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Constructional Project PROJECT BY TIM BLYTHMAN 433MHz Transmitter Module The 433-434MHz LPD433 (low interference potential device) band is used by many devices for radio control. There is no need for a class licence, so low-cost 433MHz modules make it easy to create a wireless link. As it can be difficult to get these modules with a consistent interface and good performance, we have created our own version, which is a direct replacement for commercial equivalents. W e’ve published many projects that operate on the 433MHz LIPD band. Many include a transmitter and receiver module pair, or one or the other to interface with existing equipment. The existence of drop-in radio modules has made this quite easy. In the UK, regulation of radio spectrum is handled by Ofcom. Licence-exempt operation of lowpower devices in the 433MHz band is permitted under Ofcom’s Short Range Device (SRD) framework, as defined in its Interface Requirement documents (notably IR2030), which align with European CEPT and ETSI standards. The SRD allocation covers the frequency range from 433.05MHz to 434.79MHz, meaning that the band is actually centred closer to 434MHz than 433MHz. Many devices operate at the commonly used nominal frequency of 433.92MHz. Under these provisions, no individual radio licence is required, provided the device complies with the prescribed technical conditions. These include limits on effective radiated power, duty cycle, modulation type, occupied bandwidth and spurious emissions, as specified in the relevant ETSI standards referenced by Ofcom. Typical licence-exempt devices in this band operate at low transmit power (often around 10mW EIRP, depending on device category). SRD operation in the UK is on a non-protected basis. Devices are not 46 guaranteed freedom from interference and must tolerate it if it occurs. The 433MHz band is shared with other services, including licensed amateur radio users operating in the adjacent 70cm band, who may legally transmit at much higher power levels. In practice, this means that devices such as garage door remotes, wireless sensors and key fobs generally coexist without problems, but interference is possible, particularly in built-up areas or where many devices share the same frequency. Provided the relevant technical limits are met, anyone may operate a compliant transmitter in this band in the UK without applying for or paying for a radio licence. Past projects Nearly all of our projects using 433/434MHz transmitters and receivers have used prebuilt modules. However, we recently started incor- porating discrete transmitter circuits into some of our designs. The Remote Control Range Extender from January 2023 and the Secure Remote Switch from December 2024 and January 2025 both did so. They incorporate custom circuitry built around an RF chip, the MICRF113 ASK Transmitter IC. ASK is short for ‘amplitude-shift keying’, where digital data is encoded as different amplitude levels of the carrier wave. To ensure all these projects could still be built, we decided to design this drop-in replacement for the tiny transmitter modules. Our Transmitter design has the same size, shape and pinout, so is interchangeable. Later we’ll look at some minor differences from the modules; they are mostly improvements made possible by the MICRF113 chip. Module overview We also thought that this was a Fig.1: a typical application for these sorts of modules is to create a wireless link. The DATA signal is usually a bit stream with a specific type of serial encoding, rather than a plain HIGH/LOW digital level. The antenna is often a quarterwave monopole in the form of a piece of wire about 170mm long. Practical Electronics | May | 2026 433.9MHz Transmitter Module Fig.2: the Transmitter circuit mostly consists of the recommended RF capacitors and inductors for the MICRF113, along with a 13.56MHz crystal to set the carrier frequency. We added a voltage regulator and DATA input protection circuitry to allow operation with 3.3V or 5V systems. good time to take a deeper look at these modules, their general operating parameters and what their limitations are. There are of course many cheap versions available from online sellers, and the available information can be limited. The transmitter module is our focus in this article, although we will look at how the receiver extracts the signal of interest from the busy radio spectrum. This information is presented at the end of the article, in case you are only interested in building the Transmitter. Fig.1 shows a typical arrangement used to provide a wireless link. It is as simple as supplying power to the transmitter module and providing a data signal. What goes in the DATA pin on the transmitter generally comes out at the DATA pin on the receiver, subject to the limitations of the modules, which we also discuss later. Transmission over distances of up to 100m are possible in open air. Usually, the data is an encoded packet reporting a status or a command. The packets are brief (perhaps 0.1s in duration) and are only transmitted occasionally, which helps to avoid interference with other equipment. Even if one packet interferes with another device, it’s unlikely the intervals between packets will match exactly, so they will only interfere