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Tools and measuring circuit diagrams

Build your own electronics instrument for testing components and circuits, or temperature measuring device...

Decibel Meter

The circuit below responds to sound pressure levels from about 60 to 70 dB. The sound is picked up by an 8 ohm speaker, amplified by a transistor stage and one LM324 op-amp section.
[View Decibel Meter circuit diagram]

Low Frequency Sinewave Generators

The two circuits below illustrate generating low frequency sinewaves by shifting the phase of the signal through an RC network so that oscillation occurs where the total phase shift is 360 degrees.
[View Low Frequency Sinewave Generators circuit diagram]

A simple Remote control Tester

Here is a handy gadget for test- ing of infrared (IR) based re- mote control transmitters used for TVs and VCRs etc.
[View A simple Remote control Tester circuit diagram]

Sawtooth wave generator

Sawtooth wave generators using opamp are very common. But the disadvantage is that it requires a bipolar power supply. A sawtooth wave generator can be built using a simple 555 timer IC and a transistor as shown in the circuit diagram.
[View Sawtooth wave generator circuit diagram]

Simple variable frequency oscillator

This is a very simple circuit utilizing a 555 timer IC to generate square wave of frequency that can be adjusted by a potentiometer.
[View Simple variable frequency oscillator circuit diagram]

Pot plant water tester

This simple device checks if their is water in a pot plant. You stick the two probes(paperclips)into the pot plant and if the LED lights, it means there is water in the pot plant.
[View Pot plant water tester circuit diagram]

Simple IF Signal Generator

Here is a versatile circuit of IF signal generator which may be of interest to radio hobbyists and professionals alike.
[View Simple IF Signal Generator circuit diagram]

Color Sensor

Colour sensor is an interesting project for hobbyists. The cir- cuit can sense eight colours, i.e. blue, green and red (primary colours); magenta, yellow and cyan (secondary colours); and black and white.
[View Color Sensor circuit diagram]

Magnetic proximity sensors

Here is an interesting circuit for a magnetic proximity switch which can be used in various applications. The magnetic proximity switch circuit, in principle, consists of a reed switch at its heart. When a magnet is brought in the vicinity of the sensor (reed switch), it operates and controls the rest of the switching circuit.
[View Magnetic proximity sensors circuit diagram]

Zener Diode Tester

Here is a handy zener diode tester which tests zener diodes with breakdown voltages extending up to 120 volts. The main advantage of this circuit is that it works with a voltage as low as 6V DC and consumes less than 8 mA current.
[View Zener Diode Tester circuit diagram]

Contactless Mains Voltage Indicator

This is a CMOS IC (CD4033) based circuit which can be used to detect presence of mains AC voltage without any electrical contact with the conductor carrying AC current/voltage.
[View Contactless Mains Voltage Indicator circuit diagram]

A simple Remote control Tester

Here is a handy gadget for test- ing of infrared (IR) based re- mote control transmitters used for TVs and VCRs etc. The IR signals from a remote control transmitter are sensed by the IR sensor module in the tester and its output at pin 2 goes low.
[View A simple Remote control Tester circuit diagram]

Off line Telephone tester

H ere is a circuit of an off-line telephone tester which does not require any telephone line for testing a telephone instrument. The circuit is so simple that it can be easily assembled even by a novice having very little knowledge of electronics.
[View Off line Telephone tester circuit diagram]

PC based Frequency Meter

Here is a simple technique for measuring frequencies over quite a wide frequency range and with acceptable accuracy limits using a PC. It follows the basic technique of measuring low frequencies, i.e. at low frequency, period is measured for a complete wave and frequency is calculated from the measured time-period.
[View PC based Frequency Meter circuit diagram]

3 Channel Spectrum Analyzer

This 3 channel 15 LED spectrum analyzer is the perfect addition to any audio amp project.
[View 3 Channel Spectrum Analyzer circuit diagram]

4 - Channel Digital Logic Analyzer

This is one of the simplest circuit for a PC parallel port based DLA. The Circuit is mainly used to protect the port which is fabricated on the motherboard itself.
[View 4 - Channel Digital Logic Analyzer circuit diagram]

Electronic Stethoscope

Stethoscopes are not only useful for doctors, but home mechanics, exterminators, spying and any number of other uses. Standard stethoscopes provide no amplification which limits their use. This circuit uses op-amps to greatly amplify a standard stethoscope, and includes a low pass filter to remove background noise.
[View Electronic Stethoscope circuit diagram]

Air Flow Detector

This simple circuit uses an incandescent lamp to detect airflow.
[View Air Flow Detector circuit diagram]

Salt Taster

This circuit was designed to detect the approximate percentage of salt contained in a liquid. After careful setting it can be useful to persons needing a quick, rough indication of the salt content in liquid foods for diet purposes etc.
[View Salt Taster circuit diagram]

