How to make simple AM detector using an LED?

Question

I'm new to amateur radio, and as a beginner project, I wanted to use some of the basic components I have to create an AM receiver.

Anyway, what I'm trying to do is create a simple AM receiver that simply detects if a radio station is transmitting, or not. I'm not interested in listening to that station, I just want my receiver to detect the radio waves (basically, I'm just trying to detect a carrier wave).

I've spent a lot of time researching how to make what I'm trying to make, but all of the websites I visited explain how to make radio receivers with speakers (such as crystal radios, or very complicated AM receivers). For example, I tried building this one by modifying it (aka replacing the speaker with an LED), but it doesn't work even after tinkering around for a couple days, so I thought I'd ask you guys for some help :)

To be more specific, I would like to receive the AM signal/carrierwave using an LC circuit to detect a specific frequency, and then amplify that signal to drive an LED (since the station's AM signals are too low: around 50-100 μV).

I'm pretty new to radio and circuitry, but I've done some research and I think I have a general understanding of what's going on. Regardless, I was hoping to make a detector that is super simple, using basic components such as those used on the site I linked. Lastly, for the sake of simplicity, we can leave the LC circuit permanently tuned to, let's say, 1000 KHz (also because I don't have variable capacitors or inductors). I've struggled with this for a while, so I truly appreciate guidance from you guys!

EDIT 1: When I say that replacing this circuit with an LED didn't work, what actually happened was that the LED simply turned on the whole time, regardless of whether or not the receiver was picking up a signal (which was confirmed by disconnecting the antenna and LC part). I deduced that the reason for this to happen is that since there is a constant current running in the base of the NPN transistor, there is also a constant current running thorugh it's collector and emitter, and therefore the LED has current running through it, keeping it on. I haven't been able to solve this issue, so if anyone knows how to keep it off (until it receives a signal), let me know!!

Here's a picture of the circuit I tried:

Answer 1

Partial answer (what's going on, but not a good recommendation for how to fix it):

the LED simply turned on the whole time

This circuit puts DC across the speaker terminals. You don't need to analyze the amplifier at all to see this: just note that the power supply must pass through the node at the bottom side of the speaker (and the 22k resistor) and therefore the voltage drop across the 22k resistor also appears at the speaker.

This is fine for a 'crystal earpiece' (piezoelectric crystal) because it is electrically like a capacitor, and does not pass DC and even if it did, does not make sound from it (other than a click on power-up). But your LED passes DC and lights up.

(If you were to use a dynamic (magnetic coil based) speaker rather than a piezo speaker (such as normal headphones), the DC would pass through it, and this would change the characteristics of the circuit and waste power, but it would still not make sound other than a power-on click.)

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I'm not well-versed in transistor amplifier circuits to tell you how to modify the circuit to provide carrier detection without a perpetual DC offset. If you were to add a high-pass filter before the LED, you could get a circuit which would detect the audio signal, but that's not the same thing as detecting the carrier.

Answer 2

Good morning F-16Falcon, Immediately i read you question, I thought about a Roimeter, and lucking in internet i found this: https://radioclubplacetas.cubava.cu/2016/11/17/medidor-de-roe-optico/

It's an optical Stationary waves meter.Its simple, cheap and do that you need. What do you think?

Answer 3

My experience is that an LED doesn't act like a common diode -- that is, it's not much use as a rectifier, or especially as a detector, because it requires too much forward voltage and has too much drop.

If you wanted to rectify something like 6 VAC, an LED might work, but when you're trying to detect a few microvolts, it won't do anything by itself; it'll act like an open circuit. Your circuit above may be attempting to correct this problem with a DC bias (which needs to be critically close to, but below the LED's forward voltage drop, so the LED will "switch on" and "switch off" as the incoming RF voltage fluctuates).

Beyond that problem, however, once the LED does "switch on", so much of the energy it passes goes into light production that there's nothing left to drive your audio amplifier.

If you change the LED to a low-bias germanium diode made for detector use, or zero-bias FET, and alter the biasing to suit the new component, you might get a functional radio (though your single transistor isn't well matched to the high impedance of a crystal earphone -- you'd be better with a low impedance headphone).

As noted in comments, however, if you're attempting to just give a visual indication whether the tuned station is in fact transmitting, you need the output of the receiver to control the LED. To do that, you need to start with a working receiver, in this case, using the original diode specified for the circuit you built. Once you have that, you can experiment with replacing the speaker or earphone with an LED (using the level control to determine when it lights) or a circuit that will drive the LED.