occasionally. The software at each end is often Practical Electronics | May | 2026 designed to encode an identity (possibly using DIP switches or the like), as well as the command or data. This is so that a similar nearby transmitter does not trigger a receiver unless both share the same identity value. Some codes also include a checksum to validate the integrity of the data. Circuit details Much of the circuitry needed, shown in Fig.2, is outlined in the data sheet for the MICRF113 chip. It should come as no surprise that it is also similar to the Discrete Transmitter for the Secure Remote Switch. As well as IC1, the MICRF113, there is 13.56MHz crystal X1, along with its requisite load capacitors, operating as a frequency reference. IC1 uses an internal ×32 PLL (phase-locked loop) to multiply that up to 433.92MHz. The capacitors and inductors on the PAOUT pin (which delivers the RF signal) form a signal matching network to limit the transmitted harmonics. The LINK resistor should normally be a 0W jumper, but a resistor can be used instead to reduce the output RF power and thus the necessary supply current. There are more details on this in the MICRF113 data sheet. A solder blob across its pads would also work! The MICRF113 has an operating supply voltage range of 1.8-3.6V; we have chosen a nominal 3.3V and added a voltage regulator to provide this. The current demand is about 2mA in the quiescent state and 12mA when transmitting, so a small regulator is all that is needed. We used the MCP1700, which can handle up to 6V at its input, as we expect in most cases it will be receiving 5V. Its dropout voltage at the expected load levels is less than 0.1V, so it won’t cause a problem if a lower supply voltage like 3.3V is used instead. The two 1μF capacitors are required by the regulator for stability, while the 100nF capacitor provides local supply bypassing for IC1. Finally, the incoming data signal (from a microcontroller or the like) comes through a series resistor with a shunt diode to limit the voltage at the ASK pin. Thus, our Transmitter is compatible with 5V and 3.3V supply and logic levels. Assembly The Transmitter has been designed to fit in the same compact footprint as similar commercial modules, which tend to have similar pinouts. To keep this device a similar size to commercial offerings, the assembly involves working with small SMD parts. The MICRF113 chip comes in the SOT-23-6 package, while most of the passives are M2012 (0805 imperial) at 2 × 1.2mm, with one slightly smaller (M1608/0603 or 1.6 × 0.8mm) part. You’ll need the standard SMD gear such as flux paste, tweezers and so forth. A magnifier and good light will be a great help with such small parts. Fume extraction is always recommended when working with flux paste, and you should have a solvent to clean up the flux residue, too. We’ve managed to fit all the parts on one side, so you can use some Blu-Tack or similar to keep small the PCB in position on your workbench. Solder wicking braid will be handy to remove any solder bridges that might occur. We’ve managed to fit practically all the component designators on the PCB silkscreen, but it’s best to refer to the overlay diagram (Fig.3) and accompanying photo to confirm the location of the components. Start by spreading a thin layer of flux paste on all the PCB pads. IC1 has the smallest pin pitch, so place it first. Its pin 1 designator is very small, so you will probably need to examine it under magnification to confirm the correct orientation. It must be aligned 47 Constructional Project You can also check your assembly against the photo here (shown enlarged and at actual size on the left). We used a right-angled header, but you might like to use straight headers to allow the module to be mounted parallel to a PCB, like we did with the Secure Remote Switch. We’ve designed our Transmitter to be a drop-in replacement for the prebuilt module shown in the centre and on the right, which is a fairly common type. It is even the same size with the same pinout. Fig.3: the PCB is quite small, but we managed to fit most of the component designators on the PCB silkscreen. You can check the components on the overlay diagram here as you go. Fig.4: with the components fitted as shown here, you can probe for continuity at the marked pads, which will indicate whether the inductors have been soldered correctly. 48 with the matching mark on the PCB silkscreen, which is near L1. If you have the PCB upright, with X1 at the top and the external connections at the bottom, the text on IC1 should be upright, too. Clean the tip of your iron and add some fresh solder. Tack one lead and confirm that the others are flat and within their pads. If needed, adjust the part position by using the iron to remelt the solder. Then solder the remaining leads. Use solder-wicking braid to remove any solder bridges as you go. It will be trickier as more components are added. Add a little more flux paste, press the braid against the bridge with the iron and then slowly draw both away when the excess solder is taken up. Next, fit the two SOT-23-3 parts. D1 is near IC1, while REG1 is near the external connector. Fortunately, they should both only fit one way. You can use the same idea; tack one lead, adjust and then solder the remaining leads. These parts have leads