Viesor Labs

Please, do not expect anything serious from this post. This project was more a practical joke than anything usable more than once.

Last spring we were heading for Virginia to camp. At the last moment the preparations revealed an absence of any flash light in our gear. A decision was made to build one. Here is what was available:

1. Boat engine starter battery (enough for about a week of continuous operation ) 8V
2. High intensity LEDs - 10 pcs.
3. Power cord
4. Electrical switch
5. PVC pipe (generally used for plumbing)
6. Metallic candy can (would serve as a reflector)

First the LEDs were inserted in the can using some random pattern of my choice. No particular logic was used -  just something that would be able to shine through the aperture of the candy can lid. In addition, four smaller holes were drilled in order to fix the “reflector” on the top of the potential flash light handle.

Connecting the LEDs

This part could be done in numerous ways.  Most of people know, that, in general, electrical circuits can be connected either in parallel, in series, or something like combination of the two. Before I continue, I’d like to give some information on the important LED electrical characteristics. I am not going to touch on the intensity and view angle characteristics as those can be understood without clarification.

When one designs a circuit containing an LED (light emission diode) he should posses the following information: available power (voltage and current limit), forward voltage for chosen LED, maximum forward current for chosen LED.  I should add that it all applies for designing slow frequency circuits. When frequency goes into kHz or even MHz ranges things become different. However, for a typical “indicator” application those frequencies are not useful (human’s eye is not capable of seeing refresh rates that high), unless, one is working with PWM (pulse width modulation) intensity control.

Forward Voltage - is a voltage drop (V) developed on the LED when it is connected in series with a resistor (R). See picture above. One can find it in the data-sheet for a particular LED. It is dictated by the physics of a charge motion in the semiconductor junction. This voltage ideally is independent of the battery voltage in the operating range. However, if the battery voltage drops lower than the Forward Voltage the latter will drop as well.

Forward current - is a current that flows through an LED during operation. This is the most important parameter as exceeding the maximum value of which can and most likely will damage LED permanently.

I have mentioned that knowing power source voltage and current limits was important for a simple reason that if your battery voltage was lower than Forward voltage - you would never be able to reach full intensity using it. Same applies to the current.

Back to the project …

A choice had to be made about what connection to go with for the flash light project. Forward voltage of the LEDs I had was on the order of 3.2.V, battery voltage was 8 V. The best choice was to connect two LEDs in series with one resistor. Why?! Because if I connected only one LED in series with the resistor then the voltage drop on the resistor would be 8V-3.2V=4.8V which multiplied by forward current would represent the amount of power just being wasted for no reason. In other words, the efficiency would be extremely low. However, when you connect two LEDs in series you get only 8V-2×3.2V=1.6V which is 3 times smaller.

Knowing that the maximum forward current for the LED was 30mA I chose 25mA just to be safe. Because the current is constant in every part of a circuit parts of which are connected in series, all that I had to do was to divide 1.6V by 0.025A to get 64 Ohms resistor value. The closest standard resistor value is 68 Ohms.

Finally, I connected each of 5 pairs of LEDs to their own “buddy” resistor and, then, all five in parallel. In other words, five circuits, consisting of two LEDs and one resistor each, were connected in parallel through a switch to the battery terminals.

Flash light Housing

After a PVC pipe was cut to a necessary length a hole for the switch was cut out and a cable strain relief was constructed by simply drilling two holes and passing the cable trough them - friction lock.

Next, all aforementioned electrical connections were finished, battery terminals were soldered, and the “handle” was bolted to the “reflector”.

Finally, some silicone was applied to make everything more hermetical.

Connection to the battery …

et voilà … hit the switch :D

Basic calculations (more like assessment) showed that the constructed flash light was capable of shining light for 7 days straight which was way more than we hoped to camp for. The biggest inconvience that was revealed by the actual use of this “creativity masterpiece” was a heavy battery and a poor reflector - no woder no one is using candy cans in manufacturing of flash lights!

Tags: camping, flash light, joke, led