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I am trying to build a circuit that turns on a LED diode when it detects AM radio waves, in order to create a "radio wave detector".

Before I explain further, please note that I am not trying to power the LED solely using RF power from the air. In fact, this wouldn't be possible because I read that radio waves that we pick up are typically around 50-100 μV. Thus, it wouldn't be possible to power the LED using just this power.

Instead, what I want to do is amplify the relatively faint AM radio signal using extremely basic components like a transistors (i.e. BC547) and 9V battery, so that the LED turns on when the circuit picks up AM radio waves.

I'm pretty new to electronics, and so for the sake of simplicity, we can keep this circuit permanently tuned to 1000 KHz (as an example) so that we don't need variable inductors or capacitors for the LC circuit. Furthermore, I don't need any fancy features - just a most basic AM radio wave detector using some simple parts (no IC's please!).

I spent a lot of time on the internet trying to learn how to do this, but I haven't been able to find any examples of this. Something useful that I did find, however, is a "single-transistor radio" (ex: this link). I followed instructions for these types of radios (but I replaced the speaker with an LED), my LED just remains lit the whole time, regardless of whether or not the antenna picks up radio waves. I just want it to light up when it picks up radio waves (which can be tested by connecting and disconnecting the antenna?).

How do I build a switch that can be activated with RF?

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    \$\begingroup\$ How are you going to avoid any random AM signal from triggering your circuit? This isn't a site to request schematics, that is off topic, its a site for design help. Also, most amplifiers come in IC's the design would be vastly simplified if opamps are allowed (which come in IC's) \$\endgroup\$ – Voltage Spike Jul 19 at 2:30
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    \$\begingroup\$ The output is designed for a high-impedance load of 600 ohms or more. You would need a DC peak detector followed by a comparator like a LM339 which can drive an LED in series with a resistor, however control of rf signal level is needed. \$\endgroup\$ – Sparky256 Jul 19 at 2:40
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    \$\begingroup\$ You need a way to weaken random rf noise at the input. A suggestion would be to try a 1 K resistor in series with the diode. Local stations are likely saturating the transistor input. \$\endgroup\$ – Sparky256 Jul 19 at 2:49
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    \$\begingroup\$ The answers that say that he cannot do this are RUBBISH. He needs some assistance - not uninformed wet blanketry. \$\endgroup\$ – Russell McMahon Jul 19 at 12:35
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    \$\begingroup\$ Some guidance on down and close voting seems in order - this question is just one example of the negative power of people who will not read and/or understand a question. || The downvotes and close votes and negative comments are bizarre. Some reflect a lack of reading the question correctly. The question is a good one which can be answered within the scope covered. Some clarification was provided in comments - but not so much as to transform the original question. \$\endgroup\$ – Russell McMahon Jul 20 at 1:32
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The following lists a few simple AM receivers of potential use for your application.
In each case an AC coupled audio signal is the usually provided output. You will need to either observe changes in the operating point of the following audio stage with an unmodulated carrier is received (see below) OR derive output from the preceding stage. A simple comparator (see below) can then be used to provide a high/low signal.

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Here is a better starting circuit than many from here - which includes a good description and components list.
It is "better than many" as it is "regenerative" - the audio is fed back from Q2 via R1 to the input stage so that the pair act both as RF and audio amplifier.

enter image description here

Audio only is fed via C3 to Q3.
Here the fun begins.
You will see a change with unmodulated carrier but I do not know of what magnitude.
The voltage across R5 will vary and may be used to trigger a comparator. It MAY prove easier to use the signal level change at Q2 collector, DC couple that to a Q3 stage and then use a comparator.
(Sorry for the uncertainties - easier to play and observe than cudgel brain on practical results of regen with unmodulated carrier).

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A comparator consisting of a "long tailed pair" will allow a variable DC level signal to turn an LED on and off. Many long tailed pair references here

This simple circuit from here will work very well as a DC comparator. The webpage provides a good discussion of LTPs.

