BA1404 stereo FM transmitter circuit diagram


BA1404 stereo FM transmitter circuit diagram, transmit from your portable audio equipment to your car radio, low-power FM transmitters are ideal for wireless transmission of audio signal to FM receivers. Especially in this case, the BA1404 FM stereo transmitter.

The heart of this circuit is the BA1404 integrated circuit, being a mini FM transmitter, which already has the necessary internal circuitry for FM transmission. Only a few external components need to be added. So, it can transmit from your Smartphone,  MP3 player, MP4, iPod, computer, disc man, or any other audio source, making it wireless.

If you have assembled other transistorized FM transmitters, you will see that when you assemble this one with the BA1404 integrated, the sound quality and frequency stability is excellent. In addition, it is easy to assemble.

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BA1404 stereo FM transmitter circuit diagram schematic

Ba1404 Stereo Fm Transmitter Circuit Diagram Schematic
Ba1404 Stereo Fm Transmitter Circuit Diagram Schematic

How does the ba1404 FM transmitter circuit work?

The power supply to the integrated circuit is 1.3 to 2.5 volts, note that if the voltage goes over 3 volts the IC burn. To power the circuit, we use an LM317L integrated circuit configured for about 2.4V. If you prefer, you can change the resistors to have a higher voltage. For example, by changing R1 to 330 ohms, we get 2.5V at the output. If you prefer, use this calculator for LM317.

The circuit formed by C1, L1, C2, C3 is a filter (optional) for transients that may come from our power supply. But prefer to use sources with good filtering.

R7/P2 and R8/P1 form the circuit that will limit the audio signal to the integrated circuit. According to the datasheet, this value should be 0.5V (500mV), so adjust the trimpot so that the audio signal is clear when transmitting. You may need to change the 27K resistors.

C12/R9 and R10/C13 form the pre-emphasis circuit, to calculate the value use the following formula Ct=RxC.

Capacitors C14 and C15 together with the IC input resistance (impedance) form a high pass filter, allowing the signal to pass above 30Hz. The input impedance is 540 Ω, for 30Hz frequency, the calculator value for the capacitor is about 10uF.

Capacitors C16 and C17 form a low-pass filter, for FM 15KHz, by calculations we arrive at about 20nF. You can use values of 18nF for the tolerance of the components, thus guaranteeing that it will pass the 15KHz range.

C19 is the bias capacitor for the audio stage signal and C18 is the bias capacitor for the 38KHz oscillator. R11 and R12 form the balance circuit for the multiplexed signal, use resistors with equal values or, if you prefer to adjust the balance, place a trimpot there.

The audio signal from each channel is amplified separately at 37dB and the signal applied to the multiplexer, the crystal oscillator connected to pins 5 and 6, create a 38Khz subcarrier and 19KhZ pilot signal with the same phase, but with 1/2 cycle delay. The audio signal and the 38Khz carrier are balanced and modulated in the multiplexer. Thus, we will have at the output the L + R signal, the 38Khz subcarrier and two carriers, the L-R. The multiplexed signal is output at pin 14. The 19KHz pilot signal is generated by dividing the crystal frequency by 2, this signal is output on pin 13.

The components connected to pins 13 and 14 mixes the signal, generating the MPX composite signal delivered to pin 12.

The integrated circuit has internally a special transistor to work with high frequencies, forming a Collpits oscillator. The components connected between pins 9 and 10 complete the FM modulator circuit. With the values in the circuit, we have covered the FM range. You may need to change capacitor values to achieve the result. Use ceramic capacitors of type NP0, to prevent the frequency from varying due to temperature change.

The FM signal is delivered on pin 7, you may need to change the value of C31 to get better power, values between 4.7pF and 15pF can be used. You can also apply this signal to a linear RF amplifier.

Q1 is the amplifier for an electret microphone (Optional)

Components for mounting the FM stereo transmitter

The antenna can be either a telescopic antenna or a 15 to 30 cm piece of hard-wire.
All components are easy to find, except for the Ba1404 IC and the 38kHz crystal, which can be found in many internet stores.

If you cannot find the BA1404 FM transmitter IC and the 38kHz crystal to assemble the stereo transmitter, please contact me and I will try to help you. Before contacting you, please search the sources I mentioned. You can find this circuit in kit form at Shopee, Mercado Livre or AliExpress, which makes assembly easier, since it comes with all the components to assemble the project.

Model of the coils that should be used in the fm transmitter.

The RF coil is usually a critical component in FM transmitter assemblies, the ideal coil for this circuit is a 3.5 turn ferrite core found in FM radios. Below you can see the model of this coil.

