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: enter image description here

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    $\begingroup$ When you say you "tried building … replacing the speaker with an LED" did you first build it as designed to make sure the rest of the build worked? (If it didn't work, then it needs troubleshooting; if it does work but doesn't light the LED then probably the problem is that the amplifier doesn't have enough voltage gain to meet the LED's forward voltage.) $\endgroup$ – Kevin Reid AG6YO Jul 25 '19 at 21:42
  • $\begingroup$ @KevinReidAG6YO Hi Kevin, thanks for your reply. I built it as designed, but I know why the LED replacement didn't work. I've edited my post to include an explanation of what happened with the LED. $\endgroup$ – F16Falcon Jul 25 '19 at 21:49
  • $\begingroup$ What kind of "speaker" did you use? A crystal earpiece has a very high impedance, far higher than than an 8 ohm speaker. Mouser sells those crystal earpieces, and that's what I needed for mine. $\endgroup$ – Mike Waters Jul 25 '19 at 21:50
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    $\begingroup$ @MikeWaters Yeah, that's what I saw in some of the guides online. However, I am not trying to listen to the stations, but just detect their carrier waves (by driving an LED). $\endgroup$ – F16Falcon Jul 25 '19 at 22:01
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    $\begingroup$ As someone who has troubleshot circuits for a living (amongst other things) for 20 years, I can tell you that you are far better off making the circuit work as designed first, and then modifying it, rather than modifying it first and then wondering why it doesn't work. Get it working 100% as advertised, change one or two parts, see if it works as expected, figure out why if it doesn't, repeat. $\endgroup$ – rclocher3 Jul 25 '19 at 22:31

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.)

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.


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?

  • $\begingroup$ Hi Dardo, I took a look at the website, and unfortunately, it's not quite exactly what I'm looking for. I appreciate you taking the time to post, though! $\endgroup$ – F16Falcon Jul 26 '19 at 21:38
  • $\begingroup$ Im though that you need to know when the AM signal is on. This a simple way to do it. I'm wrong? $\endgroup$ – Dardo Bagnis Jul 26 '19 at 22:49

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.

  • $\begingroup$ Hi Zeiss, I appreciate you taking the time to answer. Actually, I am trying to get rid of the speaker (I won't listen to the station). All I want to do is see visually if the station is transmitting or not. It's a dumb project - I know- but it's something that'll help me learn. So, regarding the voltage drop, if the red light's drop is 1.8V, how do I ensure that the bias is critically close to it? I think that's what I'm having a hard time with; I tried different resistors, but can't seem to get it to work. $\endgroup$ – F16Falcon Jul 26 '19 at 13:15
  • $\begingroup$ Also, I'm not that sure, but shouldn't the biasing values not really matter as long as there are resistors in series with the LED (which values can be set to ensure that the voltage across the LED is just below its minimum voltage)? $\endgroup$ – F16Falcon Jul 26 '19 at 13:16
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    $\begingroup$ @F16Falcon As an "on the air detector" using an LED indicator, you probably need a working receiver and use the ouput of the AF amp to light the LED. That is, the LED will replace your speaker (you may need another stage of amplification to get the voltage and current the LED requires, however). An adjustment to keep the level low enough that the LED is off when you're getting only noise, and voila! $\endgroup$ – Zeiss Ikon Jul 26 '19 at 13:18
  • $\begingroup$ Ah, that makes logical sense to me. So for the adjustment, do you think putting a resistor in series with the LED would do the trick? Or do I need to adjust the biasing resistor? $\endgroup$ – F16Falcon Jul 26 '19 at 13:23
  • $\begingroup$ You'd want to adjust the output of the audio amp. Like a volume control. I'm not a transistor guy much, but I think that would require adjusting the bias on the transistor rather than on the detector (a crystal diode detector usually doesn't need or want a bias). $\endgroup$ – Zeiss Ikon Jul 26 '19 at 13:36

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