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A simple AM transmitter demonstrates how sound can ride on a radio-frequency carrier: a crystal oscillator generates the carrier, and audio coupled through a transformer changes its amplitude. The Science Buddies example uses a 1 MHz oscillator and an AM radio to make the effect observable. It is an educational circuit, not proof of a particular transmission range or regulatory compliance.
How the transmitter carries sound
AM stands for amplitude modulation. The carrier is a steady radio-frequency oscillation; the audio signal varies the carrier’s amplitude. That variation forms an envelope that follows the sound, which an AM receiver can detect and reproduce.
In the Science Buddies circuit, the crystal oscillator supplies a fixed-frequency carrier. Audio from a 3.5 mm source passes through a 1000 Ω-to-8 Ω transformer. The transformer couples a stepped-up audio signal into the oscillator’s power path, varying the oscillator’s output amplitude. The oscillator output connection also serves as the antenna connection in that particular layout. An AM radio tuned to the carrier can receive the transmitted audio. Science Buddies’ project guide provides the circuit and assembly details.
What each block does
- Oscillator: creates the radio-frequency carrier. The example uses a 1 MHz crystal oscillator.
- Audio transformer: couples the audio signal into the oscillator supply so the carrier amplitude varies with the sound.
- Antenna connection: the guide identifies the oscillator output pin as the antenna connection for this circuit.
- AM receiver: tuned to the carrier, it makes the transmitted audio audible.
These blocks—oscillator, amplitude modulator, and antenna-coupling network—are also useful labels for explaining transmitter designs more generally, as in the UC Davis ECE manual. They do not imply that every transmitter uses the same circuit.
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- Complete DIY Transmitter Kit for Learning & Experiment: This radio medium wave transmitter kit includes all necessary components for building your own AM transmitter. Perfect for school science experiments, electronics education, and amateur radio enthusiasts. Understand the principles of sound modulation and high-frequency signal generation through hands-on assembly.
- Adjustable Frequency 530‑1600KHZ with Stable Oscillation: The built-in common base modulation transformer oscillation circuit generates stable high-frequency equal amplitude signals. Adjust the CV to set your desired frequency across the entire medium wave band (530‑1600KHZ). Includes positive feedback network and high-frequency bypass capacitors for reliable performance.
- Sound Amplification & High-Frequency Modulation: Features IC1 sound amplifier chip with volume potentiometer (SW1) for audio input control. The high-frequency modulation circuit (Q2) further amplifies signals for clear transmission. Adjust SW2 and SW3 to fine-tune sound quality and voltage for optimal AM modulation.
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Parts in the published example
The Science Buddies build lists these components:
- Solderless breadboard
- 4×AA battery holder and four AA cells, for 6 V total
- 1 MHz full-can crystal oscillator
- 1000 Ω-to-8 Ω audio transformer
- 1 kΩ resistor and 8 Ω resistor
- Audio connection and jumper wires
- AM radio receiver
Follow the guide’s circuit layout and oscillator pinout. A component with a similar name may differ in package, pin arrangement, or transformer winding characteristics, so check the actual part against the circuit before assembling it. The cited guide’s part numbers and sourcing details may change over time.
How to demonstrate the effect
- Assemble the cited circuit: use the breadboard layout and pinout in the Science Buddies instructions, rather than assuming another oscillator module has the same connections.
- Connect an audio source: feed audio through the specified 1000 Ω-to-8 Ω transformer connection.
- Use an AM radio as the receiver: tune it to the carrier frequency and listen for the audio. The project documents this as a receiver demonstration, not a measured range or signal-quality test.
- Explain the observation: describe the audio as changing the amplitude, or envelope, of the carrier. Avoid claiming a transmission distance, output power, or sound quality that has not been measured.
What the 1 MHz frequency means for U.S. rules
The example’s 1 MHz carrier falls within the 510–1705 kHz range addressed by U.S. 47 CFR §15.219. In the 2026 edition, that rule specifies that input power to the final RF stage, excluding filament or heater power, must not exceed 100 milliwatts; the combined length of the transmission line, antenna, and any ground lead must not exceed 3 meters; and emissions below 510 kHz or above 1705 kHz must be attenuated at least 20 dB below the unmodulated carrier.