that are quite spread out, so they should not bridge easily. Next, fit the two inductors. These will be a bit fiddly, since their leads are only on the undersides. You will need to apply the iron to the PCB pad and add solder, allowing it to melt and flow onto the leads. The smaller 68nH part is L2, which sits between the 5pF and 12pF capacitors, while 470nH inductor L1 is between IC1 and REG1. While L2 is the smaller M1608/0603sized part, we have used M2012/0805 pads to make soldering it easier. All the remaining two-lead parts are in M2012/0805 packages. If you are unsure that the parts have been soldered correctly, you can check this with a multimeter set to continuity or resistance modes. The inductors have low DC resistance, so both should read well under 10W. With none of the surrounding parts fitted, they are safe to probe. For L2, probe the adjacent pads on the 12pF and 5pF capacitors, as shown in Fig.4. Using a nearby pad eliminates the chance of a false positive in the event that the component is connected to the solder in the joint but not the pad below. For L1, try the other end of the 12pF capacitor and the end of the 0W link next to L1. If you get a low resistance reading across each inductor then all is well. If not, try adding some flux paste to each joint and reflow the solder with your iron before checking again. The passives Fit the crystal, X1, next; it is unpolarised, as are the other remaining parts. Its leads are quite small, so you might need to use a similar soldering technique to the inductors. The larger PCB pads should make this easy, although you won’t be able to test for continuity in the same fashion. Be careful not to mix up the capacitors. The two 18pF capacitors Practical Electronics | May | 2026 433.9MHz Transmitter Module are adjacent to the crystal and then, on the left of the PCB, are the 12pF capacitor above L2 and the 5pF crystal below it. The two 1μF capacitors are near REG1, while the 100nF part is next to IC1. Move on to install the 4.7kW resistor in the bottom-right corner and the 0W link nearby. Finally, fit the header of your choice; we used a right-angle header to match the prebuilt transmitters. Use a solvent to thoroughly clean the flux from the PCB and allow it to dry. Inspect your soldering with a magnifier and confirm that all the components are soldered correctly with no bridges. Testing If you wish to proceed cautiously, you should power up the Transmitter from a current-limited supply. The Transmitter should draw around 3mA while idle or 15mA when transmitting. Something simple, like a 330W resistor in series with a 5V supply, would also be suitable. Add a jumper wire or similar between the GND and DATA pins to ensure that the Transmitter is initially in the idle state. Then apply power and measure voltage across the resistor; it should be no more than around 1V. The next step is to apply a waveform to the DATA pin and see that it is received correctly. You may have a project planned for the Transmitter, in which case you should simply connect it and try it out. Another simple test we tried can be done with a piezo transducer and a 433MHz receiver module. Wire up the receiver module so that the piezo is connected between its DATA output and GND, then connect 5V power. You might not need an antenna to test over short distances, such as on a workbench. The piezo should emit a sound like white noise or static; this indicates that it is picking up normal background RF noise. If you then power on the Transmitter and drive its DATA input high, the noise should cease as that signal overwhelms the background noise and saturates the receiver’s automatic gain control (AGC). Driving the DATA pin low should similarly cause the background noise to resume. Applying a 1kHz square wave to the DATA pin should cause a high-pitched noise to be emitted Practical Electronics | May | 2026 from the piezo. Note that you should always make sure the DATA pin is driven, since it is a high-impedance input and could otherwise float to an unknown level. Conclusion You can read more about some of our comparative tests in the text below, but we have found our Transmitter to be just as good, if not better than, other similar transmitter modules it is a drop-in replacement for. It is capable of transmitting at 10mW, which could fall foul of the EIRP restrictions if used with a highly directional antenna. So we recommend sticking to simple antenna designs, such as a quarter-wave dipole, (~170mm for 433MHz) to ensure that you do not exceed the licence limits. If you must use a directional antenna, replace the 0W link with a resistor to reduce the output power (refer to the MICRF113 data sheet). A detailed analysis of 433MHz modules We performed some testing on these modules to ensure our replacement performed at least as well. We purchased a transmitter and receiver module pair from a local electronics shop; these will be our test subjects, alongside our new Transmitter. The information on their website indicates a maximum supply current of 10mA and a maximum output power of 3dBm (or 2mW) for the transmitter. The suggested data rate is 300bps to 10kbps. While the 2mW might sound like quite a low transmission power level, the EIRP (equivalent isotropically radiated power) could be higher. It is calculated as though the