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By Charles Kosina, VK3BAR Low-power, digitally-controlled FM TRANSMITTER » Short-range stereo FM transmitter tuneable from 87.5MHz to 108MHz in 100kHz/1MHz steps » Variable RF output in eight steps, under the 25μW legal limit with the specified wire antenna » Variable audio gain in eight steps » Usable range: about 10m » Power: various battery options, 1-3 cells, primary or rechargeable » Current consumption: about 26mA with a 4.2V battery » Optional integrated battery charger » A small OLED screen to show the status » Inexpensive and straightforward to build This small FM broadcast-band transmitter has frequency, gain and output power controls. It shows the current frequency and other settings on a small OLED and runs from a battery of three AA cells or similar. T his FM Transmitter uses just a few parts as it’s based on a small, inexpensive, digitally controlled stereo FM transmitter module. The only major parts required in addition to that are an Atmel microcontroller, a small OLED screen, a small switchmode boost module, a rotary encoder and two potentiometers. Such a module allows you to test FM radios, play music remotely, or use an FM radio as a basic type of intercom. The transmitter module uses a KT0803L chip and is not expensive; you can order it from the AliExpress link in the parts list or our Online Shop. However, it is not a ‘plug and play’ device. It has 18 8-bit registers that need to be set up each time it’s powered on. These are all accessed over an I2C two-wire serial bus. To choose an appropriate microcontroller, we need to consider how many I/O pins are required. The rotary shaft encoder requires three pins, the I2C interface needs two, the power and audio level pots need two analog inputs, plus one for battery voltage measurement. That’s a total of eight I/O pins needed. There are plenty of inexpensive microcontrollers that will handle that. I chose the ATMega328 in a 52 Silicon Chip 32-pin TQFP package as I have several available but the ATMega168 or even ATMega8 can be used as well. They all have more than enough pins for the task. To expand the battery choices, I added a boost converter module, which gives a 5V DC output for an input ranging from 1.5V to 4.5V. This means it can run from two or three AA alkaline cells (one would work, but it would be ‘flat’ very quickly). The advantage of using three over two is that they can be discharged down to 0.5V each. Cells that have been discarded from other equipment can be used. Rechargeable batteries can be used too, either three 1.2V rechargeable NiMH cells or a single Lithium-­ ion cell. In both cases, they can be recharged from an external 5V supply. The current drain with a 4.2V battery is 26mA. As battery voltage goes down, the current drain will rise in proportion. If using alkaline cells, once each gets below 1V, it will be discharged fairly rapidly. This project’s inspiration This project was inspired by a competition being run by the Historical Radio Society of Australia (HRSA) Australia's electronics magazine to build a low-power AM transmitter using valves. The original design was published in Electronics Australia in May 1989. Such a device could be used to connect to a sound source and transmit to nearby AM receivers on an otherwise unused frequency. The May 1989 circuit is shown in Fig.1. It is an extremely simple design: two inputs are mixed and amplified by a triode, which is coupled to a second triode that modulates the third triode acting as an AM broadcast band oscillator. It obviously works, but there are limitations. The antenna is connected to the oscillator coil and would detune the frequency. The oscillator coil has to be wound with about 100 turns. Great care has to be taken working with a circuit running at such a high voltage (250V). I briefly considered building it as I have a sufficient stock of valves in shoe boxes. But these days, I no longer have the high-voltage capacitors and 1-2W resistors, as most of my designs run from low voltages (typically 3.3V or 5V) and use surface-mount components. I thought about making a solid-state transmitter on the FM broadcast band rather than AM. I quickly sketched up siliconchip.com.au Fig.1: the May 1989 low-power AM transmitter circuit that inspired this project. Triodes V2a & V2b are stacked in series; the HT supply passes through the secondary of