How to turn on LED using RF as a catalyst?

Question

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?

Answer 1

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.

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.

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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.

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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

Answer 2

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 :)

^{simulate this circuit – Schematic created using CircuitLab}

Answer 3

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.

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.

Answer 4

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.

Answer 5

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.