In this simple case, when say TR1 base is at a higher voltage than TR2 base, TR1 turns on, taking the majority of the current via R1 (which is a current source in more complex designs) and regeneratively proting Tr1 on and Tr1 off. So, Voltage at TR1_C (V_TR1_C)falls and V_TR2_C rises and an LED may be drive thereby.

enter image description here


An even simpler regenerative receiver.
Replace LM386 stage as above. From here but registration required. Detail may or may not be available. Note regen control seems to be by moving a pickup loop on tuned circuit.

enter image description here

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Also worth a look - a "superregenerative" receiver. Input is driven in and out of regeneration at above audible (usually :-) ) rate to allow sensitivity to be optimised. From here

enter image description here

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    \$\begingroup\$ These circuits will not help the asker do anything but waste hours or days in frustration. The places where they are actually useful are in applications where the signal is keyed on and off, and these compromise-laden receivers are followed by something looking for that particular pattern amidst the output which they will otherwise generate on their own. \$\endgroup\$ – Chris Stratton Jul 20 at 2:01
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    \$\begingroup\$ @ChrisStratton Comment taken under advisement :-). I know how to do it "properly". Doing it improperly in a manner that works well enough is the challenge. I was looking for something bearably simple that gave him something better than a "Crystal set". If he'd been willing to use ICs then there are some suitable simple one chippers. Something with an AGC signal output 'would be nice'. || I believe that I can make something along the above lines work well enough for his purposes. || I'll try to get to having a play "sometime soon". \$\endgroup\$ – Russell McMahon Jul 20 at 6:03
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    \$\begingroup\$ It's particularly the tendency of regens (at least when not getting ongoing manual adjustment) to "invent" output in the absence of signal which will be challenging to the asker's goal. When they're used in automatic receivers, they are looking for a particular pattern of on-off keying, not the strength of a steady carrier. To measure the strength of a steady carrier you generally either need to be so close that you don't care about anything else, or to have an architecture with stable gain and filters to reject everything else. \$\endgroup\$ – Chris Stratton Jul 20 at 16:05
  • \$\begingroup\$ Thanks for the taking the time to write, Russell. I'll learn what I need using your post for guidance. However, when you say that it might be easier with a single IC, could you explain further? If it is indeed easier with an IC, I'd love to hear the explanation; I can create a new question post to separate it from this post, too. Let me know if you're able to! \$\endgroup\$ – F16Falcon Jul 20 at 16:56
  • \$\begingroup\$ @ChrisStratton Hi Chris, in that case, do you think there might be a better way to do this? I'm open to new ideas! \$\endgroup\$ – F16Falcon Jul 20 at 16:57
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You first need to make sure that the signal your antenna receives is the signal you intend.

  • So a band-pass filter is needed for getting only your (maybe also others..) produced frequency.

  • You need to amplify this signal significantly for it to be workable, so you will need op-amps (or another amp topology like "class A"). In the schematic the op-amp will not recreate the negative part of the incoming radio sine-wave.

  • The amplified signal can be fed to a comparator. The comparators output is ON if the positive terminal is higher than the negative terminal.

There will be a lot of tuning of the values to get this to work :)

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ A good answer even if the OP does not have the skill to understand it. Note that the TL0xx series are jfet op-amps meant for bi-polar power supplies. Please select a single-supply op-amp. Also the LED needs a series resistor. \$\endgroup\$ – Sparky256 Jul 19 at 7:04
  • \$\begingroup\$ Thank you. I edited the answer to rectify the good points you made. \$\endgroup\$ – Bonnevie Jul 19 at 7:41
  • \$\begingroup\$ Hi Bonnevie, thanks for sharing this circuit! I tried it on a circuit simulator and it works well (according to the sim) for detecting even the small 50uV AM waves. However, I'm having an issue: when the circuit is activated once by introducing an AM source, even after the AM source is removed, the LED continues to flash. Do you know why this is the case? If you'd like, I can create a new question/post so that you get appropriate credit (and also because it's a separate question too?). Again, thanks for sharing! \$\endgroup\$ – F16Falcon Jul 20 at 17:05
  • \$\begingroup\$ Actually, I think I solved the issue: I connected a resistor at the junction between C2 and L2, and then connected the resistor to ground. And this seems to have solved the issue (so now when the AM source is removed, the LED stops flashing). Do you think this is a good way to do this? \$\endgroup\$ – F16Falcon Jul 20 at 17:17
  • \$\begingroup\$ Your "attenuate" labels are the wrong way around. A low pass filter attenuates higher frequencies ….while a high pass filter attenuates lower frequencies. Also the use of a opamp as a rectifier at 1 MHz requires opamps with very high GBP/GBW products.Even a 10MHz GBP gives very low gain of about 10 at 1 MHz. You really need to go to 50-100 MHz GBP to get any worthwhile open loop gain \$\endgroup\$ – Jack Creasey Jul 20 at 18:29
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How to turn on an LED using a received RF signal at 1000 kHz?