maximum signal strength (which might only occur in one direction) was radiated in all directions. In fact, the EIRP can be no less than the actual power. For a theoretical isotropic (outputting the same power in all directions) antenna, the figures will be the same. A highly directional antenna will have higher EIRP since the radiation is concentrated. Fortunately, we nearly always use non-directional antennas with these modules. The typical gain of quarter-­ wave monopole antennas is no more than 3dBi, which is about a factor of 2, keeping the EIRP to around 4mW, well under the 10mW limit. The MICRF113 in the Transmitter is capable of more power, but it too is unlikely to fall foul of the restrictions with a quarter-wave monopole antenna. The current draw on the MICRF113 peaks at around 13mA during transmission, so it does draw slightly more current for a substantially higher output power. Receiver operation If you have ever monitored the output of the receiver modules while nothing is transmitting, you might have Parts List – 433MHz Transmitter Module 1 double-sided PCB coded 15103251 measuring 19 × 15mm 1 4-way pin header, straight or right-angled to suit application (CON1) 1 13.56MHz 5.0 × 3.2mm surface-mounting crystal (X1) [Abracon ABM3-13.560MHZ-B2-T] 1 470nH inductor, M2012/0805 size, >434MHz SRF (L1) [Coilcraft 0805HT-R47TJLB or Murata LQW21HNR47J00L] 1 68nH inductor, M1608/0603 size, >434MHz SRF (L2) [Bourns CW16080868NJ, Coilcraft 0603CS-68NXJLU or Murata LQW18AS68NJ00D] 1 4.7kW M2012/0805 size SMD resistor, ⅛W 1 0W M2012/0805 size SMD resistor or value to suit (see text) Semiconductors 1 MICRF113YM6 SOT-23 ASK transmitter IC, SOT-23-6 (IC1) 1 BAT54C/BAT54S/BAT54 200mA 25V schottky diode, SOT-23 (D1) 1 MCP1700-3302 3.3V LDO voltage regulator, SOT-23 (REG1) Capacitors (all SMD M2012/0805 size, 50V ceramic unless noted) 2 1μF 50V X7R 1 100nF 50V X7R 2 18pF C0G/NP0 (or to suit crystal) SC7430 Kit (~£12 + postage): 1 12pF C0G/NP0 includes all the parts listed here 1 5pF C0G/NP0 49 Constructional Project Fig.5: examples of various encodings that are used with RF (and IR) systems. The pulses are different to encode a 0 or 1 while maintaining a duty cycle near 50%. Manchester encoding is often decoded by looking for the rising or falling edge in the middle of the bit time, rather than the pulse length or spacing. Scope 1: in the absence of a strong, nearby transmitter, the receiver modules will deliver noise. When connected to a piezo transducer or similar, it sounds like white noise. noticed the signal is just noise; there is no always low or high idle state. Scope 1 shows a scope grab of the receiver output when no transmission is occurring. This is due to the way that the receivers resolve signals with different strengths. During transmission, they must be able to deliver a valid signal, whether the transmitter is nearby or far away. In other words, they must be able to cope with receiving weak or strong signals and produce the same output. The operation of many infrared (IR) receivers is much the same too, although most IR receivers have extra circuitry to suppress the output noise during idle periods. Though the modules are described as using ASK modulation, it is typically the most simple form called OOK (on-off keying). With OOK, one of the levels is ‘carrier on’ and the other is ‘carrier off’. Here, the carrier refers to the underlying 434MHz signal. The raw data is also further encoded with the likes of pulse-length, pulsewidth or pulse-distance modulation (as also used in IR remote controls). Manchester encoding is another system that is also used in these scenarios. Fig.5 shows representative waveforms of some of these encodings. The simple on-off nature of the data means that the receiver only needs to recognise two signal levels, and this is done with the assistance of automatic gain control (AGC). This is much the same principle that ensures that nearby and far AM radio stations are received at the same volume. The AGC takes an average of the incoming signal strength, compares it to the instantaneous strength and adjusts the receiver gain to compensate. The AGC response time will also dictate a minimum data rate; if the receiver sees a carrier on state for too long, it will saturate and start producing noise. This is why the various encoding schemes have a duty cycle close to 50%. It means that the carrier on and carrier off levels are a similar distance from the average that the AGC sees, and both output levels are decoded correctly. Analog behaviour Scope 2: the current draw (and thus output power) of the commercial transmitter module shows an analog response to a triangle wave, suggesting it is capable of amplitude modulation (AM) to some extent. 