Field-strength meter

This is a very straightforward circuit. The first stage acts as a crystal receiver.
[View Field-strength meter circuit diagram]

Lightning Detector

Here is a VLF receiver tuned to 300 kHz designed to detect the crackle of approaching lightning. A bright lamp flashes in sychrony with the lightning bolts indicating the proximity and intensity of the storm.
[View Lightning Detector circuit diagram]

Electromagnetic field detector

This lovely circuit is a real gem! Easy to assemble and more sensitive than many commercial devices available.
[View Electromagnetic field detector circuit diagram]

Adjustable High-Low Frequency Sine wave generator

This circuit uses the versatile MAX038 function generator. Although in this circuit some of the advanced characteristics of this IC are disabled, you can generate Sine, Triangle, Square waves
[View Adjustable High-Low Frequency Sine wave generator circuit diagram]

Pulse-Generator and Signal-Tracer

This simple circuit generates narrow pulses at about 700-800Hz frequency. The pulses, containing harmonics up to the MHz region, can be injected into audio or radio-frequency stages of amplifiers, receivers and the like for testing purposes.
[View Pulse-Generator and Signal-Tracer circuit diagram]

Low-distortion Audio-range Oscillator

Producing low-distortion sine waves, this oscillator operates over the range 16 to 22000 Hz.
[View Low-distortion Audio-range Oscillator circuit diagram]

Self-powered Sine to Square wave Converter

This circuit is intended to provide good square waves converting a sine wave picked-up from an existing generator. Its major feature consists in the fact that no power-source is needed: thus it can be simply connected between a sine wave generator and the device under test.
[View Self-powered Sine to Square wave Converter circuit diagram]

1KHz Sinewave Generator

This circuit generates a good 1KHz sinewave using the inverted Wien bridge configuration. Features a variable output, low distortion and low output impedance in order to obtain good overload capability.
[View 1KHz Sinewave Generator circuit diagram]

Simple but reliable car battery tester

This circuit uses the popular and easy to find LM3914 IC. This IC is very simple to drive, needs no voltage regulators (it has a built in voltage regulator) and can be powered from almost every source.
[View Simple but reliable car battery tester circuit diagram]

Digital Radar Speedometer

This circuit is a Digital Radar Speedometer. It allows us to evaluate the speed of any object moving, especially cars and other vehicles. The speed is calculated in kilometers per hour (KPH).
[View Digital Radar Speedometer circuit diagram]

Logic Probe

This circuit is a Logic Probe. It indicates the logic state of the node of any TTL logic circuit. To do that, we have to supply the probe with the same power of the circuit that we want to analyse: same Vcc and same GND.
[View Logic Probe circuit diagram]

Voltage Monitor

When the input voltage is 0 the LED glows. The LED stops glowing when the voltage rises to the level determined by R2. Reverse + and - pins to reverse operating mode.
[View Voltage Monitor circuit diagram]

Oscilloscope testing module

I call this the huntron circuit because a company by that name made a similar device. It is useful for trouble-shooting.
[View Oscilloscope testing module circuit diagram]

Crystal Tester

This circuit enables you to test quartz resonators at the range values from 32kHz to 24MHz. Confirmation of good state of quartz resonator is done by diode signalling LED and acoustic signal.
[View Crystal Tester circuit diagram]


This is a variation on the astable multivibrator. Circuit was recently developed to test for N-mosfets(the power kind e.g irf830)
[View Mosfet TESTER circuit diagram]

Beeper to find short circuits

This circuit uses two 741s or equivalent op amp to form a beeper that is used to detect short circuits.
[View Beeper to find short circuits circuit diagram]

Picoammeter circuit with 4 ranges

This circuit uses a CA3420 BiMOS op amp to form a picoammeter with 4 ranges. The exceptionally low input current (typically 0.2pA) makes the CA3420 highly suited for use in a picoammeter circuit.
[View Picoammeter circuit with 4 ranges circuit diagram]

Triangle-Squarewave Generator

Here is a simple triangle/squarewave generator using a common 1458 dual op-amp that can be used from very low frequencies to about 10 Khz.
[View Triangle-Squarewave Generator circuit diagram]

Transistor Tester

This simple circuit has helped me out on many occasions. It is able to check transistors, in the circuit, down to 40 ohms across the collector-base or base-emitter junctions. It can also check the output power transistors on amplifier circuits.
[View Transistor Tester circuit diagram]

Digital Voltmeter

The ICL7107 is a 3 1/2 digit LED A/D convertor. It contains an internal voltage reference, high isolation analog switches, sequential control logic, and the display drivers. The auto-zero adjust ensures zero reading for 0 volts input.
[View Digital Voltmeter circuit diagram]

Capacitance Meter

you will have an instrument capable of measuring all but the largest capacitors used in radio circuits.
[View Capacitance Meter circuit diagram]

XTal Tester

Tool for XTall testing. This one is MUST HAVE when you work with radio devices, RC toys...
[View XTal Tester circuit diagram]

