1uS Light Pulse Receiver Circuit With Post Amp

Here’s a design circuit for light pulse receiver circuit that is used to detect very weak light pulses lasting 1uS. In the figure at below is a figure to show the schematic diagram circuit;

This circuit uses a tuned LC feedback network to provide high sensitivity while giving high ambient light immunity. A post voltage amplifier is included with a gain of about X20. The circuit is described in more detail in the receiver section of my Handbook of Optical Through the Air Communications.  Note: The LF357 op amp is no longer available, this circuit is for reference only.

Single Chip Stereo FM Transmitter Circuit

ROHM (www.rohm.com) is originally a resistor producer company, but finally expand their business and produce monolithic IC, and this one used in our circuit is the example. The internal structure of this FM transmitter integrated circuit consist of stereo modulator that creates a stereo composite signal, an FM modulator that modulate a carrier frequency with the composite signal, and an RF amplifier that provide enough power to be transmitted through antenna. This is the figure of the circuit;

The core of this stereo FM transmitter is BA1404 integrated circuit chip. from ROHM. This FM transmitter is ideal for wireless microphone, or for audio interface and distribution for home or car appliance. For example, you can now play your portable mp3/mp4 player on your old car radio sound system that doesn’t have line-input plug. This stereo FM transmitter chip is designed for 75-108 FM band, and you can adjust the operation by trimming the LC network connected to pin 10 of this IC chip. To ease the adjustment, you can use a 22-33p variable capacitor for the 15p capacitor connected to pin 10.  Finally, this stereo FM transmitter works with only 1.5-3V power supply, ideal for battery operation.  More than 3.5V supply voltage could burn this FM transmitter circuit. [Circuit diagram source: ROHM Application Notes]

Infrared (IR) Proximity (Distance) Sensor Circuit

The photodiode can be used to detect IR reflected from the object. However, IR produced by the ambient conditions also detected by the photodiode. This IR noise must be filtered to prevent false detections. Usually, the LED’s IR signal is modulated with a convenient frequency and then using that modulation to detect only IR to filter the IR noise. So, the photodiode will detect only IR reflected from the object. This method can be simplified by using the IR proximity sensor that has simple receiver and transmitter sections. This is the figure of the circuit;

The transmitter of this circuit consists of a 940nm IR LED (IR11-21C). To turn ON and OFF the IR LED, the a 10kHz oscillator frequency is used. we can control the detection and the level of transmitted power by varying the LED current. The transmit pulses of this transmitter have a small duty cycle (typically 10%), to save power.

The Op amp is biased at 2.5V because there no input IR signal present. The op amp output varies around 2.5V with a dynamic range of 5V because of a 10kHz IR signal incident. The output drives a simple diode detector. A DC signal proportional to its amplitude can be provided by a simple diode detector. Simple diode detector also rectifies the 10kHz signal. The output signal is an analog signal that is proportional to the distance of the object from the IR transmitter. We can fed the output signal to an ADC for further processing or use it directly. 

[Circuit source: MAXIM Application Note]

Low Cost IC Stereo Receiver Circuit

The recent availability of a broad line of truly high-performance consumer integrated circuits makes it possible to construct a high quality, low noise, low distortion and low cost AM/FM/Stereo receiver. This is the complete design circuit for AM/FM/Stereo Receiver. This is the figure of the circuit;

This circuit is using LM3089FM for control the operation in this circuit. The LM3089 FM IF System does all the major functions necessary for FM processing, including a three stage amplifier/limiter and balanced product detector, as well as an audio preamplifier. A single quadrature coil was used for ease of alignment; yielding recovered audio with THD less than 0.5%, however a double coil may be used to diminish THD to 0.1% if required. Carrier level detectors provide delayed AGC, SIGNAL strength meter drive, and adjustable inter station mute control R11. The internal AFC amplifier was used to drive the TUNING meter, giving a visual indication of center tuning. FM stereo demodulation is accomplished by the use of the LM1800 phase locked loop, thereby eliminating the need for external coils. Only two adjustments are necessary: R14, which sets the 19 kHz oscillator, and R17, which corrects which corrects for excess phase shift thru the IF stages, and yields maximum channel separation. Automatic stereo/ monaural switching is built-in, and may be used in lieu of mechanical switching if desired. The open collector lamp driver is used to light a LED whenever a stereo station is encountered.

