Author Topic: Illuminated Switch / Fire LED on OKR/OKL/Raptor Mods (Read 7917 times)

werkkrew · « **on:** July 30, 2014, 09:33:00 AM »

Hey all -

I have a question about how to correctly wire a fire led or illuminated switch on an OKL/OKR/Raptor based circuit. The LED will light up while the unit is being fired.

When using a circuit with a non rated switch there is not enough power coming across the switch to make the LED light at full brightness, in my particular setup (OKL2-T/20 w/non-rated switch behind a 5.6v Zener) the LED is extremely dim.

On my breadboard I tried wiring the switch LED segment between the (+) of the 510 and ground and it lit up great but that was without any load on the circuit. I'm afraid with a load it might burn out the LED but I'm not willing to try since the buttons are a little pricy (about $11 each).

Here is the specific button I am using: http://www.digikey.com/product-search/en?KeyWords=PV6F240SS-331&WT.z_header=link

Thanks in advance!

CraigHB · « **Reply #1 on:** July 30, 2014, 02:29:13 PM »

You have to switch battery power to the LED. You can't drive an LED off the atomizer output since the voltage range is too wide. You'll either burn out the LED at higher voltages or fail to light it at lower ones.

Also, an LED must have a bias resistor or more simply, a resistor in series with the LED. Without one, the LED will burn out with a voltage much over 2V applied between the leads.

If someone doesn't offer some help, I can give you some if you post the circuit you are using. I don't recall the circuit off-hand.

werkkrew · « **Reply #2 on:** July 30, 2014, 02:36:15 PM »

Thanks!

Here is the circuit I am using, most of it was courtesy of Mamu with some tweaks of my own. The fire LED is actually built into the switch but I moved it out on my diagram and it is not currently showing as connected to anything, hence this thread.

http://i.imgur.com/64sEIHG.png

CraigHB · « **Reply #3 on:** July 30, 2014, 02:57:30 PM »

Nice drawing, I think that's one of the cleanest I've seen on the forum here.

Anyway, your problem is you do not have access to switched battery power. You've got a Zener on one side of the trigger switch and a 1k pull down on the other side of the switch. You need an unobstructed path to battery positive through the switch to power the LED.

What you can do is swap the position of the trigger switch and Zener to gain direct access to battery positive through the trigger switch. So after swapping the position of those two components, run your positive LED/bias resistor to the switch terminal and the negative side to ground.

Check your switch can handle the load current for the LED.

220 Ohm is low for 6-8.4V. It should be more like 470 or 680 Ohm. You should put an ammeter inline with your LED to verify you are not overloading it at maximal battery voltage. LEDs are current driven devices and they can not be driven past their maximal current tolerance. Usually its 20mA, but it can vary. Check your data sheet.

If you want to take it a step further, you can add a linear regulator to maintain constant voltage and thereby a constant brightness at the LED regardless of battery voltage. Use something like a 5V or 3.3V LDO (low dropout linear regulator) with a properly valued bias resistor, it will be somewhat lower with a lower regulated voltage, probably that 220 Ohms you're showing.

werkkrew · « **Reply #4 on:** July 30, 2014, 03:14:39 PM »

The switch is rated at 2A so that shouldn't be a problem with the load from the LED.

According to this site, and using a (confirmed on data sheet) 20mA current on the LED, the resistor value I would want for a red LED @ 8.4v would be 330 ohm.

If I am understanding you correctly the finalized circuit would look like this, correct?

http://i.imgur.com/BR7mdwL.png

Is the direction of the zener correct? Does this layout maintain the switch as having a non load rated requirement (aside from the load of the LED itself)?

CraigHB · « **Reply #5 on:** July 30, 2014, 04:02:06 PM »

That should do it.

There's actually a number of factors that determine the optimal value for a LED bias resistor. First, that switch may have series or parallel LEDs that are going to make a large difference there. You have to look at the data sheet to see what's going on with that.

Also, various LEDs have various voltage drops for a given current. Then there's the supply voltage and maximal loading for the LED which varies. It's possible to calculate all that stuff on paper and I do on occasion, but you really have to do a current check in-circuit to verify the value you select.

Checking currents should not be a big deal, any decent DMM is going to provide an ammeter. If you don't have one, then I highly recommend you spring the $50 for a decent brand name DMM.

Any component you put in a circuit is going to have load considerations. Even with the 1k pulldown, you have a load on the Zener that is batt voltage minus breakdown voltage divided by 1k times breakdown voltage. For example, the load on that Zener for worst case is 8.4V minus 5.6V divided by 1k times 5.6V which is about 16mW. Not a big amount, but very small SMD Zeners may only be rated for 50mW or 100mW.

The 1k resistor sees 2.8mA worst case so power consumption there is 2.8mA squared times resistance which is about 8mW, no where near the 250mW a typical through hole resistor can tolerate, but still there's a power consumption there. Small SMD resistors can tolerate as little as 50mW so if the voltage was 8.4V, current would be 8.4mA and power would be about 70mW overloading a resistor like that.

You always have to consider power ratings for any component you use except when using large resistances that are obviously not going to result in very much current flow. For me, I break out the calculator for any resistance under 10 kOhms.

werkkrew · « **Reply #6 on:** July 31, 2014, 10:18:15 AM »

I think there was an error in the last diagram I posted, I haven't built the circuit yet but I believe that for the LED to light when the mod is fired it needs to look like this:

Visus · « **Reply #7 on:** August 01, 2014, 08:20:47 AM »

Nice schematic werk, ya nc or no may make huge difference

gonna save that one too
you guys so squeamish with the spec sheet that mofo will do 20.2A thru ~32A depending on -2% failure load lol
Ya that one is on edge already --very efficient board for our use stays above 93% between 3.3v --5v

vapeviking · « **Reply #8 on:** April 07, 2015, 07:31:13 AM »

I've just finished my first build and I'm currently researching my next build:

I was thinking of using the same switch. Am I correct in assuming the bias resistor need to be connected "in front of" the zener? Should it be connected to the On/Off pin or the Vin pin? Should I ground it to the board or the battery sled?