# Just starting making electronics at home

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#### thejuggla1

##### New Member
I did some of this at school, but now that schools out I wanna make stuff at home. Right now I wanna make something very basic(I think) but forgot some of the stuff, and also have a few questions I don't know about.

First off I would like to know how many LEDs I can power with one 220 Ohm Resister, do they need a resistor for each one, or can I get a bunch off one resistor? Or can I do something like I wanna power 2 LEDs, I put in a 110 Resistor?

And how do I go about making these LEDs(9v) be able to plug into the wall? I guessing I need to add some parts to lower the voltage from a normal house plug?

Say I setup 10 LEDs, I want to make them have a dimmer, so I get a Potentiometer correct? Do I put at the where the positive comes in before any resistors or LEDs.

Edit: Another question, say I put 50+ of these LEDs together, will they need bigger wiring because I am drawing more power? What about heat, do I need a heat sink?

Edit: One other question, I came across these LEDs at the electronics store call Ultra Red Blue ect. If I put a 'Ultra' Red and 'Ultra' Blue LED next to each other does this give me UV rays? Or does the 'Ultra' mean something different in this case?

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#### Hero999

##### Banned
First off I would like to know how many LEDs I can power with one 220 Ohm Resister, do they need a resistor for each one, or can I get a bunch off one resistor? Or can I do something like I wanna power 2 LEDs, I put in a 110 Resistor?
Use the flowing formula to calculate the resistor value.

$R = \frac{Vin- Vf}{If}$

Vf is the forward voltage, If is the current and Vin is the supply voltage.

Suppose you have a 9V battery, a green LED with a forward voltage of 2.2V and you need a forward current of 10mA.

$R = \frac{9-2.2}{0.01} = 680$

And how do I go about making these LEDs(9v) be able to plug into the wall? I guessing I need to add some parts to lower the voltage from a normal house plug?
Use a mains adaptor (Wall wart) to convert the mains voltage to a safe DC voltage such as 12V.

Beware that unregulated mains adaptors give higher voltages at lower currents so expect >20V from an unregulated 12V adaptor at light loads and take it into account when calculating the resistor value. Your best bet is to measure the voltage with a multimeter.

Say I setup 10 LEDs, I want to make them have a dimmer, so I get a Potentiometer correct? Do I put at the where the positive comes in before any resistors or LEDs.
It doesn't matter whether the potentiometer comes before or after the LEDs. Look up Ohm's law, the current in all parts of a series circuit is equal.

Using a potentiometer isn't the best way of controlling the brightness a variable constant current source of PWM is better.

Edit: Another question, say I put 50+ of these LEDs together, will they need bigger wiring because I am drawing more power? What about heat, do I need a heat sink?
That depends on the current, if each LED is only 10mA then it's only 500mA so the wire can be thin, if each is 300mA then the total current will be 15A so the cable needs to be thicker.

Edit: One other question, I came across these LEDs at the electronics store call Ultra Red Blue ect. If I put a 'Ultra' Red and 'Ultra' Blue LED next to each other does this give me UV rays? Or does the 'Ultra' mean something different in this case?
No, it just gives you magenta or purple light (depending on the mix).

Look up the electromagnetic spectrum and you'll find that UV is radiation with a wavelength shorter than about 400nm. The red LEDs emit radiation of about 660nm and the blue emit radiation of about 450nm.

Ultra is just a marketing term and means nothing.

You can buy UV LEDs but they cost more than normal LEDs.

#### Hero999

##### Banned
Here's a simple variable constant current circuit for varying the brightness of an LED.

The maximum current is about 20mA.

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#### thejuggla1

##### New Member
Do you know how much UV LEDs will cost?

"Suppose you have a 9V battery, a green LED with a forward voltage of 2.2V and you need a forward current of 10mA.

R = \frac{9-2.2}{0.01} = 680"

Kinda confuses about this, I can get a LED to run off any Voltage as long as I have the right resistor?

