Op Amp Digital to Analog Converter Circuit

This is a design of the simple 4-bit digital-to-analog converter.  It is actually just a variant of a simple op amp summer circuit, i.e., an operational amplifier configured to output a voltage that is proportional to the sum of the input voltages. Here’s the figure of the circuit;

In this circuit, the inputs are binary weighted with respect to each other, with the binary weighting of the inputs achieved by the R-2R ladder resistor network at the non-inverting input of the op-amp. As its name implies, the R-2R network consists of resistors with only two values, R and 2R (10K and 20K, respectively, in the circuit shown).  The input SN to bit N is '1' if it is connected to a voltage VR and '0' if it is grounded. The number of bits of this DAC may be increased by connecting more switches with corresponding R/2R resistors.

LM134-LM10 Thermometer/Temperature Sensor Circuit

This circuit is use to build a thermometer which has -55 to 150°C sensing range.  The ideal meter for this circuit is a 0-200uA digital ampere meter, which can show both positive and negative polarity. This will make the circuit suitable for indicating temperatures below 0°C. Here’s the figure of the circuit;

This circuit is based on LM134 and LM10 that has basically a current source with very accurate and consistent temperature coefficient, so many temperature sensing application find it suitable for the sensor. [Circuit diagram source: National Semiconductor Notes]

LM10 Single Cell Microphone Amplifier Circuit

This is a design circuit for a Single-cell microphone amplifier circuit. This circuit produces a voltage gain of 60 dB at 500Ω load with bandwidth 5 kHz. This circuit can provide a 60dB gain up to 10 kHz if it is unloaded. This circuit has a 10 kΩ input impedance. Here’s the figure of the circuit;

This circuit cannot neither produces an output swing closer than 800mV or below 150mV to the supply. However this circuit still has an input noise of 40 to 50 nV/√Hz. [Circuit diagram source: National Semiconductor Application Notes]

Subwoofer Filter Circuit

This is a circuit of subwoofer active filter circuit is a 24 dB octave filter with a Bessel character and cutoff frequency of 200 Hz. So, if you are interested in experimenting with audio circuits in subwoofer range, this circuit is for you. This is the figure of the circuit;

In subwoofer range, all audio frequencies below 200 Hz can be fed to a single speaker box since the human directional perception of sound diminishes at this frequency range. The normal stereo signals above 200 Hz can be fed to two satellite speaker boxes. How does the subwoofer filter works: A1 and A2 buffer the signals coming from right and left channels. Op amp combinations A2/A4 and A9/A10 function as the high pass filters. The outputs are then connected to the final amplifiers of the battelite boxes. Signals from both channels are fed to A5. Op amps A6/A7 function as the low pass filter, A8 as the output amplifier for the subwoofer signal.

The signal level can be balanced between the subwoofer and the satellite lines. The power needed for this filter circuit must ne a symmetrical power supply. The op amps can have either JFET or bipolar inputs.

Smart Heater Controller Circuit

This is a design circuit of the electronic heater controller presented here is built around the renowned 3-Pin Integrated Temperature Sensor LM35 (IC1) from NSC. Besides, a popular BiMos Op-amp CA3140 (IC2) is used to sense the status of the temperature sensor IC1, which also controls a solid-state switch formed by a high power Triac BT136(T1). This is the figure of the circuit;

 

Resistive type electric heater at the output of T1 turns to ON and to OFF states as instructed by the control circuit. This gadget can be used as an efficient and safe heater in living rooms, incubators, heavy electric/electronic instrument etc. Normally, when the temperature is below a set value (Decided by multi-turn preset pot P1), voltage at the inverting input (pin2) of IC1 is lower than the level at the non-inverting terminal (pin3). So, the comparator output (at pin 6) of IC1 goes high and T1 is triggered to supply mains power to the desired heater element.

When the temperature increases above the set value, say 50-60 degree centigrade, the inverting pin of IC1 also goes above the non-inverting pin and hence the comparator output falls. This stops triggering of T1 preventing the mains supply from reaching the heater element. Fortunately, the threshold value is user-controllable and can be set anywhere between 0 to 100 Degree centigrade.

