Subwoofer Filter Circuit

If the audio device has a bass tone quality that is less kicking, to overcome this problem is to add an active subwoofer / crossover filter so that the tone other than the low frequency will not be passed to the power amplifier and the sound coming out of the speaker will appear even more. This the figure of the schematic diagram.

Low Pass Filter Circuit with Enhanced Step Response

Effect on the system’s time-domain response is a common problem when designing low pass filters. The system may fail to recognize significant changes in time because pushing the cut-off frequency lower slows the step response. This is the figure of the circuit that present the low pass filter circuit;

On this circuit diagram, lower cut-off frequency is allowed without sacrificing the step-response time. The delta (difference) between the filter’s input and output is monitored by window comparator. The filter increases its slew rate by increasing its cut-off frequency an order of magnitude when the delta exceeds 50mV. Low pass-filtered by R4 and C3 is the original signal which is produce a cut-off frequency (312Hz) that reduces sensitivity to momentary glitches. The window-comparator input is drove by the filtered input. Comparator U2A or U2B will assert its output low if the input is outside the 50mV window. The low output drives Q5 into cutoff, causing its collector to presume a high impedance. 

The filter’s cutoff frequency increases by ten times because the Q5 collector no longer grounds capacitor C2. The cutoff frequency throttles back to its quiescent state When the system output changes to within 50mV of the system input. This circuit diagram is configured for very low cutoff frequency, but changing C1 and C2 can rescale the configuration to higher frequency, where the oscillation frequency fOSC (in kHz) is 30 x 103/COSC (in pF) and the cutoff frequency is fOSC/100. For different window values in which the delta equals the resistance multiplied by 115µA, we can modify R2 and R3. The type of comparator must be an open-drain type.

Multiplexer with Limiter and Low Pass Filter

This is a design circuit for stereo encoder. This circuit is using BH1417 Stereo Encoder. This circuit using with pre-emphasis, limiter so that the music can be transmitted at the same audio level, low pass filter that blocks any audio signals above 15 KHz to prevent any RF interference and crystal based stereo encoder for stereo transmission. This is the figure of the circuit.

The BH1417 single chip IC can be supplied with 6 - 15V voltage, consumes only around 25mA while providing very sound quality and improved 40dB channel separation. The IC is only available in SOP22 IC case and this may be an inconvenience for some folks. On the other hand, because the chip is smaller than regular DIP-based ICs it is possible to fit the entire stereo coder on a small PCB. This IC is requires 7.6MHz crystal oscillator which is pretty hard to find. The good news is that you can use 7.68 MHz crystal instead. In fact our BH1417 stereo encoder prototype uses 7.68 MHz crystal. This has absolutely no effect on stereo encoding process, we have tested it and stereo sound is crystal clear.

Single Op Amp Band Pass Filter

This is a circuit design for a band pass filter. A band pass filter passes a range of frequencies while rejecting frequencies outside the upper and lower limits of the pass band. The range of frequencies to be passed is called the pass band and extends from a point below the center frequency to a point above the center frequency where the output voltage falls about 70% of the output voltage at the center frequency. This is the figure of the circuit.


The filter bandwidth (BW) is the difference between the upper and lower pass band frequencies. The quality factors, or Q of the filter is a measure of the distance between the upper and lower frequency points and is defined as (Center Frequency / BW) so that as the pass band gets narrower around the same center frequency, the Q factor becomes higher. For a single op-amp band pass filter with both capacitors the same value, the Q factor must be greater than the square root of half the gain, so that a gain of 98 would require a Q factor of 7 or more.

Video Limiter Circuit

This circuit is use to avoid exceeding luminance reference level standard or to avoid exceeding the input range of digitizer (ADC), video signal is often needed to be limited. The simple way to do this is by hard limiting the signal in the positive direction (white peak clipping), but this method completely destroy all information contained in the clipped region. This circuit is based on LT1228 IC’s. This is the figure of the circuit.


The better way to limit the signal is while preserving all information contained in the signal is by soft limiting the signal, where the signal will be compressed at the above threshold region. The LT1228 is used here in a slightly unusual, closed-loop configuration. The gain of the closed-loop is set by the feedback and gain resistors (RF and RG) and the open-loop gain by the trans-conductance of the first stage times the gain of the CFA. The level at which the limiting action begins is adjusted by varying the set -current into pin 5 of the trans-conductance amplifier.
[Schematic diagram source: Linear Technology Application Notes]

Phase Control Circuit

This is a simple design schematic circuit for phase control circuit. The circuit can be used to control the power delivered to an AC load. The phase control circuit can control the AC waveform, cutting the cycle to give full cycle, half cycle, zero cycle, or somewhere in between. You can say this circuit is similar to a dimmer circuit, but the switching is synchronized with the zero crossing of the waveform. This circuit is works using based on IC U208B. This is the figure of the circuit.