[Schematic diagram source: National Semiconductor Notes]

Radio Collar Transmitter Circuit

This is a circuit diagram for radio transmitter suitable for installation in a series of radio transmitting collars. This circuit is work with based on 555 timer and JFET. This is the figure of the circuit.

The pulse in the FM band between 88 MHz to 105 Mhz, which can be adjusted. Ne IC 555 are cable as astable multi vibrator to produce tone.L1, C3 and Q1 form a modulator. End of Q2 to the power amplification. For inductors made of 5 rounds, he was 5 mm insulated copper wire on a standard ball pen refill. Remove the refill and making a tap on the center coil varies transmission frequency. To experiment with the number of turns. Careful!!! What might once have had to turn the tap on center. Use 50 cm long insulated copper wire as an antenna.

Part:
R1…………………………………. 10K
R2 …………………………………1 M Ohm
R3 ………………………………….330 Ohm
R4 …………………………………..1 M Ohm
R5……………………………………. 330 Ohm
R6 ……………………………………200 Ohm POT
C1 ……………………………………..0.1 uF Ceramic
C2…………………………………….. 0.01 uF Ceramic
C3…………………………………….. 13 pF Ceramic
C4, C5, C6, C7 ……………………..0.01 uF Ceramic
Q1, Q2 ………………………………..2N4392 JFET
D1 ……………………………………….1N 914 Diode
IC 1……………………………………… NE 555

Current Loop Transmitter Circuit for Temperature Sensor

This circuit provide current loop transmitter for temperature sensor. Current loop interface has been widely used in industrial environment because it’s robustness. This is the figure of the circuit.


The temperature measurement is done by LM35 temperature sensor chip. You can use general silicone diode such as 1N4001. The current controller function is done by LM317 current/voltage regulator. This circuit will draw a consistent current proportional to the temperature being measured, regardless the supply voltage variation caused by noise or long wire’s temperature-dependent resistance variation.

The AM Transmitter Circuit for 500 KHz to 1600 KHz

This circuit is design for AM transmitter circuit. This circuit is operate for medium wave band for amplitude modulation. The operation of the circuit is in below.


The circuit is in two half, an audio amplifier and an RF oscillator. The oscillator is built around Q1 and associated components. The tank circuit L1 and VC1 is tunable from about 500 KHz to 1600 KHz. These components can be used from an old MW radio, if available. Q1 needs regenerative feedback to oscillate and this is achieved by connecting the base and collector of Q1 to opposite ends of the tank circuit. The 1nF capacitor C7, couples signals from the base to the top of L1, and C2, 100pF ensures that the oscillation is passed from collector, to the emitter, and via the internal base emitter resistance of the transistor, back to the base again. Resistor R2 has an important role in this circuit. It ensures that the oscillation will not be shunted to ground via the very low internal emitter resistance, re of Q1, and also increases the input impedance so that the modulation signal will not be shunted. Oscillation frequency is adjusted with VC1.

The Q2 is wired as a common emitter amplifier, C5 decoupling the emitter resistor and realizing full gain of this stage. The microphone is an electret condenser mic and the amount of AM modulation is adjusted with the 4.7k preset resistor P1. An antenna is not needed, but 30cm of wire may be used at the collector to increase transmitter range.

AM Receiver Circuit Using Transistor

This is the simple design and sensitivity and selectivity of the receiver are good. This circuit is use a compact three transistor, regenerative receiver with fixed feedback. The circuit is based on transistor as core of the operation. The transistor that is used is BC549. This is the figure of the circuit.