High Impedance Voltmeter Circuit

This is the design circuit off ideal voltmeter has infinite input impedance, meaning that it draws zero current from the circuit under test. This way, there will be no "impact" on the circuit as the voltage is being measured. The more current a voltmeter draws from the circuit under test, the more the measured voltage will "sag" under the loading effect of the meter, like a tire-pressure gauge releasing air out of the tire being measured: the more air released from the tire, the more the tire's pressure will be impacted in the act of measurement. This loading is more pronounced on circuits of high resistance, like the voltage divider made of 1 MΩ resistors. This is the figure of the circuit;

 

If you were to build a simple 0-15 volt range voltmeter by connecting the 1 mA meter movement in series with the 15 kΩ precision resistor, and try to use this voltmeter to measure the voltages at TP1, TP2, or TP3 (with respect to ground), you'd encounter severe measurement errors induced by meter "impact:" If we were to increase the meter's input impedance, we would diminish its current draw or "load" on the circuit under test and consequently improve its measurement accuracy. An op-amp with high-impedance inputs (using a JFET transistor input stage rather than a BJT input stage) works well for this application.

Note that the meter movement is part of the op-amp's feedback loop from output to inverting input. This circuit drives the meter movement with a current proportional to the voltage impressed at the non-inverting (+) input, the requisite current supplied directly from the batteries through the op-amp's power supply pins, not from the circuit under test through the test probe. The meter's range is set by the resistor connecting the inverting (-) input to ground.

Build the op-amp meter circuit as shown and re-take voltage measurements at TP1, TP2, and TP3. You should enjoy far better success this time, with the meter movement accurately measuring these voltages (approximately 3, 6, and 9 volts, respectively).

Audio Controlled Incandescent Lamp Light Controller

This is a design circuit for an audio-controlled lamp circuit. This circuit requires low voltage input such as pre-amplifiers, tone control, or general audio line level output. It’s also possible to feed the input with signal from small power amplifier output, or high power amplifier witk low volume level. The characteristic of lamp dimming (incandescent lamp) will look like coming from proportional controller since the switching rate of the TRIAC would be much higher than the lamp dimming response. This is the figure of the circuit;

 

If the lamp won’t go off after the audio signal back down to zero, then try to adjust the potentiometer. You can use filament transformer (usually used for tube), or you can also use small tv-antenna-booster transformer (20v-220V), or other small transformer below 500mA. Don’t worry if you can’t fine transformer with exact primary-secondary ratio, since the potentiometer is there for fine adjustment, and you can always change the input voltage level of the audio input signal.

555 Timer Voltage Controlled Oscillator Circuit

This is a circuit of a voltage-controlled oscillator (VCO) that uses the 555 timer IC as the main component. As expected, the 555 timer is configured as an astable multi vibrator to be able to serve as an oscillator. An astable multi vibrator is just a timing circuit whose output oscillates between 'low' and 'high' continuously, in effect generating a train of pulses. This is the figure of the circuit;

The difference of this circuit with the basic 555 astable circuit is that its 555's pin 5 is tied to an external voltage source.  Pin 5 is the 555's control voltage pin, which allows the user to directly adjust the threshold voltages to which the pin 2/pin 6 input voltages are compared by the 555's internal comparators.  Since the outputs of these comparators control the internal flip-flop that toggles the output of the 555, adjusting the pin 5 control voltage also adjusts the frequency at which the 555 toggles its output. Increasing the input voltage at pin 5 decreases the output oscillation frequency while decreasing the input voltage increases the output oscillation frequency.

