LED's come in all shapes, sizes and colors. And, they draw different currents for the different colors in the same size LED.

I don't know if you'll light any LED's with a 1/2 volt.

Most fun way to play with this is buy some LED's in the size you're intested in, get a pkg. of alligator clip leads and a 25k wire-wound potentiometer from Radio Shack and power them up with a 9 volt battery. Put an AMMETER in series with it to measure current as well as recording the voltage needed to produce the light intensity you want.

The packages usually tell maximum current draws but you must test it for yourself. LED's begin glowing with little current draw but increases dramatically as you approach their maximum voltage application.

I'm an HVAC controls guy and I use a lot of LED's with a 1.2 k 1/2w resistor in series with standard JUMBO LED's applied to typical 24 VAC heating systems control circuits. They never burn out.

I drill my holes for the LED's to stick thru, solder a resistor onto one leg, connect the wires and then "pot" the whole thing in with a hot glue gun, works great!

you can light LEDs with 1/2 volt, but you must use a charge pump.

This charges 4-5 capacitors in parallel to .5 volts and then switches them into series to give 2.5 volts, which it then runs into a capacitor to store it as 2.5V and the LED runs from that. Some white leds take 3 or more volts so you need a different string.

There are ICs made to do this.

You can also have a small switcher made for this small task. google for both of these

There are no LEDs on the market that can be driven from 0.5V. The normal way to handle this is to use a boost power supply in constant current mode to drive the LED directly with (no current limit resistor). The output voltage will be the LED voltage rating which is determined by the type of LED and to a much lesser degree the constant current value. Your power supply voltage has little meaning in this scenario BUT: LEDs range from 1.2V - 5V (there are greater than 5V LEDs aswell). Depending on the LED you will require 10mA - 350mA with the average blue/white/red LEDs requiring 20mA. BUT, it is very difficult to design a boost power supply utilizing a 0.5Vsupply because most boost supplies require 1.2v or greater at their input. Additionally, consider that if you could use a bootstrapped design to provide the boost power supply's required voltage, from its own output, then in order to go from 0.5v to say 3v for a singleblue LED at a constant LED current of 20mA then, not accounting for efficiency, you would draw 120mA from your 0.5V supply to power the single LED. If your 0.5V is from a small solar cell you don't have much hope of developing 20mA in full daylight let alone 120mA. I can't imagine many other sources of 0.5 volts. Now if you account for efficiency and you get say 80% efficiency from your supply then additional current is drawn from the power supply to make up for the 20% losses. The 120mA would be greater than 144mA + the current to run the boost converter. So for each LED driven, 144mA would be drawn from the 0.5V supply. If you could find a bootstrapped boost converter IC could your 0.5V power supply handle the current draw? If so, search the internet for constant current LED driver power supply schemes and use the power supply's data sheet to adapt the scheme to the power supply you've located. Good Luck

John

The boost supplies that use FETs work at .5 volts, as they are not prey to the .7 volts drop of a silicon junction. To get to 3.5 volts from .5 will need a number of stages and you reach a point of diminishing returns when you start with .5 volts.

http://www.maxim-ic.com/appnotes10.cfm/ac_pk/20/asc_pk/117

http://www.google.com/search?hl=en&q=%22charge+pump%22+%2B%22power+supply%22+%2B%22low+voltage%22+%2B.5v&btnG=Search

Just a question about your power source. 0.5 V sounds suspiciously like the voltage output by one silicon solar cell at full illumination. Is that your power source?

-e

I asked a friend of mine in the LED business. He said he had not seen a charge pump that would work on .5 volts, so it might not be out there as a product.

If it is a silicon source, you might be better making it out of three cells to get 1.5 volts as there are lots of charge pumps that work down to .9 volts

Thanks for the Input from all respondents, I have to admit to a little trickery in that I have already solved and patented a device that addresses this problem. In fact it appears that my circuit works down to + or - 0.35 Volts and will drive any colour LED and an IR type in series for each possible polarity of current. Thus it has the added bonus of being an LED woring in the current-mode and almost invunerable to over-current and over-voltage while producing volt-free signals as well.

In all this device behaves as almost solid-wire that radiates light (any colour & the I,red part of an optocoupler *2 for each +/-) on the presence and polarity of a current over the range of 3mA to 10 Amperes ac or dc. In so doing it makes electric current in a single wire something to be viewed by the eye and sensed with a high degree of isolation.

There is much more to say on how this device can be employed but for now please just consider how useful such a device would be? intermittent faults can be eyeballed for instance, multiple conditions, combined I/O over same 2-wire circuit? and many more simple solutions to otherwise complex problems.

A Bi-colour LED + series Bi optocoupler that is photo-emissive from a fraction of + or -1 volt, with a threshold of illumination at +or-3mA, and a current carrying capacity of + or -10 Amperes.

Im looking for a company interested in such a device and wanting to profit from a licencing deal. just imagine how many wires there are in the world that no-one can see whats happening in the circuit unless they use this tech.

Excuse my phishing for ideas but you have to try everything these days to get the attention of manufacturers. Oh by the way, this device won a Silver medal at the International Inventions Fair Geneva Switzerland recently.

I was to busy feeding patent agents to go myself and had to rely on the DTI people to present it, otherwise Im sure I would have got the Gold. Incidently the DTI people did not bring any contact info. back from interested judges or companies???? but thats government agencies for you.

Anyone interested?

Please call Bob Gardner

+44(0)1446 741180

Mob. +44(0)7887705480

Pictures. Lots of pictures.

Please?

-e

INDI-LINK makes light of current events, fractional Vf & 500 times normal current rating.

Electricity and current flow is normally invisible to the eye, inaudible to the ear and touched at your peril. This explains why so many people find electricity and it working's a mysterious phenomenon, even by those who work with it regularly. Indi-Link, a novel LED device has changed all that, helping to make the functional condition of just about any electric circuit something to be viewed by the eye and relayed to sensitive machines with isolation. This device can be invasively placed into any electrical circuit whereby it will allow the passage of a wide range of current, while turning a fraction of the current into Photons of any selective wavelength

SENSE and SEE current

with INDI-LINK, the INDIcating wire LINK.

Sense threshold 0.003 A

Max current 10Amps

Line Insertion loss

+or- 0.35-1.2. Volts Max.

Depending on current level

Line powered AC or DC

Current mode operation

Detects/Displays Current

1. Presence
2. Polarity
3. Phase X-over

Produces

2 colour LED display

+ 2 opto-isolated volt-free outputs.

Max. Line voltage limited

by opto-isolator insulation specification. Tested to 5 kVolt

Can be resistively shunted

or

Magnetically Powered via Current Transformer in AC applications to extend operating current range to hundreds of Amperes

See and monitor conditions, even intermittent faults on a continuous basis.

MTBF in excess of 100,000hrs, a true fit and forget solution.

Command and Monitor in closed –loop, devices such as remote solenoids over normal two wire circuit.

Conformance and Safety Integrity levels easier to Achieve, Test and Prove

(relevant to IEC 61508 & 61511 directives)

Will try to send pictures soon.

My power source is the forward volts drop of 1 silicon Power diode, 2 are configured in inverse parallel for bi-directional I flow.

please give e-mail address if you want more.

With some smart designing using depletion-mode fets and coupled inductors you could (?) come up with enough voltage to drive a real (enhanced) mode Mosfet's gate and then use convensional step-up conversion. Haven't tried it though. Is this your idea? One problem I see there, is that it may be sensible to dV/dT ramp of input voltage.

#@^*! again. By sensible i meant sensitive.