T1 to the anode of V2a, then through V2 to its cathode and onto V2b’s anode. So V2b’s anode current comes from V2a’s cathode. Thus, the audio signal at V2b’s grid modulates V2a’s supply voltage and so the output amplitude. Fig.2: the analog FM transmitter circuit I originally considered building before I settled on the digitally controlled version. Lacking a stereo modulator, it necessarily mixed the two input channels down to mono and would have been fiddly to set up. such a transmitter using a couple of transistors, which is shown in Fig.2. The oscillator is a JFET and its output is amplified by an NPN transistor. A pi-coupler on the output is connected to an antenna. Two inputs are provided, which could be two channels of a stereo source. They are applied to a varicap to produce the frequency modulation. I went as far as building a prototype of this circuit, but abandoned it for the following reasons. Setting the frequency with a slug in the oscillator coil would have been a fiddly job, and getting the right level of modulation would require a lot of trial-and-error. Also, it would only produce a mono FM signal; one of the benefits of FM radio is that it supports stereo encoding. I then thought about the fact that there are plenty of FM transmitters cheaply available at various shops, designed to plug into a car power socket, for listening to music in a car siliconchip.com.au with an FM radio. I could use one of those ‘off the shelf’. Looking on AliExpress, I came across a very simple module that transmits a stereo FM signal, leading to this project. Circuit details The full circuit is shown in Fig.2. There are three main parts: an FM transmitter module (MOD1), a microcontroller to configure it and provide the user interface (IC1) and a power supply (REG1). Both the FM transmitter (MOD1) and OLED screen (OLED1) are controlled over a shared I2C bus using two wires: SDA (data) and SCL (clock). These are driven by the hardware I2C interface within the microcontroller. An 8MHz crystal has been provided, connected between pins 7 (XTAL1) and 8 (XTAL2), with appropriate load capacitors. This isn’t strictly necessary as the microcontroller has an 8MHz internal RC oscillator and the frequency is not critical. However, if Australia's electronics magazine you set the ATmega328 fuses to use an external crystal and none is present, it’s effectively ‘bricked’, so it’s safer to have one. Rotary encoder RE1 is used to set the frequency. It has two Gray code outputs, A & B, plus an internal switch that’s activated when the knob is pressed in. All three outputs switch to ground when active, so they have 33kW pull-up resistors and 100nF capacitors to ground for debouncing. They are sensed and decoded by IC1’s PD2-PD4 digital inputs. The gain and power output potentiometers (VR1 & VR2) connect across the 5V supply, and their wipers connect to two analog inputs pin on IC1 with 100nF filter capacitors. IC1 uses its internal analog-to-­ digital converter (ADC) to convert the wiper voltages to a number from 0-1023 corresponding to the rotational position of those pots and then sends appropriate commands to the FM transmitter. September 2026  53 Power supply Whatever battery is used, its voltage will typically be in the range of 1.5V to 4.5V. This is fed to the input of switch-mode regulator module REG1 via power switch S1. REG1 produces a 5V output as long as the battery is within the 1.5-4.5V range, which powers IC1, MOD1 and OLED1. CON1, D1, D2 and the 10W resistor provide a means to recharge the battery without removing it. If using a rechargeable battery, you have two options: one Li-ion/LiPo cell (3.7V nominal, 4.2V fully charged) or three NiMH cells in series (3.6V [1.2V per cell] nominal, 4.2V [1.4V per cell] fully charged). In both cases, the 10W resistor limits the charge current while the diodes prevent the battery from being charged above 4.1V – it’s critical that Li-ion/ Table 1: Chip ATmega328PB ATmega168(P) ATmega8 Fuse Extended Byte 0xF5 0xF9 × Fuse High Byte 0xD1 0xD5 0xD1 Fuse Low Byte 0xFF 0xFF 0xFF (Low Byte int. osc.) 0xE2 0xE2 0xE4 LiPo types are not charged above 4.2V. That requires a well-regulated 5V DC supply to be used for charging, as the charge termination voltage depends on the source voltage being close to 5V (5.1V maximum). If a rechargeable battery is not used, these parts can be left off. Alternatively, you could opt for external