You ask a relatively pointed question, but present no real information on what you want to detect. There is signal level to consider along with modulation.

You then continue saying that you built an AM radio based on this link. But this does not work for you. Perhaps you should focus on that problem rather than simply look for another solution and abandon what does not work for you.

If you managed to build the one transistor radio and get audio out without knowing HOW the circuit works then this shows the first and largest gap in your knowledge.

Without some understanding of HOW a circuit works or WHY when the earphone was replaced with an LED it was always ON, you will always be stumbling. The answer by the way why is obvious, but seemingly not to you. One can therefore deduce that you have no idea HOW the radio/amplifier works.

Here's the circuit you said you built marked up with what you need to research/learn.

enter image description here

You should research and learn each of the following:

  1. Antennas (particularly look at ferrite cored antennas)
  2. Tuned circuits and transformer coupling ratios
  3. Demodulation (particularly why this type of circuit would NOT work for just an RF signal detector ….ie carrier wave with no modulation
  4. AC coupling
  5. A basic one transistor amplifier
  6. An audio transducer (in this case specifically a Piezoelectric Crystal Speaker and its properties)

Even if you just start with #5 you would then know why the LED you inserted is always ON. That would be great progress on your learning path.

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  • \$\begingroup\$ Hi Jack, I appreciate you taking the time to answer; upvoted! Regarding why the LED is always on, I think it's because the transistor has a constant (but small) current running to its base, which allows current to pass through the E and C, which keeps the LED on. Is this correct? If so, that's what I was wondering about - how can I overcome this so that the LED stays off, until the antenna picks up radio signals? If I'm wrong, could you explain? Thanks! \$\endgroup\$ – F16Falcon Jul 19 at 20:34
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    \$\begingroup\$ @F16Falcon Correct, progress being made. I'll add some more to my answer. \$\endgroup\$ – Jack Creasey Jul 20 at 20:45
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I suggest you use the circuit provided by Jack Creasey, throw away the diode, and replace the collector circuit (22K in parallel with Earphone) with 1Kohm in series with LED.

This CommonEmitter amplifier may have lots of input capacitance, because of Miller Effect. Inserting a 2nd transistor, in a cascade use, will greatly reduce that wasting of precious RF energy.

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You cannot design anything until you know how things work and don’t work and therefore can define the design specs.

In your case your only practical option is to use the audio out out of portable radio to drive an LED for which there are many ways and repeated questions on this task to repeat how to drive an LED again.

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    \$\begingroup\$ Hi, thanks for your response. I'm just getting into electronics, and read/thought this is a great way to start. However, I'd respectfully disagree that I need to know how everything works to start designing - in fact, I started learning because I wanted to build an radio. I didn't know how, but I knew what I wanted to do, and learned my way there. Now I know basics of electronics and how to make a transmitter, and am currently working on a receiver/detector, but am having a bit of trouble, which is why I asked here. I don't want to use a premade/portable radio, but build my own simple one :) \$\endgroup\$ – F16Falcon Jul 19 at 3:10
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    \$\begingroup\$ m.youtube.com/watch?v=Xyh-JpWdGmQ \$\endgroup\$ – hacktastical Jul 19 at 7:28
  • \$\begingroup\$ your question is a binary one, how to trigger an LED event with a radio yet show aptitude on how a radio works or why it fails. you need a kit solution and book that explains how to design/test and verify the performance specs , not a design guide. you need to understand how AM RF tuners work with dual variable caps , 455 kHz IF, demod and AGC. until then go read how a 6 transistor radio works and how it can fail. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 19 at 10:13
  • \$\begingroup\$ @hacktastical not being negative, but would you tell your wife how to escape in a Tiger tank without training? or let her flounder about until hit by the Magnificant Seven. basic training is to start reading about how it works and why it works. not how you drive a tank to turn on an LED \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 19 at 10:17

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