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Old Nov 23, 2012, 04:02 PM
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United States, OH, West Unity
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Help!
relay connected to rx

I found some led flashlights on sale at dollar general. They have 9 leds, and a cylindrical, removable 3 AAA battery holder.
I want to use two of them on a project buggy (at least the internals)
I figure if I used the guts from them, including the battery holders, I wouldn't have to worry about resistors and such.
So, I could obviously use a mircro servo and a mircro switch. I looked into using a servo as an LED controller, but I don't want the servo to power the lights.
So my idea was to modify a servo as a light controller, and instead of it powering the lights, have it activate a relay, with the relay completing the circuit between the lights and the battery holders.
Does this sound feasible? I'm pretty sure it will work, but I wanted to check first. and the big question...what kind of relay do I need?
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Old Nov 23, 2012, 04:32 PM
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Here's a commercial solution. You could spend this much rolling your own before you're through, and it may or may not work well.
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Old Nov 23, 2012, 06:31 PM
Stuart
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Quote:
Originally Posted by poomwah View Post
I figure if I used the guts from them, including the battery holders, I wouldn't have to worry about resistors and such.
Without the resistors, what are you going to use to limit the LED current ?
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Old Nov 23, 2012, 08:03 PM
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Quote:
Originally Posted by poomwah View Post
So my idea was to modify a servo as a light controller, and instead of it powering the lights, have it activate a relay, with the relay completing the circuit between the lights and the battery holders.
Does this sound feasible? I'm pretty sure it will work, but I wanted to check first. and the big question...what kind of relay do I need?
I don't think you need a relay. When a servo motor rotates one way, one of the motor leads, call it "A", is connected to + voltage and the other lead, call it "B", is connected to ground. When the motor rotates the other way, "A" is connected to ground and "B" is connected to the + voltage. This happens through transistors that I believe ought to be able to handle the current of your LEDs.

To make the servo PCB operate your LEDs, disconnect the servo motor and remove it, and substitute a voltage divider for the potentiometer so that the servo PCB always detects that the servo is centered. A pair of 2.2k, 2.7k or 3.3k, 1/8 watt resistors should do it.



Connect the positive side of the LED assembly permanently to the model's positive battery voltage and the negative side to either of the motor terminals of the Servo PCB. You should be able to turn the lights on and off with a switch on your radio. If the switch works backward then reverse the channel in your radio programming.

I can't guarantee that this will work, but if you use an old servo with stripped gears then you're really not risking much by trying it.
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Old Nov 23, 2012, 09:43 PM
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United States, OH, West Unity
Joined Oct 2007
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srnet, I figure the led array inside the flashlights must already have the proper resistors for the voltage and current that the batteries in the holder would provide.
Its not that I was trying to eliminate resistors, just that i was trying to avoid having to add any.

Mike,
I see how the servo would work to power the leds. The problem I'm seeing is that I don't want to draw my power from the servo, I want to have a separate battery for the lights. If I use a pair of the flashlghts, it would be a total of 18 led's. 9 in each array. I'm really doubting that the servo would put out enough juice for that many.

and now the more I think about it, a relay would be a pain in the butt to try to drive from a servo, since the power is constant and the only thing that changes is the polarity, there would be nothing to trigger the relay would there :\

I wish I had thought about this BEFORE I blew my budget on other things :P but they are fun things , hehe (new radio, and my first brushless system EVER)
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Old Nov 23, 2012, 10:59 PM
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Quote:
Originally Posted by poomwah View Post
Mike,
I see how the servo would work to power the leds. The problem I'm seeing is that I don't want to draw my power from the servo, I want to have a separate battery for the lights. If I use a pair of the flashlghts, it would be a total of 18 led's. 9 in each array. I'm really doubting that the servo would put out enough juice for that many.
You could still have a separate battery for the lights if your two batteries share a common ground, but if you want to use a relay then that's fine too.
Quote:
and now the more I think about it, a relay would be a pain in the butt to try to drive from a servo, since the power is constant and the only thing that changes is the polarity, there would be nothing to trigger the relay would there :\
You can easily drive a 5 volt relay with a servo PCB. I've done it. Connect a 1N4004 diode across the relay coil to prevent reverse voltage from damaging the servo PCB, connect the cathode side directly to +5 volts (or +4.8 volts), and the anode side to either of the motor connections on the servo PCB. The other motor connection will remain open. I guarantee that will work fine. A good relay to use is the one I use for my Winch Solenoid Safety Buzzer. It's Radio Shack part #275-232.



