I saw a video on youtube, where a guy used a high intensity diode as his diode for a crystal radio, and the stronger the signal, the brighter the LED burned. Can anyone tell me if you could use such a diode as a signal strength indicator for a regen radio? And, How would you connect the LED to the radio? Lenny

Double post

LED intensity is proportional to the current going through it. A red LED won't show any light at all until you get to about 1.7V across it. In my experience, it takes about 3 mA on most LEDs to see any observable light.

I would say a LED is not an easy choice for a signal strength indicator. You will need to design a driver circuit to boost the signal and bias the LED to the edge of illumination.

As to using an LED in a crystal set.... if you live 3 miles from a 20 KW station, you might get a LED to light. I made my LED crystal set with a battery to boost the voltage on the LED and then the incoming signal (if it's pretty strong) will get the LED to light or flicker.
http://www.avagotech.com/docs/5989-1711EN
Rich

If this is the video you saw

http://www.youtube.com/watch?v=gcui0K7JZXA

Note that he is a mile away from a commercial radio station and that the radio picks up signal at almost any setting of the variable capacitor; the signal can be increased by tuning the cap, but the fact that he gets audio and light at alll settings, indicates he is practically on top of the local station's antenna.

My nearest station KSRO, about 8 miles away, will give me almost, but not quite, enough energy to light an LED using a reasonable coil and capacitor, plus a 100 ft. antenna.

Rich

Yep! That`s the video. He may have explained what he was doing, and I do not remember all the details. So much for that!!!!!!!!!!Lenny

Many High Intensity white LED's will give a visible light at 50uA or even lower. The colored ones with clear bodies will light up at approx 100uA and will make a nice indicator on a front panel at 200uA.

You may get a visible light at 50 uA, but a white LED requires 3.6 V before it illuminates. That actually makes it more difficult to use in a "crystal" radio. A red LED starts at about 1.7V.

Any LED can work with a suitable amplifier.... say a single transistor with a battery power supply.

Rich

Several years ago we discussed this. My results on a local station.

Not be true for all, or is their a voltage decrease once conducting???

White hi LED with approx 11ma flowing through it...



Tom

I just looked at 3 different white LED data sheets.

You are right! Here is one mfr. who "bins" white LEDs and the lowest Vf range is about 2.8V.

http://www.everlight.com/datasheets/61238.pdf

I usually plan on 3.6V as that will light most all of them. The important thing is to check the current; that determines the brightness. Most LED driver ICs provide a constant current, or have a dimming feature.

Rich

The so-called forward voltage drop Vf across a diode is not a fixed point where a diode suddenly starts conducting. Any diode will conduct when forward biased with even a couple of millivolts. Current draw increases gradually as voltage increases. However, as the voltage increases beyond the Vf point, then current starts to increase dramatically. So an LED can generate light even if the applied voltage is below Vf.

I use a couple of hi-intensity blue LEDS as a MIDI bus status monitor in my synth lab.

I used two ordinary green LEDs in series, connected straight across the MIDI signal bus, until the "superblues" came along and I installed a couple of those. They were actually too bright, so I had to wire a resistor in series with the LEDS.

Thus:



The monitor is mounted in a frame on the wall. This isn't a very good pic, but you get the general idea. When the bus is hot, the lights glow faintly; any signal flashes them brightly. With an involved sequencer program driving the bus, they stay bright.

Larry