Voltage Controlled Stereo Volume Control Using LM13600 OTA

This is the design circuit for control our audio using a DAC which is controlled by a microcontroller, or we can use mono potentiometer with RC filter to minimize the scratched sound produced by a dirty potentiometer (I see this method in one of SONY audio products). Using a voltage-controlled volume control enables you to design your own tremolo effect, noise gate, dynamic compressor-expander effect and many more! This is the figure of the circuit;

This circuit uses excellent matching of the two LM13600 operational trans-conductance amplifier (OTA), providing 0.3dB channel-to-channel gain tracking (typical) volume control. Potentiometer RP is used to adjust the output offset voltage, and can be substituted by two 510R fixed resistors if you use the circuit in AC-coupled application. With the value shown in the diagram, the gain of the amplifier (Vo/Vin) = 940*IABC. [Circuit diagram source: National Semiconductor Application Notes]

Mic/Guitar Compressor Circuit Using Transistor Bias Control

This is a design circuit for compressor circuit that can be used for dynamic mic, condenser mic, or electric guitar pickup. This circuit doesn’t produce very good sound output, but it has very stable amplitude/volume. This compressor circuit design is unique since it uses base current control for manipulating the gain. This is the figure of the circuit;

The first and second transistor is an ordinary pre-amplifier, but the first transistor’s bias is controlled by the output signal to get a negative feedback. The third transistor is employed to control the dc voltage level at the junction of 1M resistor and 10uF cap that provide the first transistor bias current. The control mechanism is also unique, where  a full dc level at the capacitor is discharged at each half cycle of output when  it exceed the base forward bias. The gain control method by decreasing the base bias current of first transistor near its cut-off point produce a distortion, which might be unacceptable by some people especially for music/singing, but  can be acceptable for public address.  For electric guitar application, this compressor can be a good option since the distortion characteristic gives a unique effect, and the overshoot and undershoot of the amplitude stabilization gives something similar to “wow” effect from old recording.

Ice Warning And Light Reminder Circuit

This is a design circuit for very simple ice warning and lights reminder electronic. This device will tell a driver if his lights should be on and will warn him if the outside temperature is nearing zero by lighting a LED and sounding a buzzer. This is the figure of the circuit;

Using the VR1 you can adjusts sensitivity for temperature and using VR2 you can adjust sensitivity for the light. Both thermistor and LDR should be well protected. More high gain NPN transistors will work for this electronic project. For this electronic project you can use BC108 type transistor or some other NPN high gain transistor. This ice warning and lights reminder electronic project must be powered from a 12 volt DC power supply circuit.

Color Organ Electronic Circuit Using LM3909 IC

Using LM 3909 LED flasher IC, can be designed various electronic projects. As you can see in this circuit diagram, using the LM3909 LED flasher IC can be designed a very simple color organ. But this circuit is not complicated, it use just some common active audio filters for filtering audio signals and a part formed by a LM3909 IC. This is the figure of the circuit;

 

All of three active filters drives the audio spectrum into three bands drive rectifiers and then drive IC2,3 and 4, flashing the LEDs at 6 Hz. D4,D5 and D6 should be three different colors for best effect. In the table bellow you can see all electronic parts required by this color organ project.

Two Stage Phono Pre-Amplifier Circuit with Very Accurate RIAA Response Curve

This is a design circuit that can be used to produce a pre-amplifier with accurate RIAA response, this circuit use two stages of amplifier. This circuit is more complex than single op-amp RIAA pre-amp, but the performance is excellent in terms of accurate frequency response. This is the figure of the circuit;

The circuit uses LM833 high performance op-amp from National Semiconductor, giving the best performance of 0.1 dB response curve error, compared to the standard RIAA curve. [Circuit diagram source: National Semiconductor Application Notes]

Signal Conditioning Amplifier Circuit for Piezofilm Sensor

This circuit is design for the signal can be converted by piezoelectric films in many ways such as thermal to electrical (temperature sensor), mechanical to electrical (microphone), and electrical to mechanical (a loudspeaker). The circuit is signal conditioning amplifier for piezofilm sensor which uses three op amps and has a high-input-impedance differential charge. This is the figure of the circuit;

This circuit uses a voltage source with a capacitor in series as the electrical analog of a piezofilm sensor. The differential charge amplifier is endowed by a dual op amp (IC1) with low supply current and single-supply operation. The input common-mode voltage  is set by a small bypass capacitor (C3), R2, R1 at the mid-supply level. The C1 and C2 are used to set AC gain for the differential stage. A gain of C1/CEQ is 96.

