Help with electronics order please. Not sure if I have everything I need.

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Crazylegsmurphy

Active Member
Hello. I have two electonics projects that I would like to attempt to dip my feet into things. I am in the process of doing an order online for parts but I was hoping you guys could help me make sure I'm getting everything I need?

They are simple circuts. Both run off a watch battery.

The first is two Adafruit Sequin ( https://www.mouser.ca/ProductDetail/485-1758/ ) LEDs run off a watch battery.

The second is a single Bar Array LED ( https://www.mouser.ca/ProductDetail/630-HLMP-2300/ ) run off a watch battery.

I am also building a Ghostbusters trap, but I am using different LEDs than the plans. I am using following based on the recommendation of another person ( https://www.mouser.ca/ProductDetail/485-2860/ ). He mentioned that he used a 470 ohm resistor on the first LED to protect it. Is this appropriate? ( https://www.mouser.ca/ProductDetail/603-CFR-25JT-52-470R/)

My question is, do I need any resistors for these or am I good?
 

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zapwizard

Sr Member
RPF PREMIUM MEMBER
I suggest looking up a LED/Resistor tutorial. Sparkfun has a good one here: Light-Emitting Diodes (LEDs) - learn.sparkfun.com

The reason you don't see many folks using resistor when using coin cell batteries is either the battery can't provide enough current to worry about a resistor, or the LED voltage is very close or lower than the battery. (Like running a 3.3V LED on a 3V battery)

The Adafruit parts you linked to already have resistors and can run between 3 to 6V. (There are better specs on Adafruits website for those parts)

The Broadcom LED bar however is just a series of raw LEDs in an array. You will need a resistor to limit the current. The resistor value you chose is based on the voltage you are driving it with. See the tutorial above to learn how to calculate this.
 

Crazylegsmurphy

Active Member
I suggest looking up a LED/Resistor tutorial. Sparkfun has a good one here: Light-Emitting Diodes (LEDs) - learn.sparkfun.com

The reason you don't see many folks using resistor when using coin cell batteries is either the battery can't provide enough current to worry about a resistor, or the LED voltage is very close or lower than the battery. (Like running a 3.3V LED on a 3V battery)

The Adafruit parts you linked to already have resistors and can run between 3 to 6V. (There are better specs on Adafruits website for those parts)

The Broadcom LED bar however is just a series of raw LEDs in an array. You will need a resistor to limit the current. The resistor value you chose is based on the voltage you are driving it with. See the tutorial above to learn how to calculate this.
Thank you for the reply.

I am new to this, so bear with me.

I was sure the Sequins (tiny LEDs) had a built in resistor which is why I wanted to double check. The Neopixel (bigger one) was what was confusing me. I'm not sure why the guy I was talking to suggested an additional 470ohm resistor. I put a few in the cart because they're only a few cents, but I wanted to be sure it was even needed.

The last one is where I was getting confused. I saw a guy online hook a button/watch battery directly to one of the LEDs and it seemed to work. The data sheet seems to suggest they pull 5V each (if I'm reading it correctly) so I assumed the reason it worked is because the LED couldn't actually draw enough from the watch battery to be any harm? The issue with this Array LED is that I don't have the space for a larger battery.

I'll take a look at the link you posted and see if I can make heads or tales with it.
 

zapwizard

Sr Member
RPF PREMIUM MEMBER
Mouser.com is a good supplier, but Adafruit has much more info on the Neopixel on their website here: NeoPixel Jewel - 7 x 5050 RGBW LED w/ Integrated Drivers
"Each LED is addressable as the driver chip is inside the LED. Each one has ~18mA constant current drive so the color will be very consistent even if the voltage varies, and no external choke resistors are required making the design slim. Power the whole thing with 5VDC (4-7V works) and you're ready to rock."

They list it as running at 4-7V. 3.3V might be able to still run the Neopixel but the LEDs probably won't be at full brightness.
If thickness isn't an issue you can get coin cells that are twice as thick and run 6V, or you can stack two 3V coin cells to get 6V.

Regarding the broadcom LED bar, it has a spec of 2V @ 20mA. A 3V coin cell battery often can't provide much more than 10 or 15mA, so in effect the battery is self-limiting and so can't damage the LED despite the higher voltage.

However, the correct thing to do is to put in a resistor. If your source voltage is 3V, then you do 3V - 2V to get 1V left over. That will be the voltage across the resistor. To get 20mA you do 1V/0.02A = 50 Ohms.

If your source voltage was 5V then you do 5V - 2V to get 3V left over. 3V/0.02A = 150 Ohms. The guy who suggested 180 was probably running off 5V.
 

Crazylegsmurphy

Active Member
Mouser.com is a good supplier, but Adafruit has much more info on the Neopixel on their website here: NeoPixel Jewel - 7 x 5050 RGBW LED w/ Integrated Drivers


They list it as running at 4-7V. 3.3V might be able to still run the Neopixel but the LEDs probably won't be at full brightness.
If thickness isn't an issue you can get coin cells that are twice as thick and run 6V, or you can stack two 3V coin cells to get 6V.

