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# Tag Info

46

Actually, yes, a receiver can affect the transmitter. Passive RFID is based on this principle. However, RFID only works at very close distances, where the receiver is absorbing something on the order of 10-4 to 10-5 of the transmitter's signal. In other words, the transmitter is sending out hundreds of milliwatts, while the receiver is absorbing a few ...

45

It seems to me that you are probably using a music source that is stereo and, instead of it delivering a mono signal as you expected (L plus R), the connection of the transformer to L and R channels results in L minus R. Given that vocals and bass are usually equally mixed left and right in a stereo mix, the transformer connection you have used will only ...

35

You could potentially break some radio transmitters by operating them without the antenna connected. Several things combine to make this possible. First, it's difficult to make power at RF frequencies, so the power devices are often fairly fragile, and run near their limiting conditions. Secondly, radio signals passing down a transmission line behave in a ...

26

Ordinary UARTs have to be pre-configured with the desired baud rate (as well as word length, stop bits, parity, etc) traditionally by a human. For several decades now though there have been implementations of "auto baud" detection found in some settings, which typically works by timing key features of the waveform to deduce the baud rate. Early versions ...

22

It's technically possible to detect radio receivers if they are Superheterodyne receivers that use RF mixing to downmix the received signal to a well known intermediate frequency. You can scan for this frequency using a directional antenna and count the receivers around you. Though this doesn't sound like what you're inferring since the transmitter can't ...

15

Just to complement the excellent answer of Neil_UK and stress the fact that at RF frequencies voltages and currents don't really behave as those nice entities you know from KCL and KVL. You must drop Kirchhoff's laws and get your hands dirty with transmission lines theory, where the same concepts of voltage and current become a lot weirder! In other words,...

12

Is it possible to build a very minimalist radio transmitter that can be built only with passive components, what would be the schematics for that? Sure. You can switch transients into a resonant circuit, as others have mentioned. The trouble here is that you end also transmitting a lot of those transients, which means a lot of broad-band noise. There's also ...

12

The key to all of this is "impedance matching". You need the amplifier to think it is driving a low impedance (so it can source plenty of current from the 5 V supply, and thus generate a lot of power). Then you "magically" need to transform those currents to drive 50 ohms at a much higher voltage. This is done with an impedance matching network. When you ...

12

The inductor in the plastic tube is not an inductor. It's the inductor core. The loops of wire are the coil around that core. It's an adjustable inductor. Fixed to its given value with hot glue.

11

You could think along these lines: - simulate this circuit – Schematic created using CircuitLab The data you send needs to be DC balanced i.e. something like Manchester encoded or scrambled. The amplitude of the data needs to be significantly smaller than 5V - maybe 1Vp-p. You'll need a comparator/data slicer on the data out signal to recover the ...

9

Blue stage: tuned RF amplifier; Orange stage: General purpose amplifier; Green stage: Relay driver; Pink line: high current drive route on relay turned on. On first stage DC point is given by resistors, taking capacitors as open circuits and taking inductors as short circuits. On output dc bias is 6.1V and collector voltage dc point is 5.5V. Ic = 410uA ...

9

The plastic sleeve is not active, it just holds the core and allows it to be adjusted by screwing in or out, to tune the transmitter. The two loops of wire are the antenna windings. Most likely the one surrounding the core is the tuned circuit, while the other provides positive feedback to sustain the RF oscillation. And the whole assembly can indeed act ...

9

No. There is no way for an AM or FM transmitter to determine how many people are listening. They provide exactly the same power output at carrier whether there are a million receivers within 1 mile or zero. Digital transmissions that require a subscription can on the other hand possibly know how many receivers there are, if there is a two way verification ...

9

Q1 amplifies audio with a gain of approximately R4/R5. This audio feeds Q2 (an oscillator) and modulates its base-collector miller capacitance thus producing a varying output frequency. Q3 is an RF amplifier to provide more RF signal to the antenna. That's the intention but there is a problem (as mentioned), Q1 won't work and indeed C4 should either be ...

9

First of all is it possible to create 10 GHz wave on a PCB? Any clue on how to do something like, regarding the circuitry? If by "creating" you mean propagating and/or radiating, the answer is yes. However, you need to use specific board substrates like Rogers 4000 series, with low losses and controlled dielectric constant up to that frequency. Your ...