The tuned circuit is designed for medium wave, but the circuit will work up to much higher frequencies if a different tuning coil and capacitor are used. Q1 and Q2 form a compound transistor pair featuring high gain and very high input impedance. This is necessary so as not to unduly load the tank circuit. Q1 operates in emitter follower, Q2 common emitter, self stabilizing bias is via the 120k resistor and the tuning coil. As Q2 operates in common emitter its base voltage will be a V be drop higher than ground or about 0.71V in my test sample. The 120k resistor provides regenerative feedback, between Q2 output and the tank circuit input and its value affects the overall performance of the whole circuit.

The tuning coil can be salvaged from an old AM receiver. However to make your own wind about 50 to 60 turns of 26 swg enamel coated copper wire over a 3/8 inch ferrite rod about 3 inches long. This circuit is powered by 9 VDC.

FM Transmitter Circuit Type Versatile

This is a design circuit diagram of a versatile FM transmitter. This circuit doesn’t have a coil. The circuit is simple and easy to assemble. This circuit is work based on gate logic concept. This is the figure of the circuit.


The gate N1 acts as a buffer for strengthening the signals from the condenser microphone. The inverter N2 with its associated components forms a radio frequency oscillator in the FM region. The varicaps diode BB109 is used for frequency modulating the audio signal to the carrier wave generated by the oscillator. Inverters N4 t0 N6 are used to drive the antenna. As the N4, N5, N6 are connected in parallel their effective output impedance is very less and can easily drive the antenna. All electrolytic capacitors must be rated 10V.

This circuit is use a 10 cm long wire as antenna. Gates N1 to N6 belong to same IC CD4069. The battery can be a 9V transistor radio battery. Adapters are not recommended because they would induce noise in the circuit.

AM Receiver Circuit Using Transistor

This is the simple design and sensitivity and selectivity of the receiver are good. This circuit is use a compact three transistor, regenerative receiver with fixed feedback. The circuit is based on transistor as core of the operation. The transistor that is used is BC549. This is the figure of the circuit.


The tuned circuit is designed for medium wave, but the circuit will work up to much higher frequencies if a different tuning coil and capacitor are used. Q1 and Q2 form a compound transistor pair featuring high gain and very high input impedance. This is necessary so as not to unduly load the tank circuit. Q1 operates in emitter follower, Q2 common emitter, self stabilizing bias is via the 120k resistor and the tuning coil. As Q2 operates in common emitter its base voltage will be a V be drop higher than ground or about 0.71V in my test sample. The 120k resistor provides regenerative feedback, between Q2 output and the tank circuit input and its value affects the overall performance of the whole circuit.

The tuning coil can be salvaged from an old AM receiver. However to make your own wind about 50 to 60 turns of 26 swg enamel coated copper wire over a 3/8 inch ferrite rod about 3 inches long. This circuit is powered by 9 VDC.

AM Receiver Circuit Using Transistor

This is the simple design and sensitivity and selectivity of the receiver are good. This circuit is use a compact three transistor, regenerative receiver with fixed feedback. The circuit is based on transistor as core of the operation. The transistor that is used is BC549. This is the figure of the circuit.


The tuned circuit is designed for medium wave, but the circuit will work up to much higher frequencies if a different tuning coil and capacitor are used. Q1 and Q2 form a compound transistor pair featuring high gain and very high input impedance. This is necessary so as not to unduly load the tank circuit. Q1 operates in emitter follower, Q2 common emitter, self stabilizing bias is via the 120k resistor and the tuning coil. As Q2 operates in common emitter its base voltage will be a V be drop higher than ground or about 0.71V in my test sample. The 120k resistor provides regenerative feedback, between Q2 output and the tank circuit input and its value affects the overall performance of the whole circuit.

The tuning coil can be salvaged from an old AM receiver. However to make your own wind about 50 to 60 turns of 26 swg enamel coated copper wire over a 3/8 inch ferrite rod about 3 inches long. This circuit is powered by 9 VDC.

Medium Power FM Transmitter Circuit Using Transistor

This is a design circuit diagram of a moderate power FM transmitter circuit. This circuit is built operation by two transistors. This is a figure of complete circuit.