Regarding the broadcom LED bar, it has a spec of 2V @ 20mA. A 3V coin cell battery often can't provide much more than 10 or 15mA, so in effect the battery is self-limiting and so can't damage the LED despite the higher voltage.

However, the correct thing to do is to put in a resistor. If your source voltage is 3V, then you do 3V - 2V to get 1V left over. That will be the voltage across the resistor. To get 20mA you do 1V/0.02A = 50 Ohms.

If your source voltage was 5V then you do 5V - 2V to get 3V left over. 3V/0.02A = 150 Ohms. The guy who suggested 180 was probably running off 5V.
Thanks.

I may have not been clear. The Neopixels (large ones) are in the Ghostbusters trap and will be running off a large battery. My only concern with those ones is whether the additional 470ohm resistor is needed and why it was recommended at all.

The smaller Sequin ones seem to have their own resistor which means I should be able to hook them directly to the watch battery without issue if I am understanding correctly. (I'm only using two).

As for the Broadcom... I'll show you what I'm attempting.


At the very bottom you can see he just hooks it directly to the battery (only one of the LEDs though). But I see what you're saying, so a 50ohms resistor is a best practice anyway?
 

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zapwizard

Sr Member
RPF PREMIUM MEMBER
If your large battery voltage is between 4V to 7V you don't need to worry about the Neopixels. Neopixels are a self-regulating device that has LEDs, they aren't discrete LEDs.

The Sequin can be directly connected to a battery so long as that battery voltage is between 3V to 6V. I got these specs from the Adafruit product pages.

The Broadcom LED is just a single LED, called a LED bar just do to its larger size. Using a 50 ohm resistor here would be the technically correct option. It can be a tiny 1/8Watt or smaller resistor. However, this guy is probably relying on the self-limiting current that the CR2032 provides (he probably don't even realize this). Since CR2032 cells can't provide much current, the LED probably won't have any issue running without the resistor because the coin cell simply can't provide enough current to blow the LED. In reality the LED in his prop is probably being run at more than 20mA but since it isn't running all the time, the LED can't get hot enough to burn out.
 

Crazylegsmurphy

Active Member
If your large battery voltage is between 4V to 7V you don't need to worry about the Neopixels. Neopixels are a self-regulating device that has LEDs, they aren't discrete LEDs.

The Sequin can be directly connected to a battery so long as that battery voltage is between 3V to 6V. I got these specs from the Adafruit product pages.

The Broadcom LED is just a single LED, called a LED bar just do to its larger size. Using a 50 ohm resistor here would be the technically correct option. It can be a tiny 1/8Watt or smaller resistor. However, this guy is probably relying on the self-limiting current that the CR2032 provides (he probably don't even realize this). Since CR2032 cells can't provide much current, the LED probably won't have any issue running without the resistor because the coin cell simply can't provide enough current to blow the LED. In reality the LED in his prop is probably being run at more than 20mA but since it isn't running all the time, the LED can't get hot enough to burn out.
Excellent. I think I understand now.

I appreciate you taking the time. It was helpful and informative.
 

Crazylegsmurphy

Active Member
The Broadcom LED is just a single LED, called a LED bar just do to its larger size. Using a 50 ohm resistor here would be the technically correct option.

Hey Sorry to bother you again. I was reading and I just want to make sure I understand.


I was looking at the data sheet and unless I'm reading it wrong, the 2300 does seem to have two LEDs. It seems that each LED has a DC Forward Current of 30mA and Forward Voltage of 2.0 - 2.6 V.

I am horrid at math, but would it be?

Battery = 3V

LED 1 = 2V
LED 2 = 2V

... It seems that one battery would not be sufficient to power two. What I couldn't figure out is the minimum volts needed. So if I calculate one LED then it seems to come out to...

Battery = 3V

LED 1 = 2V / 30mA

Resistor = 33.3 Ohm

I think that from what I can tell the closest resistor is 39 Ohm

So something like this ( https://www.mouser.ca/ProductDetail/Yageo/CFR-25JT-52-39R/?qs=KUIzHt/e91l61Sc1v6OXyw== )

...

What would happen if I connected both? Would the two LEDs power up but dimmer? Should I see if 2 smaller batteries would stack to give each LED the 3V?

Thanks!



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zapwizard

Sr Member
RPF PREMIUM MEMBER
I see now that the datasheet calls for 30mA as the current for the -2300 part number, but you can also probably rung it at 20mA also.

Now your getting into parallel vs series circuitry:

If you wire the two LEDs in parallel then they still have a 2V voltage drop. The current will instead be doubled. They will pull 60mA. You can use a single resistor for both LEDs.

If you were to wire the LEDs in series, one into the next, then you add the voltage drop together. So a 4V drop. The current remains the same, 30mA.

If you stick with a CR2032 battery the LEDs probably won't ever be able to pull that 60mA anyways and you won't need the resistor, the battery in effect the resistor. So your probably safe to stick with the original prop design and just wire them straight to the battery.
 

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