DC power and forego the battery, powering it via CON1 instead. CON5 provides the optional 38,400 baud serial debugging interface. Only serial output is supported, using Mosfet Q1 with a 1kW pull-up resistor as an inverter and isolator. If you don’t need the debugging interface then CON5, Q1 and the pull-up resistor don’t need to be fitted. Panel preparation Before mounting parts on the PCB, use it as a drilling template. Attach it centrally to the front panel using tape (eg, masking tape). Use a 3mm drill bit to make two diagonal holes and attach the PCB to the panel with 3mm screws and nuts. Fig.3: the circuit of the final FM Transmitter design. IC1 loads the required configuration into MOD1; the audio signal is fed directly into MOD1 from an external source. Rotary encoder is used to change the configured transmission frequency, while VR1 sets the output power level and VR2 adjusts the modulation level. 54 Silicon Chip Australia's electronics magazine siliconchip.com.au Next, use a 1.5/1.6mm drill to mark the position of the four holes for the encoder, potentiometers and switch, and then a 3mm drill for the two remaining corner mounting holes. Remove the PCB and drill out the four 1.5/1.6mm holes to 6.5mm. Another hole has to be drilled in the side to line up with the 3.5mm audio socket on the Elechouse module, plus one for the barrel socket if you are using it. Programming IC1 You have three options for programming the microcontroller, IC1: 1. Purchase a pre-programmed microcontroller from our Online Shop (siliconchip.au/Shop/9/7724) and it will be ready to solder to the board. No further programming will be necessary. 2. Program it before you solder it to the board. You will need an Atmel serial programmer as well as a TQFP32 programming adaptor, such as the one we published in the October 2023 issue (siliconchip.au/Article/15977). The PCB for that adaptor is available from our Online Shop. 3. Program it after soldering it to the board. In this case, you will need a serial programmer with a six-pin socket or ribbon cable. You will also need to solder the six-pin programming header, CON2, to the board. Cheap Atmel programmers are available from AliExpress, such as this one for about $6, including postage: www.aliexpress.com/ item/1005005962442597.html Jaycar also sells a suitable program- Table 2: CKDIV8 1 Do not divide clock frequency by eight CKSEL 1111 Low-power crystal oscillator, 8-16MHz SUT 11 fast rising power, 14 clock + 4.1ms delay EESAVE 0 EEPROM memory is preserved BODLEVEL 101 Brownout detector = 2.7 All fuse field differences from defaults mer, Cat XC4627. Make sure to get the 10-pin to 6-pin adaptor (or you can make one yourself). In both cases, the required software is a free download. Three versions of the HEX file are provided in the download package (siliconchip.au/Shop/6/3643) depending on the exact microcontroller used. Be sure to select the right one. Connect the programmer to the 6-pin header (or if using the TQFP adaptor, wire it up to that board) and use the software to program the HEX file into the chip, then set the fuses to the values provided in Table 1. Note that if you’re using the TQFP adaptor, once you set the fuses, you won’t be able to make any more changes as the chip will need the crystal connected to work. Fuses All three supported chips have at least two fuse bytes (low and high), each with eight bits of configuration data. After you’ve programmed the HEX file into data memory, set them as shown in Table 1. The low byte value for internal oscillator operation is provided in case you haven’t fitted crystal X1. If you have, use the 0xFF value. How you set these depends on the software you’re using, but it should show you the values that will be programmed in hexadecimal, so make sure they match the values given. The differences from the default values are shown in Table 2. Construction The components used are a combination of through-hole and surface-­ mount devices (SMDs). Fit all the SMD devices first; most of them are on the board under the OLED display. Start with the 32-pin microcontroller, IC1, which has pins on all four sides. Make sure the microcontroller has the correct orientation and is positioned centrally on the pads before soldering it. There should be a dot or divot in one corner, indicating where pin 1 is. That corner goes at upper-left, as shown in Fig.4, the PCB overlay diagram. Tack