Operation of this circuit is explain as the voice signals picked by the microphone will be amplified by the transistor Q1.The second transistor is wired as an oscillator operating in the FM band. The output of T1 is given to the base of T2.T2 performs the modulation also. The tank circuit comprising of components L1 and C6 determines the frequency of the signal, and can be varied by adjusting C6.The capacitor C7 couples the FM signal to the antenna. The inductor L1 can be made by making 6 turns of 0.8mm enameled copper wire on a 5.5mm diameter/4.5mm length plastic former.

With a matching antenna and proper tuning this transmitter can have range up to 100 meter. The Antenna A1 can be a 1M long wire. The circuit can be powered from a9V PP3 battery.

FM Tracking Transmitter Circuit

This is a design for the tracking transmitter of audio tone in FM band frequency. The circuit can be used a signal transmitter or remote control transmitter. The circuit is use only the available components. The range of the transmitter is 100 m in the distance using 9V power supply and with a matching antenna. The circuit is built by 555 timer IC for producing the audio tone and based on JFET as core the circuit. This is figure the circuit.


The operation of the circuit is the first JFET (Q1) is wired as a Hartley oscillator which is frequency modulated by the audio tone. The second (Q2) JFET is wired as a buffer to isolate the oscillator based on Q1 from the antenna. The diode D1 is used as a varactor here. The diode is reverse biased by the ramping voltage produced at the pin 6&2 of the IC1.This results in the change of junction capacitance of reverse biased diode, which in turn alters the frequency of the oscillator to attain the frequency modulation. The inductor L1 can be made by winding 5 turns of 18 SWG enameled copper wire on a 3/8 inch long, 3/16 inch diameter plastic tube .The coil must be tapped at the center. The antenna can be a 20cm long wire.

AM Radio Circuit Using Transistor

The circuit for a powerful AM transmitter using ceramic resonator/filter of 3.587 MHz is presented here. This circuit is based on transistor for the core operation of the circuit. Resonators/filters of other frequencies such as 5.5 MHz, 7 MHz and 10.7 MHz may also be used. Use of different frequency filters/resonators will involve corresponding variation in the value of inductor used in the tank circuit of oscillator connected at the collector of transistor T1. This is the figure of the circuit;


The AF input for modulation is inserted in series with emitter of transistor T1 (and resistor R4) using a transistor radio type audio driver transformer as shown in the circuit. Modulated RF output is developed across the tank circuit which can be tuned to resonance frequency of the filter/resonator with the help of gang condenser C7. The next two stages formed using low-noise RF transistors BF495 are, in fact, connected in parallel for amplification of modulated signal coupled from collector of transistor T1 to bases of transistors T2 and T3. The combined output from collectors of T2 and T3 is fed to antenna via 100pF capacitor C4.

The circuit can be easily assembled on a general-purpose PCB. The range of the transmitter is expected to be one to two kilometers. The circuit requires regulated 9 volt power supply for its operation. Note: Dotted lined indicates additional connection if a 3-pin filter is used in place.

Low Power FM Transmitter Using SDM

This is a low-power FM transmitter using surface-mount devices (SMD) that will be received with a standard FM radio. There are many designs for small FM transmitters but they have some problems. First, you need an audio amplifier to get enough modulation. Second, the antenna is attached directly to the collector. Third, the coil L must be wound by hand and adjusted by stretching. This is the figure of the schematic.


How is the circuit work? The transmitter is consists of two stages: an oscillator and an output amplifier. Modulation is from an electric microphone but you can use a low power audio source. Transistor Q1 is a Colpitts oscillator where the frequency is determined by the parallel resonant circuit formed by inductor L, varactor V1 and capacitors C7 and C8. Q1 is a common-collector amplifier where the power gain counts. V1 is actually dual varactor that eliminates the possibility of forward conduction at the sine wave peaks.

The frequency of oscillation is set by adjusting the DC voltage on V1 with potentiometer R2. R4 and C3 form a low-pass filter to prevent RF from feeding back onto the DC. Capacitors C7 and C8 form an AC voltage divider to provide feedback at the emitter of Q1 to sustain oscillation. Modulation is done by superimposing an audio signal from the electret mic onto the DC bias applied to V1. R3 and C1 form a low-pass filter to prevent RF from feeding back to the microphone. R3, R4 and R2 form a voltage divider for the audio.