However, if you can’t find such a coil, you can make one yourself by winding 4 coils of 22 AWG copper wire over a 5 mm diameter ferrite core. To adjust the operating frequency of the stereo transmitter, use a plastic key to adjust and find a fm-free range.

If you cannot tune in the FM range 88 to 108 MHz, try changing the value of the tuning capacitor. In the last case the coil can be 4 coils of enameled wire with an air core and the capacitor a variable capacitor from 6 to 30pF. The reason for not using an air-core coil is that with changes in ambient temperature, there is a variation in the size of this coil that will interfere with frequency stability, which is not ideal in good quality FM generators.

Using it to transmit from your smartphone, iPhone, MP3 player, MP4, iPod or cell phone to your car radio.

If you want to use the stereo transmitter in your car to transmit from your MP3, iPod, cell phone, MP4 to your FM radio, you may need to decrease the power of the transmitter. To complete this, reduce the size of the antenna and use only one battery to power the circuit.

Suggested PCB for mounting the ba1404 stereo FM transmitter

Pcb Fm Stereo Transmitter Ba1404 Ic Bo
Pcb Fm Stereo Transmitter Ba1404 Ic Bo
Pcb Fm Stereo Transmitter Ba1404 Ic Component View
Pcb Fm Stereo Transmitter Ba1404 Ic Component View
Top Overlay Pcb Fm Stereo Transmitter Ba1404 Ic
Top Overlay Pcb Fm Stereo Transmitter Ba1404 Ic

This circuit is a low power FM stereo generator, but can be used with a power stage, for a version of a high power FM stereo transmitter.
For more information download the ic ba1404 datasheet in pdf and build your hi-fi stereo FM transmitter

BOM  to assemble the ba1404 FM transmitter circuit

Last update: 23/11/2021 20:13

Parts Value Description Quantity
C1, C2, C6, C7, C29, C30 10nF (103) Ceramic Capacitor 6
C3, C5 100µF/16V Electrolytic Capacitor 2
C4, C9, C14, C15, C19, C23 10µF/10V Electrolytic Capacitor 6
C8, C10, C11 100nF (104) Ceramic Capacitor 3
C12, C13 1nF  (102) Ceramic Capacitor or film 2
C16, C17, C18, C20, C25 1nF (102) Ceramic Capacitor 5
C21, C26, C31 10pF (10p) Ceramic Capacitor NP0 3
C22 220pF (221) Ceramic Capacitor NP0 1
C24 470pF (471) Ceramic Capacitor NP0 1
C27, C28 15pF (15p) Ceramic Capacitor NP0 2
IC1 LM317L Positive Adjustable Voltage Regulator IC 1
IC2 BA1404 Stereo FM transmitter IC 1
MIC1 MIC Electret microphone (optional) 1
Q1 BC546B NPN Transistor 1
Q2 38KHz Crystal 1
Resistors 1/4W 5%
R1 300 Orange, black, brown, gold 1
R2 330 Orange, orange, brown, gold 1
R3, R4 2.2K Red, red, red, gold 1
R5, R13 100k Brown, black, yellow, gold 1
R6 10k Brown, black, orange, gold 1
R7, R8 27k Red, violet, orange, gold 2
R9, R10, R11, R12 47k Yellow, violet, orange, gold 4
R14 2.7k Red, violet, red, gold 1
R15 10 Brown, black, black, gold 1
R16 270 Red, violet, brown, gold 1
CN1 DC Terminal block 2 pin 5.08 mm 1
J1 P2 stereo jack Audio Input
L2 Adjustable coil 1
L1, L3 Inductor 10µH Brown, black, black, silver 2
LSP1 Antenna Connect antenna 1
P1, P2 22k Trimpot horizontal 1
Solder, Wire, PCB, Housing, Power Supply or Battery, FM Antenna, etc.

Download the files for this assembly, including plate in PNG, PDF, POSTSCRIPT, and GERBER formats.



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Download BA1404 IC PDF datasheet

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About the author blog author. Electronics technician for the technical school of Brasilia - Brazil. Interested in electronics, circuits and technology in general.

1 thought on “BA1404 stereo FM transmitter circuit diagram”

  1. Excuseme i have a problem. I bought this same circuit in Electronics DIY. But i had a problem with the circuit noise or tuning, i dont know exactly which is the problem.
    When i am tuning using a plastic screwdriver, i hear so much noise, i cant tune the signal.

    I hope you can help me!!!

    Congratulations this is a perfect page!!!


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