Keith 737 mip

[jph]

Hi Keith, great stuff.
The image you tried to insert does not show though.
Joe

[Keith Baxter]

Hi Keith, great stuff.
The image you tried to insert does not show though.
Joe

Joe,

I posted the incorrect one so I edited with the correct one.

Keith

[jph]

Hi Keith, how are you calculating a 90 ohm resistor ? - and at what wattage ? - I cannot correlate that.
Joe

[Keith Baxter]

Hi

Prototyping. Waiting for led tactile switches.

Keith

[Keith Baxter]

Hi Keith, how are you calculating a 90 ohm resistor ? - and at what wattage ? - I cannot correlate that.
Joe

Wow Joe you picked that up.

I have not gotten the led tactile switches yet as the colors have different forwarding voltage.
I assume the wattage is 20mA

I used a forwarding voltage of 1.7 X 6 = 10.2
12v - 10.2 == 1.8
R = U / I
1.8/0.02 = 90Ω

I will do the same for the 7 nested and single LED's

I will also test the brightness before I go soldering in the resistors.
I am going to use 0.5 watt resistors. Mainly for the heat.

Keith

[jph]

Hi Keith,
You have, mostly, the resistors in parallel, so if the forward voltage is, say, 2.0V, and the desired current is 15mA, then - presuming a supply of 12v, we get 12-2 = 10v desired drop, and therefore R = 10/0.015 = 666 ohms.

Wattage of the resistor would be 0.015 x 10 = around 150mW
Joe

[SimPassion]

Hi Keith @Keith Baxter, really interested in if you will deal with leds strip behind tactile switches and if yes how you will perform it, or rather with ready made tactile ones already embedding the leds ?

[Keith Baxter]

Hi Keith,
You have, mostly, the resistors in parallel, so if the forward voltage is, say, 2.0V, and the desired current is 15mA, then - presuming a supply of 12v, we get 12-2 = 10v desired drop, and therefore R = 10/0.015 = 666 ohms.

Wattage of the resistor would be 0.015 x 10 = around 150mW
Joe

Then the AM wiki is wrong.
This is what you say.

Determine the voltage and current needed for your LED.
We’ll use the following formula to determine the resistor value: Resistor = (Battery Voltage – LED voltage) / desired LED current.
For a typical white LED that requires 10mA, powered by 12V the values are: (12-3.4)/.010=860 ohms.
To use several LEDs in parallel, sum the current values. From the example above, if we use 5 white LEDs the current requirement is 10mA x 5 = 50mA. So (12-3.4)/.050=172 ohms.

The AM wiki

Selecting your components
When an LED is conducting, it has close to zero resistance. This means that when the LED is conducting, it creates a shortcircuit. Therefore you will always need to use a resistor in series with the LED, or you'll destroy it. And that is where it gets a little bit tricky... LED's also have something called 'forward voltage'. This can be seen as the voltage that is 'lost' on the LED. A usual forward voltage is 1.7 volts, but this varies between LED's and I advice you to look it up in the specifications document of your LED. But lets say that your LED has a forward voltage of 1.7 volts, a typical (forward) current of 20 milliamps (mA), and we drive it with 5 volt from an Arduino or Raspberry Pi. To find out what resistor to use, you first subtract the 1.7 volts from the 5 volts:

5 - 1.7 = 3.3

Now we can use the formula R = U / I (resistance is voltage divided by current), and this makes:

3.3 / 0.02 = 165

We would have to use a resistor of 165Ω (Ohm). But since a 165Ω resistor does not exist, we round it up to the closest resistor value available, which is 180Ω when following the E24 series (E-series).

Placing LED's in series
Now the next explanation is optional, but it is certainly interesting for you to know. You can also place LED's in series, this can save you a lot of components when you have to drive multiple LED's together. With 5 volts you cannot do much, but with for example 12 volt, you can have up to 7 LED's in series (1.7 volt forward voltage). Let's use the 5 volt from the previous example. If we divide 5 by 1.7 we get 2.9. This means we can place two LED's in series (always round to the lowest number). With two LED's this creates a forward voltage of 3.4 volt (1.7 x 2), and if we do the same calculation:

5 - 3.4 = 1.6

We use the same R = U / I formula:

1.6 / 0.02 = 80

Now we will have to use a resistor of 80Ω. This one also does not exist, so we use 82Ω.

Keith

[Keith Baxter]

Hi Keith @Keith Baxter, really interested in if you will deal with leds strip behind tactile switches and if yes how you will perform it, or rather with ready made tactile ones already embedding the leds ?

Gilles,

It is easier to use these. I have 200 yellow on order.

https://fr.aliexpress.com/item/32762591 ... pt=glo2fra

For the panels I will use the strips that Joe talks of.

Keith

[jph]

Hi Keith, no worries,
Plug it in and try it. you could be right.
Joe