TX-200 - 200mW FM Transmitter

Here is the latest and greatly improved TX200 VFO/VCO FM transmitter. The most versatile transmitter to date that can be turned into high fidelity stereo PLL based 200mW FM transmitter. It is a perfect circuit for transmitting your music around the house and yard. TX200 uses only two coils; one in the oscillator and the other one in the 200mW VHF amplifier so it should be fairly easy for anyone to build. It also includes built-in pre-emphasis and C5 for enhanced sound quality. While assembling the transmitter care must be taken to make sure that C1 is directly connected to L1 and C9 to L2. These caps eliminate the distortions form the DC supply and improve the sound quality greatly. 9V voltage supply is also very important because it provides the exact amount of current to Q1 to produce loud and clear sound quality. I hope that you'll have as much fun as I had while building this transmitter. Enjoy! ;)

Power Meter

TX200 comes with built-in LED based power meter. This is a very helpful tool that will tell you if your transmitter's oscillator is working properly. If RF signal is transmitted the LED will illuminate. Besides that, it will also give you a quick visual way to check how much power is being transmitted. I highly recommend that you have the transmitter and power meter on the same PCB. If you like to experiment a lot, you will appreciate this inexpensive but externally helpful addition.

How to use PLL to digitally tune the frequency of the TX200 transmitter?

If you want to use Phase Locked Loop (PLL) to digitally tune the frequency of TX200 then you will need to make small changes to the transmitter's oscillator. Theoretically, change original variable frequency oscillator (VFO) to voltage controlled oscillator (VCO). PLL uses voltage to change the capacitance of the varicap diodes / frequency of the oscillator so two BB105 varicap diodes or any 15-30pF varicaps will be perfect for this.

You vary the capacitance of the varicap diode(s) by applying voltage supply between 1 - 8V. If you apply 1V the capacitance will be around 34pF, and if you apply 8-30V the capacitance will be 12pF (minimum). Now, you may wonder why do we need to start from 1V? The reason is that if you apply voltages below 1V the capacitance may go as high as 200pF and we simply can't use that. 100K resistor will be used to lower the current of the voltage, because varicaps require only 0.2mA of current to operate!

With this knowledge we replace C6 - 15pF capacitor with two varicaps and one resistor. That much is enough to be able to use 1 - 8V voltage to change the oscillator's frequency. The second change that we will need to make is to use 2pF capacitor and connect it to the Q2 transistor's collector. PLL will use this to sample the frequency and compare it with digitally selected frequency. PLL will then compare those frequencies and if they are not the same PLL with vary the voltage that is applied to the varicaps, until those two frequencies are the same.

Before you even connect PLL circuit to the TX200 transmitter you will need to test if the oscillator is working properly. We can tune through entire FM band (88-108MHz) by applying the voltage to the varicaps. To do so, connect "PLL - Voltage Control" PIN to the 10K potentiometer as shown below. With this setup you will be able to use potentiometer to change the frequency of the transmitter. If you can't go through the entire FM band or if your frequency is out completely then you will need to change the inductance of the L1 coil.

PLL Module

The schematic and instructions for building this PLL module can be found at pira.cz. After building it and doing some testing with it I was amazed by how well this Phase Locked Loop circuit performs. It's stable, compact, very easy to build and connecting it to TX200 transmitter was a piece of cake!

Make TX200 a Stereo Transmitter!

You can easily turn TX200 into a high quality stereo transmitter by connecting this stereo encoder module that you can either build it yourself or buy it as a kit. Stereo encoder kit includes all the hard to find components such as NJM2035 IC and 38KHz crystal.

TX200 Prototype Images

Where to get the Parts

If you are building the above FM Transmitter and have trouble finding components such as variable coils, BF199 transistors or ferrite beads we are distributing these components in our Electronics-DIY store.

Accurate LC Meter Capacitance Inductance Meter with 16F628 and LCD

Volt Ampere Meter with 16F876 Microcontroller and LCD display

Accurate LC Meter

Build your own Accurate LC Meter (Capacitance Inductance Meter) and start making your own coils and inductors. This LC Meter allows to measure incredibly small inductances making it perfect tool for making all types of RF coils and inductors. LC Meter can measure inductances starting from 10nH - 1000nH, 1uH - 1000uH, 1mH - 100mH and capacitances from 0.1pF up to 900nF. The circuit includes an auto ranging as well as reset switch and produces very accurate and stable readings.

PIC Volt Ampere Meter

Volt Ampere Meter measures voltage of 0-70V or 0-500V with 100mV resolution and current consumption 0-10A or more with 10mA resolution. The meter is a perfect addition to any power supply, battery chargers and other electronic projects where voltage and current must be monitored. The meter uses PIC16F876A microcontroller with 16x2 backlighted LCD.

50MHz 60MHz Frequency Meter / Counter with 16F628 & LCD

60MHz Frequency Meter / Counter

Frequency Meter / Counter measures frequency from 10Hz to 60MHz with 10Hz resolution. It is a very useful bench test equipment for testing and finding out the frequency of various devices with unknown frequency such as oscillators, radio receivers, transmitters, function generators, crystals, etc.

1Hz - 2MHz XR2206 Function Generator

1Hz - 2MHz XR2206 Function Generator produces high quality sine, square and triangle waveforms of high-stability and accuracy. The output waveforms can be both amplitude and frequency modulated. Output of 1Hz - 2MHz XR2206 Function Generator can be connected directly to 60MHz Counter for setting precise frequency output.

BA1404 HI-FI Stereo FM Transmitter

Be "On Air" with your own radio station! BA1404 HI-FI Stereo FM Transmitter broadcasts high quality stereo signal in 88MHz - 108MHz FM band. It can be connected to any type of stereo audio source such as iPod, Computer, Laptop, CD Player, Walkman, Television, Satellite Receiver, Tape Deck or other stereo system to transmit stereo sound with excellent clarity throughout your home, office, yard or camp ground.

USB IO Board

USB IO Board is a tiny spectacular little development board / parallel port replacement featuring PIC18F2455/PIC18F2550 microcontroller. USB IO Board is compatible with Windows / Mac OSX / Linux computers. When attached to Windows IO board will show up as RS232 COM port. You can control 16 individual microcontroller I/O pins by sending simple serial commands. USB IO Board is self-powered by USB port and can provide up to 500mA for electronic projects. USB IO Board is breadboard compatible.

200m 4-Channel 433MHz Wireless RF Remote Control

Having the ability to control various appliances inside or outside of your house wirelessly is a huge convenience, and can make your life much easier and fun. RF remote control provides long range of up to 200m / 650ft and can find many uses for controlling different devices, and it works even through the walls. You can control lights, fans, AC system, computer, printer, amplifier, robots, garage door, security systems, motor-driven curtains, motorized window blinds, door locks, sprinklers, motorized projection screens and anything else you can think of.

100m 4-Channel 433MHz Wireless RF Remote Control

Four button RF remote is used to turn ON / OFF four different devices independently. Any of the four outputs can be configured to work independently in either toggle or momentary mode. Outputs are buffered by BC549 NPN transistors and can drive low voltage devices directly or be connected to either 5V or 12V relays (or motors) to control appliances that use 110V / 220V mains voltage or any voltage of your choice. Multiple remote systems can be used independently to control more than four appliances in the same location by changing the address code on 433MHz receiver and remote. It is also possible to use several remotes to control the same appliance such as garage door.