How Do You Hook Up A Single LED To A Solar Panel Without Dimming

I am just a newbie when it comes to leds. What I have is a max 4vdc solar panel. I have shined every light I have on this panel and seen a max output of 3.96vdc. I would like it to power a single red led with a 1.7 forward voltage and 20mA. I would like to protect my led because. I have both indoor and outdoor uses for this application. I figure just a simple resistor will only limit the extreme side of the input. But once I take it indoors there will be too much resistance on the led to produce any light. I need an alternative. I understand that once the solar panel output is dropped a little below the 1.7v loose the light from the led and this is exactly what I want. To sum up, I need a way to light an led using a solar panel from low and high light sources with out damaging the led. Any help would be great. Thanks

tech-king10 years ago
ditrex: is the led used for illumination or as a light level indicator? my suggestion if the led is an illuminator: use a zener diode as a voltage regulator nadsoft: either attach the pannels to a board and attach the board to the roof, or perhaps lay down a layer of non-conductive silicon and attach the pannels to that.
diterex (author)  tech-king10 years ago
I want to use it for illumination purposes. I am not too familiar with the zener diodes. How would it be wired up? In series or parallel with the led? The smallest voltage I could find was 2.4v. Also Now that being said the led specs say it is typical 1.7v but max of 2.4v. I don't want to greatly shorten the life span.
Patrik diterex10 years ago
Yeah, Zener diodes below 3.3V are hard - but not impossible - to find. Here are a two sites that carry 1.8V zener's:

Mouser Electronics
Digi-Key

When you connect a diode "in reverse" (i.e. connect the - side to positive voltage and vice versa), it will not conduct electricity - that's really the whole point of a diode, right? However, increase the voltage high enough, and at some point the diode will "break down" and act as if it's shorted out.

In a Zener diode, this "breakdown" happens very abruptly, at a well-defined voltage, and is reversible. That means you can use such a diode - wired in reverse for what you would expect for a normal diode - as a simple voltage regulator.

In your case, you would put a 1.8V Zener diode in parallel (but pointing in the opposite direction!) with your 1.7V LED. If the output of your solar cell gets above 1.8V, the Zener diode in parallel with your LED will start to conduct, and keep the voltage across the LED from rising above 1.8V. If your solar panel is small enough you probably don't even need a limiting resistor in series.

Extra credit challenge: At 4V, we should be able to run two LEDs in series off this solar panel. Design an energy-efficient circuit such that only one LED is lit up when the voltage is low, but a second one lights up when the voltage gets high enough...
nadssolf10 years ago
I can't answer your questions directly, but since we're asking "How-To's"...what's the best way to attach solar panels on metal roofs?