Crystal radio to power an LED?

Question

I want to make something like a crystal radio to light up a small LED. I am not near a station, so I am guessing it wont be enough to power an LED, so what if I use a capacitor to collect the energy and then only light up the LED for a few seconds at a time?

Is this possible and if it is how can I do it?

First poster:
But what if I store the energy with a capacitor or something so that the LED lights up for a second say every, 5 minutes?

Answer 1

I've tried to find a quick-and -easy solution for what you propose, actually I'm pretty sure it's possible even if you are limited to 40uW. You can, instead of flashing it for one second every 3-400, flash it for only about 1/20th of a second evry 15-20 seconds. Same amount of energy, and it will be visible. Rather than detect the signal with a diode, step up the voltage thru an aircoil tapped choke. Rectifiy it, apply to perhaps a 10nF cap. Then the tricky part - you need some sort of triac-type of device, that once triggered by a threshold voltage will dump the charge on the cap to your LED. Thing is, you can't use bipolar transistors like a traic uses, the leakage current is too high - the cap will never charge. Perhaps a triac made out of FET's?

I built something similar years ago, it was a very small coil connected to a circuit similar to the one just descibed. It pulsed an LED at a rate somewhat proportional to the magnetic field the coil was in. I was building chargers and charging coils for battery-powered implantable medical devices which could be recharged via a magnetic field. I need a sensor for measuring and mapping in 3D field strengths at better than 1cm resolution, but the only commercial field probes I could find were at least 10 times as large. Anyway, I built it and it worked, but these were strong fields, over 100 gauss. Bipolar triac was fine. Your app is a bit more challeging. Good luck, it's an interesting project, email me if I can help.

[[REPOST]]

Don't let others discourage you, I guarentee it can be done if you really want to make it happen. Ceramic caps have leakage in the femptoamp range and can store charge for years. The trickest part will be the FET traic biasing, such that it turns on hard once triggered rather than assume a steady state. I remember having a helluva time with that.

Answer 2

If you make a standard crystal radio and for the detector employ a voltage multiplier, you might raise enough voltage - but an LED also requires current.

So you will have to trickle-charge up a larger capacitor to then discharge into the LED.

Good luck, it sounds like an interesting project to try!

Answer 3

Still will not work. The capacitor will not continually store up more energy that is impressed upon it. Electricity, like people, is lazy, it only does what it has to. 0.01 volts from a source for the next 10 billion years is all that you are going to get. You would need an amplifying device to charge pump the capacitor, which is outside of what you want to do.
Expecting the capacitor to build sufficient charge to light up the LED is like expecting the filter capacitors in a power supply to build up a charge equal to all of the energy supplied to it for as long as the power supply is turned on. That is not going to happen. So, no go, it isn't going to happen, unless you do as stated, include a charge pump which requires an external power source.

Answer 4

Aside from the problems of capacitor leakage over that time frame, if the first answer is correct - 40 uW, then an LED, which draws about 10 ma at 1.5 volts, draws 15 mW or 15000 uW. So you would have collect for 375 seconds (5 min. 15 sec.) for one second of output before losses and timer circuits, even CMOS, are going to have needs/losses

Answer 5

A normal input level to an antenna for good reception is about 40 uW. That is not enough power to light an LED. Not even close!

So, it ain't going to work, dude! Sorry.

Answer 6

It is probably possible.

The voltage applied by a radio signal to an antenna is going to be somewhere in the μV (microvolt) range. The time it would take to charge a capacitor would be extremely long... and the flash would be extremely short

Answer 7

If you use a good battery with low self-discharge, you can flash your LED on it at the same power level for years (or decades).

But why do it the easy way if one can do it the hard way?

Answer 8

you would need a large coil antenna. use a voltage doubler or something like that to increase the voltage and rectify your input.
attach your capacitor to the output.
this should work depending on your proximity to a transmission source.

Answer 9

NO, its output never reach more than 1V and its current less than 1mA.