The output of the oscillator is fed through C9 to the Q2 emitter-follower. The output of Q2 drives the antenna through C11. The Q2 emitter-follower it ensures that the oscillator is not loaded down by the impedance of the antenna and it provides power gain to drive the antenna. This is the component of the circuit.

Low Power FM Transmitter Circuit Using Transistor

The circuit of the FM transmitter is shown in the figure is simple design. The first stage is the oscillator, and is tuned with the variable capacitor. This circuit is use two transistor BC549 as op amp the input signal. Select an unused frequency, and carefully adjust C3 until the background noise stops. Because the trimmer cap is very sensitive, make the final frequency adjustment on the receiver. When assembling the circuit, make sure the rotor of C3 is connected to the +9V supply. This ensures that there will be minimal frequency disturbance when the screwdriver touches the adjustment shaft. You can use a small piece of non copper-clad circuit board to make a screwdriver - this will not alter the frequency.

How does this circuit work? The frequency stability is improved considerably by adding a capacitor from the base of Q1 to ground. This ensures that the transistor operates in true common base at RF. A value of 1nF (ceramic) as shown is suitable, and will also limit the HF response to 15 kHz, this is a benefit for a simple circuit like this, and even commercial FM is usually limited to a 15 kHz bandwidth. Q1 is the oscillator, and is a conventional design. L1 and C3 (in parallel with C2) tune the circuit to the desired frequency, and the output (from the emitter of Q1) is fed to the buffer and amplifier Q2. This isolates the antenna from the oscillator giving much better frequency stability, as well as providing considerable extra gain. L2 and C6 form a tuned collector load, and C7 helps to further isolate the circuit from the antenna, as well as preventing any possibility of short circuits should the antenna contact the grounded metal case that would normally be used for the complete transmitter.

The audio signal applied to the base of Q1 causes the frequency to change, as the transistor's collector current is modulated by the audio. This provides the frequency modulation (FM), that can be received on any standard FM band receiver. The audio input must be kept to a maximum of about 100mV, although this will vary somewhat from one unit to the next. Higher levels will cause the deviation (the maximum frequency shift) to exceed the limits in the receiver usually ± 75 kHz. With the value shown for C1, this limits the lower frequency response to about 50 Hz (based only on R1, which is somewhat pessimistic), if you need to go lower than this, then use a 1uF cap instead, which will allow a response down to at least 15 Hz. C1 may be polyester or mylar, or a 1uF electrolytic may be used, either bipolar or polarized. If polarized, the positive terminal must connect to the 10k resistor.

TDA1072A for AM receiver circuit

This TDA1072A integrated AM receiver circuit performs the active and part of the filtering functions of an AM radio receiver. TDA1072A is intended for use in mains-fed home receivers and car radios. The circuit design can be used for oscillator frequencies up to 50 MHz and can handle r. f. signals up to 500 mV. R.F. radiation and sensitivity to interference are minimized by an almost symmetrical design. The voltage-controlled oscillator provides signals with extremely low distortion and high spectral purity over the whole frequency range even when tuning with variable capacitance diodes. When required, band switching diodes can easily be applied. Selectivity is obtained using a block filter before the i.f. amplifier. The figure of the circuit is shown by the figure below;

The feature of TDA1072A is;
The inputs protected against damage by static discharge. Gain-controlled r.f. stage. Separately buffered, voltage-controlled and temperature-compensated oscillator, designed for simple coils. Internal generation of a.g.c. voltage with possibility of second-order filtering. Buffered field strength indicator driver with short-circuit protection.

Antenna directivity and gain

Antennas (aerials) do not radiate equally in all directions. It is found that all realized radio antennas radiate more in some directions than others. The actual pattern is dependent upon the type of antenna, its size, the environment and a variety of other factors. This directional pattern can be used to ensure that the power radiated is radiated in the desired directions.

