500mW FM/VHF Transmitter Amplifier Circuit

This is a circuit for power amplifier. This is the figure of the circuit;


This circuit is a high performance low noise 500mW amplifier / booster for all low power FM transmitters such as BA1404, BH1417, BH1415, 433MHz transmitter modules, etc. The amplifier chip is an integrated circuit containing multiple transistor stages and all other parts conveniently within a single small package. Boosting your FM transmitter has never been easier and the output signal can also directly drive 2n4427 or 2n3886 transistors for 1W or 5W of RF output power.

10W Power Amplifier Using TDA2003 IC’s

This is a design circuit for power amplifier that is the circuit diagram of a 10W audio amplifier using the popular TDA2003 IC from SGS Thomson. This is the figure of the circuit;


The IC can easily deliver 10W to a 4 Ohms load at 18V DC supply voltage. The IC can be also operated from 12V and that makes it applicable in car audio systems. The useful features of TDA2003 includes short circuit protection between all pins, thermal overload protection, low harmonic distortion, low cross over distortion etc.

Random Blinking LED Circuit

This is a design circuit to make the LEDs blink in a random pattern according to the slight differences in the three Schmitt Trigger oscillators. This is the figure of the circuit;


The CD4511 is BCD to 7 segment Driver, so the pattern of the led is actually the come from seven segment representation. Arrange the LEDs to be far from seven segment pattern if you want to avoid people analyze your circuit. The randomness is based on the frequency difference because the 22uF and 47k resistors that determine the frequency have at least 5% tolerance.

LED VU Meter Circuit Using LM3915 IC

This LED VU Meter (volume-unit) is capable of monitoring and displaying power levels present at the speaker terminals of an stereo audio power amplifier. The levels are displayed in ten discrete steps using 10 LEDs for each channel. This project is designed to give an approximate visual indication of the audio power output of each channel. This is the figure of the circuit;

Two external resistors (R2 & R3) programs the full scale from between 1.2V and 12V applied to pin 5. 10.5V is used to turn on all 10 LED's. The voltage required to turn on all the LEDs is set by R2 and R3. The IC develops a nominal 1.25V reference voltage (Vref) across pins 7 and 8. Since this voltage is constant then the current through R3 is also constant. This current also flows through R2. The total voltage across R2 and R3 is given by voltage. Internally this chip consists of ten voltage comparators. The non-inverting (+) input of each comparator is connected to an accurate ten-step voltage divider network. Each comparator will therefore trigger on a different comparison level. The inverting (-) inputs of each comparator are commoned together and connected to an incoming DC signal via a high impedance input buffer.

Light Dependent Resistor Circuit

The Light Dependent Resistor and a trimpot form a voltage divider which is used to apply bias to a transistor. As the LDR changes resistance the change in potential is detected by the circuit and the relay is activated. The PCB-mounted switch just interchanges the trimpot & the LDR as far as the detection circuit is concerned. So a dark activated switch becomes a light activated switch or vice versa. This is the figure of the circuit;

An LED with current limiting resistor is in parallel to the relay to give a visual indication of when the relay is turned on. The relay (Use a 5A/250VAC) can be connected to a light bulb and power supply which will light up when the environment is bright or vice versa. This circuit is satisfactory if the changes in light level to be detected are large and the transition is quick - for example, a person walking past a doorway. An inherent problem of the circuit is chattering of the relay for slowly changing light levels just at the transition point between turning on/odd and vice versa. This leads to the relay chattering as it rapidly turns on/off. This problem can be overcome in by having a hysteresis circuit using an op-amp or a Schmidt Trigger.

3V FM Transmitter Circuit

This is a design of circuit and the parts list needed to built a 3V FM Transmitter. This FM transmitter is about the simplest and most basic transmitter to build and have a useful transmitting range. It is surprisingly powerful despite its small component count and 3V operating voltage. It will easily penetrate over three floors of an apartment building and go over 300 meters in the open air. This is the figure of the circuit;


The circuit is basically a radio frequency (RF) oscillator that operates around 100 MHz. Audio picked up and amplified by the electret microphone is fed into the audio amplifier stage built around the first transistor. Output from the collector is fed into the base of the second transistor where it modulates the resonant frequency of the tank circuit (the 5 turn coil and the trimcap) by varying the junction capacitance of the transistor. Junction capacitance is a function of the potential difference applied to the base of the transistor. The tank circuit is connected in a Colpitts oscillator circuit. Components may be added to the PCB in any order. Note that the electret microphone should be inserted with the pin connected to the metal case connected to the negative rail (that is, to the ground or zero voltage side of the circuit). The coil should be about 3mm in diameter and 5 turns. The wire is tinned copper wire, 0.61 mm in diameter. After the coil in soldered into place spread the coils apart about 0.5 to 1mm so that they are not touching. (The spacing in not critical since tuning of the Tx will be done by the trim capacitor. It is quite possible, but not as convenient, to use a fixed value capacitor in place of the trim capacitor - say 47pF - and to vary the Tx frequency by simply adjusting the spacing of the coils. That is by varying L of the LC circuit rather than C.) Adding and removing the batteries acts as a switch. Connect a half or quarter wavelength antenna (length of wire) to the aerial point. At an FM frequency of 100 MHz these lengths are 150 cm and 75 cm respectively.

Small Amplifier Circuit Using Transistors

This is a design circuit for audio amplifier circuit. When audio is detected, the output is push-pull and consumes less than 3mA (with no signal) but drives the earpiece to a very loud level. It’s extremely difficult to set up because the whole circuit is DC coupled. Basically you don’t know where to start with the biasing. 8k2 between the emitter of the first transistor and 0v rail and the 470R resistor are the two most critical components. This is the figure of the circuit;


The emitter voltage on the BC 547 is set by the 8k2 across the 47u and this turns it on. To call the driver transistor, the collector is directly connected to the base of a BC 557. Current flow through the 1k and 470R resistors so that the voltage developed across each resistor turns on the two output transistors is caused by these transistors and the output of BC 557 are now turned on. The end result is mid-rail voltage on the join of the two emitters. Major negative feedback is provided by 8k2 feedback resistors while the 330p prevents high-frequency oscillations occurring.

[Circuit diagram source: Talking Electronics]

Active Crossover Circuit: Split The Audio Signal Before Amplification

You might be familiar with passive crossover network installed inside your speaker box, consist of inductors and capacitors. The problem with passive crossover network is that they dissipate the audio power, so it’s not environmentally friendly, contributing a little disaster of global warming. Moreover, the capacitor and the inductor in the passive crossover network contribute the distortion of the signal since it must drive a low impedance load (the loud speaker). The best way to get high quality audio is to separate the high and low frequencies before feeding the signal to the amplifier. The drawback is that you need two amplifiers, to amplify the low and high audio signal separately. This is the figure of the circuit;

The circuit use a constant voltage method, means that the output of high and low frequency is summed up, and then fed back to be compared with the input to make sure this sum is equal to the input signal. This method is ensures that the total response is flat, if we summed back the separated high and low frequency output. You have to use 1% tolerance for the resistors to give a precise response. According to the formula, the capacitor C should be 6.6nF to give 1kHz crossover frequency, but a 6.8nF can be used because it widely available, and the crossover frequency will be shifted to about 975Hz.

[Circuit schematic diagram source: National Semiconductor Application Notes]