Rank #2
- [SOUND AMPLIFICATION] - Built-in sound amplifier chip and volume potentiometer for adjustable sound amplification.
- [OSCILLATION SOURCE] - High frequency equal amplitude generated by a common base modulation transformer oscillation circuit.
- [HIGH FREQUENCY AMPLIFICATION] - High frequency amplitude further amplified for excellent quality.
- [HIGH FREQUENCY MODULATION] - Sound capacitors and bias resistor for sound modulation.
- [FILTER NETWORK] - Low pass network to filter out high harmonics and achieve sinusoidal waveform.
Those are conditions in a U.S. rule, not measured characteristics of this science-fair circuit and not a blanket assurance that any transmitter under a nominal power figure is compliant. Compliance depends on the complete device and its emissions. The cited project does not report a compliance evaluation, output-power measurement, or controlled performance test.
The FCC defines an intentional radiator as “a device that intentionally generates and emits radio frequency energy by radiation or induction” in 47 CFR §15.3. A separate provision, 47 CFR §15.221, addresses certain intentional radiators used for an AM broadcast station on an educational institution campus under its own conditions. It is not a general exemption for a student-built transmitter. These cited regulations are U.S.-specific; builders elsewhere should check the rules that apply in their jurisdiction.
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Best Value
- COMPACT AND PORTABLE: This AM transmitter designed in a compact size, making it easy to carry and use anywhere.
- WIDE MODULATION RANGE: With a modulation range of 600KHz‑1500KHz, this transmitter allows you to transmit your audio Signa over a wide frequency range.
- CONVENIENT POWER SUPPLY: Can be powered by 2 x AA lithium battery, which not included. And this transmitter can also be powered by an external DC jack for extended usage.
- FLEXIBLE AUDIO OUTPUT: Equipped with a 3.5mm headphone jack, this transmitter allows you to connect headphones or other audio devices for convenient listening.
- VERSATILE USAGE: This transmitter for radio testing, audio experiments, or creating your own AM broadcasting station.
Rank #4
- AMT-MW207 medium wave transmitter is a simple AM signal source suitable for amateur electronics enthusiasts and radio enthusiasts.
- Simple circuit, it is only composed of common triodes and resistance-capacitance inductive components, without audio transformers, which is easy to make.
- Good timbre, within the rated transmission distance, the sound quality is close to that of FM broadcasting, and the signal-to-noise ratio is good.
- There is no need for an external antenna (tens of meters for medium wave), and the magnetic field leaked by the magnetic rod affects the receiver, which is easy to implement and the transmission distance is relatively short.
- There are many interfaces, designed with waveform test terminals, audio sockets, external power sockets, etc., which are easy to use and expand functions.
Rank #3
- [Convenient Power Supply] Powered by 2 x AA lithium batteries (not included) or an external DC jack, this transmitter versatile and suitable for extended usage. Say goodbye to frequent battery changes!
- [Flexible Audio Output] Equipped with a 3.5mm headphone jack, this transmitter allows you to conveniently connect headphones or other audio devices for seamless listening experiences. Enjoy your favorite radio stations with ease.
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- [Compact and Portable] This AM transmitter designed to be compact and portable, making it easy to carry and use anywhere. It for on-the- audio experiments or testing.
- [Wide Modulation Range] With a modulation range of 600KHz-1500KHz, this transmitter allows you to transmit over a wide frequency range, ensuring clear and uninterrupted broadcast.
What this demonstration can—and cannot—show
- It can illustrate the relationship between audio and the changing envelope of an AM carrier.
- It can show, with a nearby AM receiver, that a tuned receiver can reproduce audio carried by a modulated signal.
- It does not establish a reliable transmission distance, efficiency, signal quality, or exact output power; the project guide supplies no such measured result.
- It does not establish regulatory compliance merely because the schematic is published or the carrier frequency is within the cited U.S. band.
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