JeffPo's LED Flashing/Blinking Marker Light Page
By Jeff Polston: Oct. 5, 2016
I've seen enough requests for devices and ideas on how to mark tripods and such at observing sessions, to keep people from bumping or tripping over them in the dark, that I thought I'd create this article on the subject. While it's been a while since I've done mobile observing, I use to do it all the time. In addition to my telescope setup (generally on a tripod), I'd also have a table for my accessories, and various power cables running to my car (for 12 volt power). It's kind of hard to see those tripod feet or cables in the dark. I'd still bump into, or trip over them, and I knew they were there. Visitors to my setup were even more prone to run into them. I needed a way to illuminate them in such a way that they were easily seen, but such that the light didn't interfere with the observing session. What I came up with was red flashing/blinking LED units.
Here you see a couple of the units I built, with parts from Radio Shack. Very simple to build, and very simple and effective to use.
Red blinking LED
Resistor of proper ohms
9 volt battery connector (hard plastic version instead of soft works better for connecting/disconnecting from battery)
Heat shrink tubing (much better than using electrical tape)
Solder and soldering iron
9 volt battery (easier to work with and you don't need a holder)
I use a red LED because red doesn't harm your night vision. While I could actually use a solid burning one, I use a blinking or flashing one because it gets your attention better. Plus having all those blinking red lights on the ground looks pretty neat.
The difference between a LED and a normal light bulb, with regards to powering it, is that a LED can't generally be hooked up directly to a battery. It needs to be wired in series with a resistor. The resistance needed can be easily calculated with the following formula:
R = (Vbattery - Vled) / (Iled),
where R is the resistance in ohms, Vbattery is the voltage of your battery, Vled is the LED's "forward voltage", and Iled is the "forward current" of the LED. The forward voltage and the forward current can be found on the LED package. Another difference is that the longer of the two wires (leads) from the LED has be connected to the positive lead from the battery. If you wire it the other way around, the LED will not illuminate.
Here's a sample calculation of one LED from Radio Shack. According to the package, it has a forward voltage of 1.85 volts. The forward current is 20 milliamps (0.02 amps). If you were using a 9 volt battery, you would need a resistance of:
R = (9 - 1.85) / (0.02) = 357.5 ohms
The resistance doesn't have to be exact. Just get something close to this range. I used ¼ watt resistors in my projects as they are very common.
I have very poor soldering skills, so if I can do it, anyone can do it. You can trim the leads of the resistor and LED a little shorter if you want too, just keep up with which lead on the LED is longer, and don't make them so short you can't work with them. You can also trim the leads from the battery connector as you see fit. You'll probably be making one longer than the other such that everything is even once it's all connected.
1) Solder one lead of the resistor to one lead of the LED. I do it to the longer lead of the LED so I can more easily keep up with which one needs to go to the positive of the battery. The resistor is bi-directional, so it doesn't matter which way you hook it up.
2) Now attach the battery connector.
a) Solder the long lead of the LED to the red lead of the battery connector. The red lead of the battery connector is the one that connects to the positive terminal of the battery. Since I've already connected the resistor to long lead of the LED, I'm really soldering the red lead of the battery connector to the resistor (again, which is already connected to the LED).
b) Solder the short lead of the LED to the black lead of the battery connector.
Here's a diagram of what your project should look like, prior to adding heat shrink tubing. The LED is that red thing on the right, and the resistor is that dumbbell shaped thing in the middle.
3) First, touch the battery connector to a 9 volt battery to make sure the LED will illuminate. If it doesn't, check your connections. Once you're sure you've got a working assembly, slip an appropriate size and length of heat shrink tubing over the LED and wiring. I leave just a little bit of it covering the base of the LED (see image). The heat shrink tubing will make the assembled flashing light stronger, and protect the soldering joints. While there are special heat guns for shrinking the tubing, I just gently pass it over the top of a lit candle, rotating as necessary. Just enough to get the tubing to shrink up and do its job. NOTE: While you may be tempted to use electrical tape for this, I don't recommend it. Electrical tape is more suited for something that isn't going to be handled much. With this device, you need something more permanent and strong, like heat shrink tubing.
That's it. You're done. Just snap the battery connector to a 9 volt battery and the LED will start blinking/flashing. You'll find that the 9 volt battery will last a very long time. In fact, when a 9 volt is too weak to power other things, it still generally has more than enough power to work a LED (as long as it's not completely drained). Make as many as you need. Make extras.
I use one flasher on the ground for each foot of my tripod legs. It's not really illuminating the legs, but rather where the legs are. Given LEDs are pretty bright, and pretty directional, I tend to point the LED downward so it doesn't "blind" anyone, or distract too much. Works really well in the grass. For cables along the ground, I place flashers alongside the cable, about every foot, to a foot and a half. You don't have to mark every inch of a cable. Once you get two or three flashers on the ground, people know there is something running linearly along the ground, even if they can't see it. The mind will fill in the blank spots.
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