You could also order one of these for $7.14 plus shipping.

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Old Nov 23, 2012, 11:14 PM
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United States, OH, West Unity
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I almost got one of those turnigy switches last night, but they were out of stock.
maybe I will just wait until the are available again.
I appreciate the other ideas you offered, but I'll probably mess them up, LOL
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Old Nov 24, 2012, 01:09 PM
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I tried using the servo to drive the led arrays.
It had enough power to light both arrays , a total of 18 LEDs. Super bright.
unfortunately though, there are no resistors in the flashlights. I figured that since they were using 4.5 volts that they'd be safe to run off the servo. well, I already burned out three LEDs :[
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Old Nov 24, 2012, 02:17 PM
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I connected a 100 ohm, 1/8 watt resistor to each LED.
http://www.ebay.com/itm/1-8W-Watt-10...-/221054491270
no burnouts.
18 5mm leds on my ugly stick wing.
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Last edited by rick.benjamin; Nov 25, 2012 at 08:13 PM. Reason: spelling
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Old Nov 24, 2012, 02:31 PM
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United States, OH, West Unity
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thanks rick, that's a quite economical solution.
The only complication is that the lights will be in "pods" so there isn't much room behind the LED's, so I couldn't attach resistors directly to the LEDs. They are currently soldered to a board, wired in parallel. Could I use one resistor further up in the power line instead of one for each LED? If so, what value should the resistor have?Since they are in parallel would I still use a 100 ohm but in a higher wattage?
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Old Nov 25, 2012, 10:20 PM
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A why and how article about current limiting with a led
http://tinkerlog.com/2009/04/05/driv...ut-a-resistor/
Some figures regarding 5mm water clear white LEDs
http://www.electronics123.com/s.nl/it.A/id.2334/.f
Here's a calculator. Plug-in the figures.
http://led.linear1.org/led.wiz

The Wiz and I think a 1 watt 68 ohm resistor will dissipate the load from 9 leds.
Ebay sells packs of 100
http://www.ebay.com/itm/100-1W-1-wat...-/150561660834
If you are somewhat near a Radio Shack, if they have them, they sell them in packs of 5, call first.
Parts Express has packs of 10 ($0.85), shipping is $4.
http://www.parts-express.com/pe/psho...003-68&scqty=1
Mouser (my favorite for components) $0.09 each, shipping $6.95
http://www.mouser.com/ProductDetail/...qqr20MLkPx0%3d
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Old Nov 25, 2012, 10:38 PM
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LED array with one resistor
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Old Nov 26, 2012, 05:25 AM
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Thanks Rick, I really appreciate it.
I'm doing something wrong with the wizard, I just wanted to check what it said for 18 leds (since I have 2 arrays of 9 each) and its showing 18 resistors :[
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Old Nov 26, 2012, 04:36 PM
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You're doing good.
I altered the 2nd pic (post 12) to show 1 resistor.
Wiz suggests that total power is 820.8 mW.
I rounded the watts to the next size resistor so it could handle nine LEDs.

18 LEDS on one resistor need an even bigger resistor as total power is Wiz'd at 1641.6 mW.
Next size resistor would be 2 watts.
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Old Nov 26, 2012, 04:56 PM
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If your diagram in your post #11 is correct then your diagram in post #12 can't possibly be correct. Assuming that the diagram in post #11 is correct, the single resistor in the diagram in post #12 would have to be 68 / 9 = about 7.5 ohms for the current through the LEDs to be the same in both circuits.

If you have trouble seeing this, consider the diagram below, where a common connection is added between all of the junctions of LEDs to resistors. Because of the symmetry, it's clear that the circuit wouldn't be affected by the added connections, yet what you'd have on the right half is nine 68 ohm resistors in parallel for a total resistance of 7.56 ohms.
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