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.
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.
3 Channel Spectrum Analyzer
This 3 channel 15 LED spectrum analyzer is the perfect addition to any audio amp project.
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.
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.
Air Flow Detector
This simple circuit uses an incandescent lamp to detect airflow.
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.
This is a very straightforward circuit. The first stage acts as a crystal receiver.
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.
Electromagnetic field detector
This lovely circuit is a real gem! Easy to assemble and more sensitive than many commercial devices available.
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
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.
Low-distortion Audio-range Oscillator
Producing low-distortion sine waves, this oscillator operates over the range 16 to 22000 Hz.
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.
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.
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.
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).
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.
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.
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.
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.
This is a variation on the astable multivibrator. Circuit was recently developed to test for N-mosfets(the power kind e.g irf830)
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.
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.
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.
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.
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.
you will have an instrument capable of measuring all but the largest capacitors used in radio circuits.
Tool for XTall testing. This one is MUST HAVE when you work with radio devices, RC toys...
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.
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.
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.
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.
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.
Digital Step-Km Counter
This circuit measures the distance covered during a walk.
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.
Simple Servo Tester
Tester for servo devices.
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.
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.
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.
Coil Coupled Operation Metal Detector
One more kind of metal detectors.
Simple connection tester built with famous 741 op-amp circuit. Very useful for every electronics workshop.
Sine Wave Generator
This oscillator produces sinusoidal wave. It's made around LF351 or TL071CN or TL081CN. A classic Wien Bridge oscillator
This device should measure electromagnetic energy, but only the one which human body absorbs.
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.
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
Beat Balance Metal Detector
A Beat Balance Metal Detector made from discrete components. Interesting design.
IR Remote Control Modulation Detector
A circuit to extract and measure the modulated carrier of an Infra Red remote control
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.
Built around a single 8038 waveform generator IC, this circuit produces sine, square or triangle waves from 20Hz to 200kHz in four switched ranges.
Digital Radar Speedometer
It allows us to evaluate the speed of any object moving, especially cars and other vehicles
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.