one pin first, then re-check the orientation and placement, verifying the pins on all four sides are correctly centred on their pads. If not, re-melt the joint and gently nudge the chip into position. Repeat until you are happy, then solder the diagonally opposite pin. Proceed to solder the pins on one of the other two sides, first adding a thin layer of flux paste along the pins. With good-quality flux paste on the pins, all you need to do is clean your soldering iron tip, add a bit of solder, then gently drag it along the pins and solder will flow onto them. Another The underside and top of the FM Transmitter PCB. Note that the antenna cable on the FM transmitter module connects to a pad on the underside of the PCB. siliconchip.com.au Australia's electronics magazine September 2026  55 Fig.4: assemble the FM Transmitter by mounting the parts on both sides as shown here. Solder the SMDs first, then the through-hole parts on the same side, then the rest. Note how the OLED screen fits over the top of IC1/X1. The antenna wire from MOD1 goes to a pad on the main PCB just under the lower-right corner. Make sure REG1 is fitted with the orientation shown. Fig.5: this shows where to drill the hole in the right side of the case so you can plug into the audio input socket of MOD1. All dimensions are in millimetres (100% scale). technique is to add a little solder to the tip, then touch the tin to the end of the pin and let the flux draw the solder onto the pin and pad. Repeat until all pins on one side are soldered. Don’t concern yourself too much if you accidentally bridge pins while soldering the first side, since it’s easy enough to fix later. Once one side is soldered, do the other side, then repeat for the other two (where you originally tacked one pin). Check carefully for bridges (eg, using a magnifier) and if you find any, add a bit of extra flux paste, then use solder-wicking braid pressed down with the tip of the soldering iron to draw off the excess solder. Once all bridges are cleared, clean off the flux residue (eg, using alcohol and a lintfree cloth) and then go over the joints again to verify they all look good. After that, use a similar technique to solder Mosfet Q1 (if you are fitting it) but it only has three pins to solder. Once that’s finished, move onto the passives (resistors and capacitors). These are not polarised so you don’t have to worry about their orientations as you solder them. 56 Silicon Chip The resistors should be marked with their values in scientific notation (eg, 33kW = 3302 or 333), while the capacitors will be unmarked but there are only two different values. Ensure the 22pF capacitors go into the right locations as marked. Through-hole parts Now install the through-hole components on the front of the board, using the front panel as a guide to make sure the two potentiometers, encoder and toggle switch are positioned accurately. The OLED screen and transmitter module are plugged into socket strips and attached by screws with standoffs. The OLED can come in slightly different sizes and can have either 2mm or 2.5mm mounting holes. Don’t try to drill out the 2mm holes to the larger size as it would most likely wreck the module. Although four mounting holes are provided for the modules, only two are actually required. Use a short piece of insulated wire to connect the ANT terminal on the transmitter module to the antenna pad on the main PCB, which is near the pads Australia's electronics magazine for the debug header mounted on the other side (CON5). Now that you’ve soldered all the parts, inspect the board for any shorts between pins and bad solder joints, and clean off any remaining flux residue. On my board, I fitted an SMA socket to connect the output to a spectrum analyser (CON3) but it is not necessary. Instead, you can simply solder a wire to the central pad provided from CON3 and attach it to an antenna socket on the back panel of the case. That can be an RCA panel-mounting socket. The external antenna just needs to be a piece of wire about a quarter of a wavelength, which is 75cm at 100MHz. There is no ground plane as such, but the shielded wire to the sound source can be looked upon as the other half of a dipole. We are not after any great efficiency here, as the required range is only some meters. Besides, the maximum allowable radiated power (EIRP) to operate in this band without a license is