Linear Resistance Meter

Most analogue multimeters 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.
[View Linear Resistance Meter circuit diagram]

IR Remote Control Tester

I have used a photodiode, SFH2030 as an infra red sensor. A MOSFET opamp, CA3140 is used in the differential mode to amplify the pulses of current from the photodiode. LED1 is an ordinary coloured led which will light when IR radiation is being received. The output of the opamp, pin 6 may be connected to a multimeter set to read DC volts.
[View IR Remote Control Tester circuit diagram]

Continuity Tester

Several simple circuits were tried -- a lamp, battery and probes still demanded the attention of the eyes; replacing the lamp with a buzzer was more successful but needed some three to four volts and gave no indication of a series semiconductor junction if the polarity was correct while the current flow was large enough to damage the more delicate devices within the circuit under test.
[View Continuity Tester circuit diagram]

Simple Lie Detector

Here's a simple lie detector that can be built in a few minutes, but can be incredibly useful when you want to know if someone is really telling you the truth.
[View Simple Lie Detector circuit diagram]

Cellular Phone calling Detector

This circuit was designed to detect when a call is incoming in a cellular phone (even when the calling tone of the device is switched-off) by means of a flashing LED.
[View Cellular Phone calling Detector circuit diagram]

Digital Step-Km Counter

This circuit measures the distance covered during a walk.
[View Digital Step-Km Counter circuit diagram]

555 stepper pulse generator

The 555 Stepper Pulse Generator kit will help you with the pulse required to drive your favorite DC Servo Motor application.
[View 555 stepper pulse generator circuit diagram]

Simple Servo Tester

Tester for servo devices.
[View Simple Servo Tester circuit diagram]

Self-powered Fast Battery-Tester

This circuit runs a fast battery test without the need of power supply or expensive moving-coil voltmeters. It has two ranges: when SW1 is set as shown in the circuit diagram, the device can test 3V to 15V batteries. When SW1 is switched to the other position, only 1.5V cells can be tested.
[View Self-powered Fast Battery-Tester circuit diagram]

Speech Recorder

The 2 pushbuttons = S1: Start/Pause. S2: Stop/Reset. If you want to play your message, put S3 at Play. Then push S1 to start playing and again to pause. If you want to delete your message press S2 twice. If you want to record a message put S3 at Rec. Then push S1 to start and S2 to stop.
[View Speech Recorder circuit diagram]

Static Electricity - Negative Ion Detector

This device can detect negative ions in air. It has very few components: one FET, two resistors and battery. You also need multimeter to complete device. This device reacts, for example, on a TV radiation or air ionizer.
[View Static Electricity - Negative Ion Detector circuit diagram]

Coil Coupled Operation Metal Detector

One more kind of metal detectors.
[View Coil Coupled Operation Metal Detector circuit diagram]

Connection Tester

Simple connection tester built with famous 741 op-amp circuit. Very useful for every electronics workshop.
[View Connection Tester circuit diagram]

Sine Wave Generator

This oscillator produces sinusoidal wave. It's made around LF351 or TL071CN or TL081CN. A classic Wien Bridge oscillator
[View Sine Wave Generator circuit diagram]


This device should measure electromagnetic energy, but only the one which human body absorbs.
[View MILLIGAUSS METER circuit diagram]

Bench Amplifier

This circuit is basically an audio amplifier, but it is placed in this category because it may be used as signal tracer. It is built around simple National Semiconductor LM386 amplifier.
[View Bench Amplifier circuit diagram]

Signal Tracer and Injector

A simple test circuit to fault find audio and radio equipment. Can be used to inject a square wave signal, rich in harmonics, or used with headphones as an audio tracer
[View Signal Tracer and Injector circuit diagram]

Beat Balance Metal Detector

A Beat Balance Metal Detector made from discrete components. Interesting design.
[View Beat Balance Metal Detector circuit diagram]

IR Remote Control Modulation Detector

A circuit to extract and measure the modulated carrier of an Infra Red remote control
[View IR Remote Control Modulation Detector circuit diagram]

Electromagnetic Field Detector

This circuit is sensitive to low frequency electromagnetic radiation and will detect for example hidden wiring or the field that encompasses a transformer.
[View Electromagnetic Field Detector circuit diagram]

Function Generator

Built around a single 8038 waveform generator IC, this circuit produces sine, square or triangle waves from 20Hz to 200kHz in four switched ranges.
[View Function Generator circuit diagram]

Digital Radar Speedometer

It allows us to evaluate the speed of any object moving, especially cars and other vehicles
[View Digital Radar Speedometer circuit diagram]

Metal Detector

A single chip metal detecor with a range of a few inches. This is useful for decting nails or screws in walls and floors, or for locating buried mains cable.
[View Metal Detector circuit diagram]

..:: Important ::..

Read disclamer, please!


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