It is normal to refer to the directional patterns and gain in terms of the transmitted signal. It is often easier to visualization the antenna is terms of its radiated power, however the antenna performs in an exactly equivalent manner for reception, having identical figures and specifications.

In order to visualization the way in which an antenna radiates a diagram known as a polar diagram is used. This is normally a two dimensional plot around an antenna showing the intensity of the radiation at each point for a particular plane. Normally the scale that is used is logarithmic so that the differences can be conveniently seen on the plot. Although the radiation pattern of the antenna varies in three dimensions, it is normal to make a plot in a particular plane, normally either horizontal or vertical as these are the two that are most used, and it simplifies the measurements and presentation.

Antennas are often categorized by the type of polar diagram they exhibit. For example an omni - directional antenna is one which radiates equally (or approximately equally) in all directions in the plane of interest. An antenna that radiates equally in all directions in all planes is called an isotropic antenna. As already mentioned it is not possible to produce one of these in reality, but it is useful as a theoretical reference for some measurements. Other antennas exhibit highly directional patterns and these may be utilitized in a number of applications. The Yagi antenna is an example of a directive antenna and possibly it is most widely used for television reception.

Antenna gain
An antenna radiates a given amount of power. This is the power dissipated in the radiation resistance of the antenna. An isotropic radiator will distribute this equally in all directions. For an antenna with a directional pattern, less power will be radiated in some directions and more in others. The fact that more power is radiated in given directions implies that it can be considered to have a gain.

The gain can be defined as a ratio of the signal transmitted in the "maximum" direction to that of a standard or reference antenna. This may sometimes be called the "forward gain". The figure that is obtained is then normally expressed in decibels (dB). In theory the standard antenna could be almost anything but two types are generally used. The most common type is a simple dipole as it is easily available and it is the basis of many other types of antenna. In this case the gain is often expressed as dBd i.e. gain expressed in decibels over a dipole. However a dipole does not radiated equally in all directions in all planes and so an isotropic source is sometimes used. In this case the gain may be specified in dBi i.e. gain in decibels over an isotropic source. The main drawback with using an isotropic source is as a reference is that it is not possible to realize them in practice and so that figures using it can only be theoretical. However it is possible to relate the two gains as a dipole has a gain of 2.1 dB over an isotropic source i.e. 2.1 dBi. In other words, figures expressed as gain over an isotropic source will be 2.1 dB higher than those relative to a dipole. When choosing an antenna and looking at the gain specifications, be sure to check whether the gain is relative to a dipole or an isotropic source.

Apart from the forward gain of an antenna another parameter which is important is the front to back ratio. This is expressed in decibels and as the name implies it is the ratio of the maximum signal in the forward direction to the signal in the opposite direction. This figure is normally expressed in decibels. It is found that the design of an antenna can be adjusted to give either maximum forward gain of the optimum front to back ratio as the two do not normally coincide exactly. For most VHF and UHF operation the design is normally optimized for the optimum forward gain as this gives the maximum radiated signal in the required direction.

Synthesized WBFM Transmitter

This project is a complete crystal-controlled Wide Band Frequency Modulated (WBFM) transmitter delivering a power output in the order of 10 mW (+10dBm) using simple components. The transmitter is based upon the Phase-Locked Loop (PLL) principle, but due to the circuit's simplicity a true "phase lock" can never be achieved.

The transmitter has both 1v peak-to-peak 'LINE' input and 10mV 'MIC' audio inputs. These will accept audio input sources from external equipment, such as hi-fi, CD and computer equipment. The microphone input also has an in-built power source to energize an 'Electret' type condenser microphone. The Radio Frequency (RF) output circuitry includes a three-pole filter for reduction of harmonics and other spurious signals. The spurious output signal level is better than -40dBc (0.0001 times the power of the wanted signal level), which makes the project suitable for driving an external power amplifier.

The transmitter is powered from a 12v supply, but it will operate from 9 Volts to 16 Volts. The DC power input is equipped with a diode (D1), which protects the transmitter in the event the supply voltage is inadvertently connected the wrong way round.