The benefit of switching the power in zero crossing condition is that the triacs doesn’t suffer power dissipation, thus increasing the overall efficiency. This phase control circuit is suitable for brushed AC motor, heater filament, or incandescent lamps. The IC U208B is designed as a phase control circuit in bipolar technology with internal supply-voltage monitoring. As the voltage is built up, uncontrolled output pulses are avoided by internal monitoring. Furthermore, it has internal-current and voltage synchronization. It is recommended as a low cost open-loop control. [Schematic diagram source: TEMIC TELEFUNKEN Microelectronic Application Notes]

Butterworth Second Order High Pass Filter Circuit

This is a circuit for high pass filter. This circuit is similar to low-pass filter circuit, but the position for resistors and capacitor are interchanged. This circuit is based on op-amp for the operation. LM833 IC is the op-amp that is used in the circuit. This is the figure of the circuit.


Similar with low pass design guide, the resistor and capacitor should be chosen according to the formula, and the resistor value should be:
· Much higher than equivalent leakage resistance of the capacitor.
· Much higher than the operational-amplifier’s (op-amp’s) input impedance.
· Doesn’t draw excessive current-violating the maximum allowed op-amp’s output current.

In general, for higher capacitor value, it is leakage current would be higher and you must use lower resistors to compensate the capacitor’s current leakage. [Schematic source: National Semiconductor's LM833 Application Notes]

High And Low Voltage Cut Off Circuit

This is a design for protection voltage. This circuit is called as high and low voltage cut off. The circuit is using time delay for cut off the voltage. This is a low cost and reliable circuit for protecting such equipments from damages. This is the figure of the circuit.


Whenever the power line is switched on it gets connected to the appliance only after a delay of a fixed time. If there is hi/low fluctuations beyond sets limits the appliance get disconnected. The system tries to connect the power back after the specific time delay, the delay being counted from the time of disconnection. If the power down time (time for which the voltage is beyond limits) is less than the delay time, the power resumes after the delay: If it is equal or more, then the power resumes directly. This circuit is using op-amp 741 and 555 IC for control the operation.

The complete circuit is consisting of various stages. They are: - Dual rail power supply, Reference voltage source, Voltage comparators for hi/low cut offs, Time delay stage and Relay driver stage. Under normal operating conditions i.e. when the input voltage is between maximum and minimum limit the output from the both the comparators are low. The transistor Q1 is OFF and the relay is in de-energized (pole connected to N/C pin) state and the output is obtained. When the input voltage is below or above the limits set by the pre-sets R8 or R9, the output of the Op-Amps goes either low or high and diodes D1 or D2 would be forward biased depending on the situation. Transistor Q1 switches ON and the flow of current from collector to emitter energizes the relay and the output is cutoff.

DC to AC Inverter Using 555 IC

This is a design for AC inverter circuit. This circuit is produces an AC output at line frequency and voltage. The circuit is using 555 IC as main control. This IC is configured a low frequency oscillator, tunable over the frequency range of 50 – 60 Hz by potentiometer R4. This is the figure of the circuit.


The principle work of the circuit is the IC feeds its output that amplified by Q1 and Q2 to input of the transformer T1. A reverse is connected filament transformer with necessary step-up turns ratio. A capacitor C4 and coil L1 filter the input to T1, assuring that it is effectively a sine wave. Adjust the value of T1 to your voltage. Replacement types for Q1 are: TIP41B, TIP41C, NTE196, ECG196, etc. Replacement types for Q2 are: TIP42B, TIP42C, NTE197, ECG197, etc. The input voltage of the circuit is anywhere from +5V to +15Volt DC.

Active Band Pass Filters

Active band pass filters are simply filters constructed by using operational amplifiers as active devices configured to simulate inductors. Active band pass filters are used largely at audio frequencies where otherwise the size of the inductor would become prohibitive. In this figure is multiple feedback band pass (MFBP) type which uses capacitors of equal value and leads us to simplified calculations.