just 25μW. You don’t need a high-­ efficiency antenna to achieve that. In fact, you don’t want a high-efficiency siliconchip.com.au antenna to ensure you aren’t exceeding the legal limit! Parts List – FM Transmitter Battery selection 1 double-sided PCB coded CSE260501C, 76 × 59.5mm 1 black front panel PCB coded CSE260502, 86.5 × 80mm, 0.8mm thick 1 5V DC regulated plugpack (optional, unless using a rechargeable battery) 1 115 × 90 × 55mm case with clear acrylic lid [AliExpress 1005009896550305] 1 3 AA battery holder OR 1 3.7V Li-ion or LiPo rechargeable cell (and holder if required) 1 panel-mount DC barrel socket (CON1) ● 1 3×2-pin header, 2.54mm pitch (CON2; optional, for programming IC1 in-circuit) 1 4-pin vertical polarised header or two 2-pin types, with matching plugs and pins (CON4) 1 3-pin vertical polarised header (CON5) ◆ 2 4-pin female socket headers (for OLED1 & MOD1) 1 Elechouse FM transmitter module (MOD1) [SC7712 or AliExpress 2840333316] 1 0.96-inch OLED display module with SSD1306 or compatible controller (OLED1) [SC6936 (white) or SC6176 (cyan)] 1 five-pin vertical rotary encoder with a 20mm-long shaft (RE1) 1 ND0205MA 3V to 5V DC step-up converter module (REG1) [SC7713 or AliExpress 1005006176918158] 1 miniature SPDT toggle switch with solder tabs (S1) 2 9mm 10kW linear PCB-mounting vertical potentiometers with 20mm shafts (VR1, VR2) 1 8MHz crystal, HC-49 (X1) 3 small knobs to suit RE1, VR1 & VR2 Hardware 4 M3 × 16mm tapped hex spacers 2 M3 × 10mm tapped hex spacers 2 12mm-long 2-3mm inner diameter untapped spacers 12 M3 × 6mm panhead machine screws 2 M2 × 16mm panhead machine screws 2 M2 hex nuts Semiconductors 1 ATmega328PB-AU microcontroller programmed with CSE0501A.HEX, TQFP-32 (IC1) ■ 1 2N7002 N-channel Mosfet, SOT-23 (Q1) ◆ 1 1N4004 400V 1A diode (D1) ● 1 1N5819 40V 1A schottky diode (D2) ● Capacitors (all SMD M2012/0805 50V X7R MLCC unless noted) 1 4.7μF 6 100nF 2 22pF NP0/C0G Resistors (all SMD M2012/0805 ±1% metal film unless noted) 3 33kW 1 1kW ◆ 1 10W 2W ±5% axial ● ■ ATmega8 or ATmega168 variants are also suitable ◆ optional; for debug interface ● for use with a rechargeable battery (3 NiMH AA cells or one Li-ion/LiPo cell) As mentioned earlier, there is a choice of batteries. In my prototype, I used a triple AA cell holder, which I attached to the inside of the case with double-sided tape. This can take either alkaline non-rechargeable cells or NiMH rechargeable cells, which can be recharged via CON1 if it’s fitted. An alternative is to use an 18650size Lithium-ion cell. These have a nominal voltage of 3.7V and a capacity of up to 3500mAh (although 2-3Ah is more typical). You may find these advertised by overseas suppliers with ludicrous capacities of up to 19,900mAh. This is sheer nonsense, so don’t buy them (if in doubt, stick with a local supplier like Jaycar or Altronics). A single 18650 cell holder can be glued inside the case. Testing If you haven’t already programmed IC1, do it now using in-circuit serial programming header CON2. Refer to the “Programming IC1” section above for instructions. Initial testing is with the board not yet installed in the case. Before connecting power, check with an ohmmeter to make sure there are no short circuits between pin pairs 1 & 2 or 3 & 4 of CON4. Apply power and you should see a splash screen showing FM TX on the top line and the version number on the bottom line. After a second, this is replaced by a four-line display. The top line shows the frequency, line 2 the power level, line 3 the audio gain, and line 4 the battery voltage. Rotating the encoder knob will change the frequency in 100kHz steps. Pressing the knob will toggle to 1MHz steps. Rotating the potentiometers will vary the power and gain levels from 1 to 8. Connect the transmitter to an audio source using a stereo cable with a 3.5mm TRS plug at one end. Find a clear channel on a nearby FM radio and tune the transmitter to that frequency. Try different transmit power settings and use the minimum that gives good results. If you have a spectrum analyser, such as a TinySA, you can fit CON3 and connect it there. You should see a spectrum like the one shown in SC Screen 1. siliconchip.com.au Screen 1: the Transmitter’s output spectrum. The second harmonic is -23dB compared to the fundamental. The EIRP is under 25μW with the specified ¼-wavelength wire antenna. Australia's electronics magazine September 2026  57