# Beginner help with LED diodes, use a resistor or not for prolonging length of time

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#### Chrissy Garrison

##### New Member
Hey to anyone willing to help me! I fibbed on being a specialist in a specific field of Electronics. My hubby is an electronic engineer but when it comes to work after his "work hours" and then it being me I get him frustrated. So I have been doing some searching for a few hours and I'm not 100% sure my findings and what I believe is going to work for what I am trying. I will attempt to make this as short as possible.
To begin I am just a stay at home "crafter". I do all types of things, however this year for xmas a big seller is an ornament. It's a "Santa Cam or Elf Watch Cam" here's an example

I have a very competitive market so I have to have my own original design. I have simple plastic ornaments and I will do everything I just really want to add a long lasting tiny red LED flashing/blinking light to make it more realistic. So I have found red 3mm LED Light Emitting Diodes and then just using AG3 Alkaline 1.5V Button Cell Batteries. This is where I am getting lost or confused. It's probably something simple and I am going to come off really dumb looking. I just don't have the mind that understands what you are all great for.
Can I buy these diodes and then just slide the button battery between making like a "throwie" taping the 2 together or use a cell button battery holder? Or do I need to buy 5mm Clear Red LED w/ Resistors with more of maybe a 3V button cell battery. I am wanting the LED/Battery last for at least a month. Am I shooting for the impossible? I just figured there are always small blinking LED's on toys etc that seem to just last forever. I just want to push the bulb through a small hole I put into the ornament.

I apologize for being long winded or completely confusing you because I have no idea what I am even doing!

#### alec_t

##### Well-Known Member
Welcome to ETO!
Standard LEDs (of whatever colour) would require additional circuitry to make them flash.
You can buy flashing LEDs with the necessary circuitry built in, but they need a supply voltage of at least 3V and may draw an average current of at least 1mA (the datasheets are unclear on this). This suggests a coin cell battery would not have enough energy to power the LED for a month.

#### Tony Stewart

##### Well-Known Member
Each colour has a slightly different voltage although Red-Yellow are 2~2.2 and Blue/White/Green 3.0~3.2 (typically and not always depending on quality). I find the best throwie solution uses Lithium primary cells because the voltage is exactly 3.0V unlike Alkaline which ranges from 1.6 (new) to 1.2V (old) making brightness impossible to be stable and eventually insufficient voltage before the battery is fully used.

So what I suggest if you want Red use 6V with two cells in series and use 3 REDs in series and if you want the other colours use one 3V.cell with no resistors required.

The current will strongly vary with voltage in this range, so a stable battery voltage is crucial. Unfortunately Lithium cells are more expensive but worth it.

My recommendation is to buy bulk CR2 Lithium cells and choose LEDs with the highest brightness in 5mm. These cost < 25 cents and perhaps <10 cents in high volume. the battery is the biggest cost. I used to sell these to a client who wanted more than (>) 10,000 mcd or 10 Candella in 30 degree.

For each reduction in lens angle by two almost doubles the brightness and visa versa. e.g. 60 deg to 30 deg about 1.8x brighter mcd.

The 5mm LEDs have an equivalent incremental series resistance of about 15 ohms, so your husband can design the pulser circuit to switch on with less than 10% of this resistance in the electronic MOSFET switch that would be good.

Assembly time and cost is inverse with the effort put into the design. So a good design requires careful planning and perhaps a small PCB.

Not all batteries are the same cost and expect cheap ones to last less but you should be able to find CR2 Lithium cells (fatter than AA but smaller than C size) for about $1usd each in packs of 10. The ornament socket must be polarized like LEDs and the negative side of some LEDs has a flat edge. A socket is hard to find for LEDs but strip header sockets are perhaps the best. Since the 5mm LED pin spacing is 0.2" and the header spacing most common is 0.1" you use the outer 2 sockets in the correct orientation. (forget button batteries, these only last a day or two depending on pulse time.) If you need advice on your choice of LEDs let me know what you choose and ask. Think big ornaments. Last edited: #### spec ##### Well-Known Member Most Helpful Member Hi Chrissy, We don't get many ladies on ETO. My wife causes havoc when she gets involved in technical matters! Interesting project; how many LEDs would you like to flash? Using high efficiency 2.5V, 2mA LEDs, and a high efficiency flashing circuit (effectively no current) and, assuming the LEDs flash on for half a second and off for half a second, the average current for each LED would be 1mA. There are 30 * 24 = 720 hours in a month, which means that a battery of 720mA/hours capacity would be required for each LED. A CR2032 3V lithium manganese oxide primary (non rechargeable) button cell has a capacity of 225 mA/hours so, for each LED, you would need three CR2032 button cells. A CR123A 3V lithium manganese primary battery has a typical capacity of 1550 A/h which translates to 1000mA/hours in a practical circuit. So one CR123 would supply one flashing LED for around 1.4 months. (corrected in view of Tony's comments in post #6) In addition to the other posts, this is just a bit of information to give you an idea of the scale of what you want to do, but you could reduce the current consumption by having just one LED on at a time. This can look quite effective if the LEDs are physically arranged so that they appear to 'chase' each-other. Alternatively you can have a single LED illuminating randomly. You could also have different color LEDs. spec Last edited: #### spec ##### Well-Known Member Most Helpful Member Last edited: #### Tony Stewart ##### Well-Known Member Most Helpful Member just a small detail spec All CR123 and CR123A batteries are exactly 3.0V because they are PRIMARY Lithium cells (non-chargeable) They do make SECONDARY or rechargeable cells in Lithium Ion Polymer or LiPo which are all 3.7V but after about 3/4 energy used, decay quickly from 3.5 to 3V then need to be recharged with a fancy LiPo charger. ($)

the LiPo equivalent is the RCR123 , which I would not recommend for this user.

PRIMARY non rechargeable cells will hold more capacity in a range of qualities, but one time use.

#### spec

##### Well-Known Member
just a small detail spec

All CR123 and CR123A batteries are exactly 3.0V because they are PRIMARY Lithium cells (non-chargeable)

They do make SECONDARY or rechargeable cells in Lithium Ion Polymer or LiPo which are all 3.7V but after about 3/4 energy used, decay quickly from 3.5 to 3V then need to be recharged with a fancy LiPo charger. (\$)

the LiPo equivalent is the RCR123 , which I would not recommend for this user.

PRIMARY non rechargeable cells will hold more capacity in a range of qualities, but one time use.
Thanks Tony: yes CR123A are lithium manganese oxide primary batteries with a typical capacity of 1.55AH, but 1Ah in a practical circuit (post #4 corrected)

I was getting confused because I have a load of LiIon (not LiPo) RCR123 rechargeables which are marked just CR123A.

As an aside, I take your point that secondary RCR123As do not have the same capacity as primary CR123As, but I have found that the discharge curve of the RCR123As follows the typical LiIon voltage curve during discharge: 4.2V charge max, 3.6V most of the time and 3V at discharge point. I thus suggest that the RCR123A would be suitable for this application. Of course, for a conservative regime you can charge to 4V and discharge to 3.2V. If you do not maximize the battery capacity, and possibly reduce battery life, a LiIon battery charger amounts to little more than a current limited constant voltage supply (just like lead acid).

spec

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