the battery internal resistance, and that as current through a diode (any diode) increases, so does the forward voltage. As the current flow nears the rating of the diode, Vf increases much more rapidly.

    Try different brands of tape, or buy it at a store that has a lot of turn-over. If electrical tape has gotten hot (like sitting in a truck or warehouse for awhile) it will smell more strongly. Electrical tape is not permanent; eventually the adhesive will fail and the tape will fall off. This happens much more quickly in environments such as automobiles.

    Rechargeable batteries are a good thing. NiMH batteries are expensive initially, but will last much longer than the older ni-cad rechargeables.
    Better to start a new topic with that idea.
  10. Wendy


    Mar 24, 2008
    Given how chinese goods have been over the last 5 or so years there is no way of knowing whether it is a problem or not. I've never run into it myself, I generally stick with 3M (no rightous reason, it is handy).

    The odd are your resistors are heating slightly (which they are designed to do) and causing the outgas. I went through the early posts on this thread, no resistors were mentioned. Even with LEDs sometimes a schematic is worth a thousand words, schematics and math are the language of electronics, no ambiguties. Leaving out the resistors is a common mistake for beginners, so if you don't mention them we don't assume they are there.

    Radio Shack is convenient, but not cheap. I am lucky to have several local electronics outlets, battery holders are around 68¢ or so. I highly advise doing the research and seeing if there are any similar outlets in your area. With a little research even mail order can be a lot cheaper, as well as several sites like Dan's Small Kits and Parts.