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#### Hero999

##### Banned
Kinda confuses about this, I can get a LED to run off any Voltage as long as I have the right resistor?
No the voltage needs to be greater than the LED's forward voltage. If you try the formula with a lower voltage you'll get a negative resistance which is impossible in reality.

#### thejuggla1

##### New Member
No the voltage needs to be greater than the LED's forward voltage. If you try the formula with a lower voltage you'll get a negative resistance which is impossible in reality.
So using your example LED above(a green LED with a forward voltage of 2.2V and you need a forward current of 10mA), it is safe to plug an LED into a into a house socket if I give it 11780 Ohms of resistance?

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#### im_in_asia_now

##### New Member
So using your example LED above(a green LED with a forward voltage of 2.2V and you need a forward current of 10mA), it is safe to plug an LED into a into a house socket if I give it 11780 Ohms of resistance?
The AC voltage straight out of the wall shouldn't be used directly with just a resistor to light LED's, you'd at least need a diode as well to block negative voltage from the negative cycle of the AC source. The LED is a diode, but it's not designed for blocking negative voltages quite like a regular diode is. Your talking about AC, so the voltage and current are oscillating between positive and negative at 60 hz.

120 V is the rms voltage of electricity from the wall, so when you do a calculation using the current in the circuit- you used 10 mA- that is also the rms current. The peak voltage from the wall is 120 / sqrt(2) or about 170 V, which is something you would want to consider when making any calculations. In this case, the peak current would be 10 / sqrt(2) or about 14 mA.

You could just use some batteries or a wall wart (AC-DC converter) like somebody else suggested, that would make it a little more basic. Plus I wouldn't really recommend using 120 V AC for your first home experiment anyways.

#### Torben

##### Well-Known Member
I second what others have said: use an old wall wart to power your projects, at least up until you've built your first few linear power supplies and understand some more of the issue involved.

Just connecting the LEDs directly to the mains would be a bad idea for the reasons given by im_in_asia_now, plus it would be dangerous since there would be no electrical isolation--which the wall wart would provide since it includes a mains transformer. Plus you don't have to use a honkin' huge resistor to waste all the power you're not using in the LED.

Regards,

Torben

#### thejuggla1

##### New Member

I gotta cut the end off revealing 2 wires? Is one positive and negative, or does that not matter? Do I have to use that or can I use other things such as my phone charger(It says input 100-240v @ .2A output 4.8V @ .9A) How many LEDs could I power off it? I'm looking to power really powerful red/blue possibly UV or other colored LEDs recommendations on those also?

Is this site reliable ULTRA VIOLET LED, 395 NM 30 DEGREE | AllElectronics.com ?

Would you be able to link me the best Blue Red and UV Leds?

Is this adaptor the one I want? http://www.allelectronics.com/make-a-store/item/DCTX-4175/7.5-VDC-TO-9-VDC-WALL-TRANSFORMER/-/1.html

How do I tell how many LEDs I can power with that?

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#### agis68

##### New Member
of course u have to consider the amp consumption of these leds....a common led of 1,7 to 2,2 V needs at least 0,5A to work... The problem with multiple leds is the amp consumption. Common leds work in 1,7 V and 0,3-0,5 Amp
but a banch of them they gonna consume far few Amps....

#### Hero999

##### Banned
If you're connecting many LEDs, it's better to connect as many of them as you can in series rather than parallel.

For exampl if you're using a 9V supply and the LEDs have a forward voltage of 3.5V, if you need four LEDs, it's better to connect them as two strings of two in series.

In this case the supply is only 4.8V so you don't have the option of connecting more than one 3.5V LED in series because it would exceed the voltage of the supply.

#### kishoretvk

##### New Member
hello guys i'm doing a simple circuit on low voltage battery beeper
i didnt under stand wat to do with the explation
can any one please give me the actually working of my project

my circuit is here below please do help me i need to give a seminar

Battery Low Voltage Beeper

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