The differential amplifier also is used to act as a first-order high-pass filter. The resistors R9, R8, R6, and R5 with IC2 perform differential-to-single-ended conversion. Using the values shown, the different gain is 20. [Circuit diagram source: maxim-ic.com]

One Second Steps 555 Timer Circuit (0 To 59 S)

This is a design circuit for the linearity of 555 IC is sufficient to make 1 second precision, and this circuit employs the feature in this design. This timer step circuit can generate a timing steps from 0 to 59 s in 1-s intervals. This is the figure of the circuit;

Flame Detector Using Platinum-Rhodium Thermocouple

In furnace operation, it is necessary to make sure that the fuel or gas is ignited properly by detecting the flame. If there’s no flame detected, the situation can be dangerous if the furnace is flooded with explosive gas, so the fuel/gas supply valve should be closed automatically to avoid catastrophic disaster.  A thermocouple can be use to detect flame. This is the figure of the circuit;

 

A platinum-rhodium thermocouple produce 8mV output at 800 °C. You can see the thermocouple is connected directly to the positive and negative inputs, look like there’s no threshold, but wait, the balance input (pin 5) is connected to the reference output and this gives the threshold. [Circuit diagram source: National Semiconductor Application Note]

Fan Speed Control: Turn On Your Fan Only When Needed

If you have to provide a fan to cool your audio power amplifier final transistor, it better to turn it off when it’s not needed. This solution is good since you don’t have to hear the fan noise when you play the music with low level volume at night, the power transistor won’t get hot. When you start turning your volume control toward maximum level, your power chip/transistor begin getting hotter and this circuit will turn on the fan to cool the heat sink. You won’t hear the fan noise because now your music is getting much louder. This is the figure of the circuit;

 

The temperature sensor (NTC, negative temperature coefficient thermistor) should be placed as close as possible to the power transistor or IC. Installing it on the heat sink is good idea, just make sure a tight thermal contact and locate it close to the power transistor/IC.

If you have a thermometer, you can set the VR1 at heat sink temperature about 70 Celsius degree. Lowering this setting it’s OK but you might hear the fan noise although you only use your amplifier to play music for mild volume level.  What if you don’t have a thermometer but you want your power amplifier safe?  The following technique can be use but use at your own risk: use your amp for loud volume and use your finger to regularly touch the heat sink and adjust VR1 to activate the fan at heat intensity  where it’s the maximum you can stand to touch.  The LED indicator will give you the information that the fan is active. Stay cool with your amplifier!

Anti-Log Converter Circuit

Anti-log or exponential generation is simply a matter of rearranging the logarithmic circuitry. The circuitry of the log converted to generate an exponential output from a linear input. This is the figure of the circuit;

The emitter of Q2 in proportion to the input voltage is driven by amplifier A1 in conjunction with transistor Q1. The collector current of Q2 varies exponentially with the emitter-base voltage. [Circuit diagram source: National Semiconductor Application Note]

Radio Frequency (RF) Watt Meter Circuit

This is a circuit for RF (radio frequency) transmitter experiment, watt meter is useful for optimizing the transmitter circuit. A simple RF watt meter circuit is shown in the schematic diagram below. Because circuit is not frequency sensitive, calibration is accurate on all HF bands. The sensitivity is affected by meter movement, number of turns in primary coil, and resistive voltage driver. This is the figure of the system;

Pots can be adjusted for full-scale values from 1-14 W with values shown on the diagram. C1 and C2 are 3-20 pF. Diodes are 1N34A, 1N60, or equivalent. L1 is 46 turns No. 28 on Amidon T-50-2 toroid, with 2 turns No. 22 between ends of L1 for L2. Connect resistive dummy load to one coax receptacle and RF power source to other to adjust, with R2 at maximum resistance. We can provide highest meter reading and make that the FWD position with place the switch on the upper position. Switch to other position, which becomes REF, and for null reading, adjust C1. Reverse RF source and load, leaving switch at FWD, and adjust C2 for null. Now, we can calibrated the Wattmeter.