Finally pick a convenient value for C which if reasonably large, leads to smaller values of resistance and consequently some aid in reducing noise. Valuable feedback (no pun intended) from readers using rate-this-page (see below and on every other page) indicates the following needs clarification. The 100 uF capacitor above is purely part of the power supply reservoir and has nothing to do with the filter itself. The two 10K resistors are part of the power supply biasing of the op amps because we are not using positive and negative power supplies. The capacitor and resistor values are simply the value of C you choose to use and the resistor values result from the following calculations. It's that simple.

1 KHz Sine Wave Generator Circuit

This is a design circuit for generates a good 1 KHz sine wave adopting the inverted Wien bridge configuration (C1-R3 & C2-R4). It features a variable output, low distortion and low output impedance in order to obtain good overload capability. The figure is shown in below.


The bulb must be a low current type (12V 40-50mA or 6V 50mA) in order to obtain good long term stability and low distortion. Distortion @ 1V RMS output is 0.15% using a 12V 40mA bulb, raising to 0.5% with a 12V 100mA one. Using a bulb differing from specifications may require a change of R6 value to 220 or 150 Ohms to ensure proper circuit's oscillation. Set R5 to read 1V RMS on an Audio mili voltmeter connected to the output with R7 rotated fully clockwise, or to view a sine wave of 2.828V Peak-to-Peak amplitude on the oscilloscope. With C1, C2 = 100nF the frequency generated is 100Hz and with C1, C2 = 1nF frequency is 10 KHz but R5 requires adjustment. High gain transistors are preferred for better performance.

Component Part:
R1 5K6
R2 1K8
R3, R4 15K
R5 500R 1/2W Trimmer Cermet
R6 330R 1/4W Resistor
R7 470R Potentiometer
C1, C2 10nF/63V
C3 100µF/25V
C4 470nF/63V
Q1, Q2 BC238 25V 100mA NPN Transistors
LP1 12V 40mA Filament Lamp Bulb (See Notes)
J1 Phone chassis Socket
SW1 SPST Slider Switch
B1 9V PP3
Clip for 9V PP3 Battery

Dew Sensitive Switch Circuit Using LM358

This is a simple design circuit that can be used to switch ON or OFF a device when the dew present in the surrounding atmosphere crosses a set value. This circuit uses a dew sensitive resistive element and a comparator based on LM 358 to perform the above said operation. This is the figure of the circuit.


In operation of the circuit, at normal condition the resistance of dew sensor element will be low and so the voltage drop across it. So the voltage at the non inverting pin of LM358 (IC1) will be less than the voltage at the inverting input of the LM358.So the output of the op amp will be low. This keeps the opto coupler (MCT2E) deactivated. When the dew increases the resistance of the element increases and so do the voltage across it. Now the voltage at the non inverting pin of LM358 (IC1) will be higher than the voltage at the inverting input of the LM358.So the output of the op amp will be switched to high. This in turn activates the opto coupler. The LED glows to indicate it. As a result we get an opto coupler activated and de activated according to the amount of dew in the atmosphere. The output pins of opto coupler pin (5&4) can be used to control the external device.

Diode D1, resistors R6&R3 and capacitor C1 is employed here to derive the power for the circuit directly from mains. The dew sensor is hard to find in market. But it can be easily obtained from a old VCR. Also the type no of the sensor is not so important here. Try with any thing you get. I used one from a old Hitachi VCR. LM 358 is a dual op amp. Here only one op amp inside it is used.

Active Band Pass Filters

Active band pass filters are simply filters constructed by using operational amplifiers as active devices configured to simulate inductors. Active band pass filters are used largely at audio frequencies where otherwise the size of the inductor would become prohibitive. In this figure is multiple feedback band pass (MFBP) type which uses capacitors of equal value and leads us to simplified calculations.


Finally pick a convenient value for C which if reasonably large, leads to smaller values of resistance and consequently some aid in reducing noise. Valuable feedback (no pun intended) from readers using rate-this-page (see below and on every other page) indicates the following needs clarification. The 100 uF capacitor above is purely part of the power supply reservoir and has nothing to do with the filter itself. The two 10K resistors are part of the power supply biasing of the op amps because we are not using positive and negative power supplies. The capacitor and resistor values are simply the value of C you choose to use and the resistor values result from the following calculations. It's that simple.

1 KHz Sine Wave Generator Circuit

This is a design circuit for generates a good 1 KHz sine wave adopting the inverted Wien bridge configuration (C1-R3 & C2-R4). It features a variable output, low distortion and low output impedance in order to obtain good overload capability. The figure is shown in below.