This figure is schematic the transmitter;

Construction should begin with the wire link WLK, all horizontally mounted resistors, then the power diode D1. WLK is formed from one of the resistor lead off-cuts. All vertically mounted resistors and ceramic capacitors are next, followed by the electrolytic capacitors and the four transistors. Finally, fir the vericap diode, D2, and the integrated circuits.

Although internally protected, the ICs should be fitted on a static protected workbench. In the absence of a suitable workbench, then aluminum foil can be taped to a work board and connected to the soldering iron Earth (Ground) via a 1M0 resistor. The ICs should be left in the packaging supplied until required. Before handling them, the worker should also be electrically connected to the workbench anti-static mat (foil) via a 1M0 resistor.

Note that C25 and C29 must be miniature capacitor types, due to space restrictions on the PCB. These two components should ideally be fitted after all the resistors and other components have been fitted, and the component bodies positioned about 3mm away from the PCB. These two capacitors should be mounted vertically, as shown to the right. Long component leads will have no effect on the operation of the project. The distance between TR2 / TR3 body and the PCB must be at least 4mm minimum and no greater than 6mm. 4mm is required for clearance between TR3 and L3.

Failure to observe this may cause the collector of TR3 to touch L3 causing a collector-emitter short circuit. A good tip is to fit TR2 and TR3 after L3 has been fitted. L3 should be fitted touching the PCB. C30 has been added to the circuit to restrict the upper operating frequency of the transmitter. 4p7 restricts this to approximately 125MHz. Without C30 fitted the transmitter is capable of operation over 150MHz. It is possible that certain manufacturers of IC1 produce a slightly less sensitive version. If this is used then C7 may be increased in value to 120pf.

Low Power FM Transmitter Schematic

The circuit of the transmitter is shown in Figure 1, and as you can see it is quite simple. The first stage is the oscillator, and is tuned with the variable capacitor. Select an unused frequency, and carefully adjust C3 until the background noise stops (you have to disable the FM receiver's mute circuit to hear this).

Because the trimmer cap is very sensitive, make the final frequency adjustment on the receiver. When assembling the circuit, make sure the rotor of C3 is connected to the +9V supply. This ensures that there will be minimal frequency disturbance when the screwdriver touches the adjustment shaft. You can use a small piece of non copper-clad circuit board to make a screwdriver - this will not alter the frequency.

The frequency stability is improved considerably by adding a capacitor from the base of Q1 to ground. This ensures that the transistor operates in true common base at RF. A value of 1nF (ceramic) as shown is suitable, and will also limit the HF response to 15 kHz - this is a benefit for a simple circuit like this, and even commercial FM is usually limited to a 15kHz bandwidth.

The Principle of works this application;
Q1 is the oscillator, and is a conventional design. L1 and C3 (in parallel with C2) tune the circuit to the desired frequency, and the output (from the emitter of Q1) is fed to the buffer and amplifier Q2. This isolates the antenna from the oscillator giving much better frequency stability, as well as providing considerable extra gain. L2 and C6 form a tuned collector load, and C7 helps to further isolate the circuit from the antenna, as well as preventing any possibility of short circuits should the antenna contact the grounded metal case that would normally be used for the complete transmitter.

The audio signal applied to the base of Q1 causes the frequency to change, as the transistor's collector current is modulated by the audio. This provides the frequency modulation (FM) that can be received on any standard FM band receiver. The audio input must be kept to a maximum of about 100mV, although this will vary somewhat from one unit to the next. Higher levels will cause the deviation (the maximum frequency shift) to exceed the limits in the receiver - usually ±75kHz.

With the value shown for C1, this limits the lower frequency response to about 50Hz (based only on R1, which is somewhat pessimistic) - if you need to go lower than this, then use a 1uF cap instead, which will allow a response down to at least 15Hz. C1 may be polyester or mylar, or a 1uF electrolytic may be used, either bipolar or polarise. If polarised, the positive terminal must connect to the 10k resistor.