Power Sequencer Circuit Using LM3880

This is a design circuit for Power Sequencer offers the easiest method to control power up and power down of multiple power supplies (switchers or linear regulators). By staggering the startup sequence, it is possible to avoid latch conditions or large in-rush currents that can affect the reliability of the system. This is the figure of the circuit;

Available in a SOT23-6 package, the Power Sequencer contains a precision enable pin and three open drain output flags. Upon enabling the LM3880 the three output flags will sequentially release, after individual time delays, permitting the connected power supplies to startup. The output flags will follow a reverse sequence during power down to avoid latch conditions. EPROM capability allows every delay and sequence to be fully adjustable. Contact National Semiconductor if a non-standard configuration is required.

Linear Resistance Meter Circuit

This is a one design circuit for analogue multi meters are capable of measuring resistance over quite a wide range of values, but are rather inconvenient in use due to the reverse reading scale which is also non-linear. This can also give poor accuracy due to cramping of the scale that occurs at the high value end of each range. This is the figure of the circuit;

This resistance meter has 5 ranges and it has a forward reading linear scale on each range. The full-scale values of the 5 ranges are 1K, 10K, 100K, 1M &10M respectively and the unit is therefore capable of reasonably accurate measurements from a few tens of ohms to ten Mega ohms. Most linear scale resistance meters including the present design, work on the principle that if a resistance is fed from a constant current source the voltage developed across that resistance is proportional to its value. For example, if a 1K resistor is fed from a 1 mA current source from Ohm’s Law it can be calculated that 1 volt will be developed across the resistor (1000 Ohms divided by 0.001 amps = 1 volt). Using the same current and resistance values of 100 ohms and 10K gives voltages of 0.1volts (100 ohms / 0.001amps = 0.1volts) and 10 volts (10000 ohms / 0.001amps = 10 volts). Thus the voltage developed across the resistor is indeed proportional to its value, and a voltmeter used to measure this voltage can in fact be calibrated in resistance, and will have the desired forward reading linear scale. One slight complication is that the voltmeter must not take a significant current or this will alter the current fed to the test resistor and impair linearity. It is therefore necessary to use a high impedance voltmeter circuit.

Electromagnetic Field Probe Circuit Using Meter Output

This circuit is designed to locate stray electromagnetic (EM) fields. It will easily detect both audio and RF signals up to frequencies of around 100kHz. Note, however that this circuit is NOT a metal detector, but will detect metal wiring if it conducting ac current. This is the figure of the circuit;

Frequency response is from 50Hz to about 100kHz gain being rolled off by the 150p capacitor, the gain of the op-amp and input capacitance of the probe cable. Stereo headphones may be used to monitor audio frequencies at the socket, SK1. The output signal from the op-amp is an ac voltage at the frequency of the electro-magnetic field. This voltage is further amplified by the BC109C transistor, before being full wave rectified and fed to the meter circuit. The meter is a small dc panel meter with a FSD of 250uA. Rectification takes place via the diodes, meter and capacitor. Switch on, plug in headphones (optional) and move the probe around. Any electrical equipment should produce a hum and indicate on the meter. It remember once building a high gain preamp (for audio use). I made a power supply in the same enclosure. The preamp worked, but suffered from an awful mains hum. This was not directly from ripple on the power supply as it was regulated and well smoothed. What I had done was built the audio circuit on a small piece of veroboard, and placed it within a distance that was less than the diameter of the transformer.

3A Simple Switcher Power Module Using 20V Maximum Input Voltage

This is a design circuit for power module that is an easy-to-use step-down DC-DC solution capable of driving up to 3A load with exceptional power conversion efficiency, line and load regulation, and output accuracy. The LMZ12003 is available in an innovative package that enhances thermal performance and allows for hand or machine soldering. This is the figure of the circuit;

The LMZ12003 can accept an input voltage rail between 4.5V and 20V and deliver an adjustable and highly accurate output voltage as low as 0.8V. The LMZ12003 only requires three external resistors and four external capacitors to complete the power solution. The LMZ12003 is a reliable and robust design with the following protection features: thermal shutdown, input under-voltage lockout, output over-voltage protection, short-circuit protection, output current limit, and allows startup into a pre-biased output. A single resistor adjusts the switching frequency up to 1 MHz.