8

$$F = \frac{1}{2\pi\sqrt{LC}}$$ L and C2 are components oscillator but this is a very awful bad circuit. 1 transistor as oscillator an as modulator is not the way to do it it isn't as stable as an regular modular transmitter for just a sine out you wont use this circuit you could look on the Internet for Colpitts oscillator Hartley oscillator clapp ...

8

Depending on the distances involved, sensing / timing precision available at master transceiver, response speed at slave transceiver, and computational power available on the master device, Time of Flight mechanisms may be usable for distance estimation. ToF measurement involves transmitting an identifiable, unique bitstream from the master transceiver, ...

8

Two UARTS "agree" on baud rate by means of documentation and by operator/user setting the baud rate by hands, including handshake protocol, stop bit size, etc.

8

Can I replace R1 with the DAC output from an AVR microcontroller? Yes you can but be aware of noise on the output of the DAC especially when changing DAC values - you get a bit of capacitive coupling from the clock and this might require a bit more filtering than just the 270 ohms and 2200 pF will bring about. For this reason, it's not generally ...

7

This is how we used to do it back in the stony ages:

7

C3 turns the circuit into an oscillator. To understand this you need to think hard about the role of the emitter and the thing you have to think hard about is this - the emitter (as well as the base) is an input to the transistor that is amplified by the transistor. An ac voltage (superimposed on a dc voltage) between base and emitter is amplified by ...

6

The problem ended up being very simple. I followed a schematic/tutorial for the FM transmitter module online. One thing they tutorial left out is that the LA pin must be held either high or low. It selects the i2c address. So, what was happening was the module was randomly changing i2c address during operation. This lead to all of the problems I had. Now, I ...

6

You've got the choke on the wrong side of the varactor. It needs to go between the audio source and the varactor, in order to keep the RF (oscillator) signal out of the audio circuit. You want the RF to get to the varactor. Also, 10µF for the DC-blocking capacitor between the varactor and the oscillator tank is way too large (although it'll probably ...

6

It's not exactly clear what you mean by "transmit a single wave, not whatever the microphone outputs". Is that OOK (On-Off Keying)? It's not worth the trouble and the money to start building a transmitter yourself (unless the purpose is in the building, not the use). Digikey lists 433MHz transmitters for around 4 dollar: This one can send data at ...

6

Neither of your circuits make any sense. That is simply not how transistors, capacitors, and crystals work. Unfortunately, they are so far off that trying to explain why they won't work is too large a task for a reasonable answer here. Also, you'd need to understand more electronics to understand the answer, and trying to teach all that electronics first ...

6

Here's how it works but a couple of factoids first: - According to this source, the most common frequency of transmission is 437 MHz Antenna gain$^1$ for a 1m dish is about $0.5\cdot\dfrac{(\pi D)^2}{\lambda^2}$ and at 437 MHz $\lambda$ is about 0.69 metres and gain will be about 10 and in dBs this also is 10dB Link loss from satellite to earth is: - ...

6

I assume you have a small beacon PCB that you haven't designed yourself, so you need to work with it as-is. if you're laying out the board, you have more options... 1. Restricting the area covered by a beacon You're onto the right track here - mounting it above the supermarket aisle, or what have you, is probably best, then you can focus the signal into a ...

6

Sounds like complete and utter cr*p for all practical purposes. The actual energy extracted by a receiver is microscopic. Though there is a story of a farmer who built a big tuned loop in order to extract free power from a nearby radio transmitter. Sufficient to distort the field pattern and be detected.

6

L1 and C4 are the parallel-resonant tank circuit for the RF oscillator, and Q1 is wired as a common-base amplifier that provides the gain required for oscillation, with the feedback (and output coupling) provided through C5. In the common-base configuration, the input is applied to the emitter and the output is taken from the collector. The base is held at ...

6

This is a variation on a Colpitt's oscillator. L1 and C4 make up the main LC resonant circuit. C5 is used to provide feedback to the emitter of the transistor which amplifies the signal and makes up for losses and the power radiated from the antenna. The transistor is in a configuration known as "common base" or "grounded base" as the signal to be ...

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