I received some LED buttons for use with my arduino kitchen timer from aliexpress today.
They were sold as "5V" buttons and several other voltage ranges, turns out that choice just means they give you different resistors along with the buttons.
I have no datasheets for them, so I want to be on the safe side.. can anyone think of a reason why 3.8k would be needed to power the LED in the switches from 5V? Normally I'd go for 100-200 ohm for blue LEDs on 5V, is there any reason not to do so here?
What happens when you use the supplied resistor?
Steve
The LED lights up, although I feel it's fairly dim. Using a 220ohm resistor, it's twice as bright or so.
Are you sure that resistor is 3.8K? That's a very odd value, not in any of the standard series that I know.
Steve
Seems like a reasonable value, readily available at Digikey (chip resistor). Maybe better with 12V than 5V tho.
(12V - 2.5V)/3800 ohm = 2.5mA
3.8 isn't in any of E12/E24/E48/E96/E192
If its a stock part not in the standard values that typically means its a commonly used value in some specialized circuit or chip, or is in a simple ratio with a standard value (for instance 5,50,500,5k... are widely available non-standard values)
3k8 is available it seems, no idea why, no obvious ratio relationship to standard values.
As for the original question - who knows, perhaps they were the cheapest in Shenzhen market at the time?
slipstick:
Are you sure that resistor is 3.8K? That's a very odd value, not in any of the standard series that I know.Steve
I agree. How did you measure it? It's probably a 3.9k
Considering it's likely a 5% or 10% tolerance part, and it measures as 3.84 and 3.85kohm for each, I took a guess at the actual value. 3.9kohm sounds plausible then.
I'm somewhat colorblind, so checking the stripes isn't a good way for me to determine it.
Most modern LEDs are much too bright at their maximum current (usually 10-20mA). I drive an LED in a commercial product I make, with 1.2mA and it's almost too bright.
In this case, bright is welcome. The LED lights up a ring around the base of the push part, so it's a fairly passive light, and since it will indicate whether the timer is running, it should be well visible. Using the 3.9k resistor makes it light up just barely, not enough to act as an indicator.
arakon:
Considering it's likely a 5% or 10% tolerance part, and it measures as 3.84 and 3.85kohm for each, I took a guess at the actual value. 3.9kohm sounds plausible then.
I'm somewhat colorblind, so checking the stripes isn't a good way for me to determine it.
Even if you have good colour vision the metal film resistors with blue bodies are hard to read as the blue
shows through making it hard to distinquish black/brown, brown/red and red/orange reliably...
390 ohms would be pretty common as a current limiting resistor, and might be easily confused with 3800.
(that's a white vs gray band, and brown vs red, right? And it would still get brighter with 220 ohms instead.)
Except I measuered them (out of circuit, of course), unless you mean the guy packing them mixed them up.