50 There are some reports of these modules being capable of transmitting and receiving analog data, such as voice Practical Electronics | May | 2026 433.9MHz Transmitter Module or audio, using AM (amplitude modulation). But it’s doesn’t appear to be possible with either the commercial receiver or our Transmitter. The commercial transmitter might be capable of AM transmission, so could be used for this purpose with an appropriate receiver. Scope grabs of our tests are shown in Scope 2 (commercial Transmitter) and Scope 3 (our Transmitter). In these, the blue trace is a triangle waveform from a signal generator, which was fed into the DATA input of the transmitter module. The red trace is the output of a nearby receiver, and the green trace is the voltage across a resistor in the transmitter’s positive supply; a crude current measuring shunt. Being in the positive supply, a lower voltage means more current being consumed by the transmitter. Assuming that the current reflects the strength of the RF transmission, we can get an idea of whether the modulation is AM or simple OOK. For Scope 2 (the commercial transmitter), we can see that the current does indeed follow the incoming signal level over a range, while Scope 3 (our Transmitter) shows a very digital response, with a hysteresis between 1.3V (falling) and 2.0V (rising). So we don’t think our module will be suitable for AM transmission. In both cases, you can see that the receiver has a very ‘digital’ response, so we don’t think it could be used for AM reception. There is what appears to be some hysteresis in the receiver output, but part of that may be delays in the receiver and its AGC response. Comparative tests To keep the tests between our module and the commercial one fair, we set everything up on a breadboard so that we could swap between the two transmitters without changing anything else. We did not attach any external antenna to the modules. The breadboard strips provide a very short antenna that was sufficient for transmission over short ranges. We looked at aspects like signal delay between the transmitter and receiver and the response to transmitting at different data rates. For all these, we used our Arduino Uno to generate a pulsewidth waveform of the type used in 433MHz applications. Each cycle is nominally 720μs, giving a 1.4kbit/s data rate. Practical Electronics | May | 2026 Scope 3: our Transmitter has a more digital response, even showing hysteresis. This should provide a cleaner signal when used in digital applications, as is usually the case for these modules. Scope 4: the blue trace shows a transmitter input, while the red trace is a receiver output. The short delay between the two is around 20-40μs and differs slightly between the modules. We used an oscilloscope to compare the data coming from the Uno to the data received by the receiver module. Scope 4 shows the delay between the input to the transmitter and the output from the receiver. For both transmitters, the rising edge was propagated more quickly than the falling edge (by about 10μs). Also, our Transmitter showed quicker response times in general; around 20μs compared to 35μs for the commercial module. This could be partially due to the higher output power of our Transmitter, but there may also be some difference in the way that the incoming signal is modulated. Our findings in the Analog behaviour section above are consistent with that. The next test involved speeding up the waveform until we started to see missed and distorted pulses. Our Transmitter did not miss a pulse until we reached cycle times under 95μs or around 10.5kbit/s, while the prebuilt module we purchased started missing pulses at around 105μs or 9.5kbit/s. Scope 5 shows the conditions we looked for. The transmitter input is delayed to align it better to the receiver’s output; you can see a few locations where the receiver waveform has stayed high when it should have gone low. SDR analysis We also examined the output of the transmitters using a software defined 51 Constructional Project Scope 5: the green trace is the transmitter input delayed slightly to roughly align it with the red trace of the receiver output. You can see that at higher pulse rates than in Scope 4, pulses are missed and the asymmetry between rising and falling edges is more pronounced. radio (SDR) receiver. The necessary hardware can be found in cheap USB TV receiver dongles. The free AirSpy SDR# program provides a way of receiving and viewing the spectrum of the RF signal. We used a low-cost DVB-T dongle with an RTL2832U chip as the receiver. Since we were performing simple comparative tests, we used the basic whip antenna included with the dongle. Screen 1 (our Transmitter) and Screen 2 (the one we bought) show the received spectra, with a displayed bandwidth covering the approximate range of the 433MHz LIPD band. The SDR is not a precision device, so the readings are simply relative to its full scale (0dB at the top of the spectra). The peak of both transmitters sits very close to 433.92MHz, as expected. Our Transmitter has a few spurs and it peaks at around -4dB, while the commercial transmitter has a wider spread and more spurs. It peaks at around -8dB. That is not surprising, given the Screen 1: the spectrum of our Transmitter is neatly confined to the 433MHz LIPD band. 52 expected output power given by the respective data sheets. Summary Our Transmitter works as a dropin replacement for a typical prebuilt transmitter; it should work in all applications that require a digital transmitter on the 433MHz LIPD band. The only thing our module can’t do that the prebuilt one can is transmit analog signals (ie, amplitude modulation). It works especially well for PE digital transmissions, though. Screen 2: the commercial transmitter module has a wider spread, more spurs and lower peak power than our Transmitter. Practical Electronics | May | 2026