The bulb must be a low current type (12V 40-50mA or 6V 50mA) in order to obtain good long term stability and low distortion. Distortion @ 1V RMS output is 0.15% using a 12V 40mA bulb, raising to 0.5% with a 12V 100mA one. Using a bulb differing from specifications may require a change of R6 value to 220 or 150 Ohms to ensure proper circuit's oscillation. Set R5 to read 1V RMS on an Audio mili voltmeter connected to the output with R7 rotated fully clockwise, or to view a sine wave of 2.828V Peak-to-Peak amplitude on the oscilloscope. With C1, C2 = 100nF the frequency generated is 100Hz and with C1, C2 = 1nF frequency is 10 KHz but R5 requires adjustment. High gain transistors are preferred for better performance.
Component Part:
R1 5K6
R2 1K8
R3, R4 15K
R5 500R 1/2W Trimmer Cermet
R6 330R 1/4W Resistor
R7 470R Potentiometer
C1, C2 10nF/63V
C3 100µF/25V
C4 470nF/63V
Q1, Q2 BC238 25V 100mA NPN Transistors
LP1 12V 40mA Filament Lamp Bulb (See Notes)
J1 Phone chassis Socket
SW1 SPST Slider Switch
B1 9V PP3
Clip for 9V PP3 Battery

1 KHz Sine Wave Generator Circuit

This is a design circuit for generates a good 1 KHz sine wave adopting the inverted Wien bridge configuration (C1-R3 & C2-R4). It features a variable output, low distortion and low output impedance in order to obtain good overload capability. The figure is shown in below.


The bulb must be a low current type (12V 40-50mA or 6V 50mA) in order to obtain good long term stability and low distortion. Distortion @ 1V RMS output is 0.15% using a 12V 40mA bulb, raising to 0.5% with a 12V 100mA one. Using a bulb differing from specifications may require a change of R6 value to 220 or 150 Ohms to ensure proper circuit's oscillation. Set R5 to read 1V RMS on an Audio mili voltmeter connected to the output with R7 rotated fully clockwise, or to view a sine wave of 2.828V Peak-to-Peak amplitude on the oscilloscope. With C1, C2 = 100nF the frequency generated is 100Hz and with C1, C2 = 1nF frequency is 10 KHz but R5 requires adjustment. High gain transistors are preferred for better performance.

Component Part:
R1 5K6
R2 1K8
R3, R4 15K
R5 500R 1/2W Trimmer Cermet
R6 330R 1/4W Resistor
R7 470R Potentiometer
C1, C2 10nF/63V
C3 100µF/25V
C4 470nF/63V
Q1, Q2 BC238 25V 100mA NPN Transistors
LP1 12V 40mA Filament Lamp Bulb (See Notes)
J1 Phone chassis Socket
SW1 SPST Slider Switch
B1 9V PP3
Clip for 9V PP3 Battery

Band Pass Filters

A band-pass filter is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range. An example of an analogue electronic band-pass filter is an RLC circuit (a resistor–inductor–capacitor circuit). These filters can also be created by combining a low-pass filter with a high-pass filter.

Band pass is an adjective that describes a type of filter or filtering process; it is frequently confused with pass band, which refers to the actual portion of affected spectrum. The two words are both compound words that follow the English rules of formation: the primary meaning is the latter part of the compound, while the modifier is the first part. Hence, one may correctly say 'A dual band pass filter has two pass bands.

An ideal band pass filter would have a completely flat pass band (e.g. with no gain/attenuation throughout) and would completely attenuate all frequencies outside the pass band. Additionally, the transition out of the pass band would be instantaneous in frequency. In practice, no band pass filter is ideal. The filter does not attenuate all frequencies outside the desired frequency range completely; in particular, there is a region just outside the intended pass band where frequencies are attenuated, but not rejected. This is known as the filter roll-off, and it is usually expressed in dB of attenuation per octave or decade of frequency. Generally, the design of a filter seeks to make the roll-off as narrow as possible, thus allowing the filter to perform as close as possible to its intended design. Often, this is achieved at the expense of pass-band or stop-band ripple.

The bandwidth of the filter is simply the difference between the upper and lower cutoff frequencies. The shape factor is the ratio of bandwidths measured using two different attenuation values to determine the cutoff frequency, e.g., a shape factor of 2:1 at 30/3 dB means the bandwidth measured between frequencies at 30 dB attenuation is twice that measured between frequencies at 3 dB attenuation.

Outside of electronics and signal processing, one example of the use of band-pass filters is in the atmospheric sciences. It is common to band-pass filter recent meteorological data with a period range of, for example, 3 to 10 days, so that only cyclones remain as fluctuations in the data fields.