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Surveillance Bug Circuits
To avoid grief when using 12-V power supplies with mobile transceivers, especially when there is a short -circuit failure of the series pass transistor, crowbar circuits provide protection by clamping the power line and blowing the fuse within microseconds of an overvoltage condition. It is a good idea to incorporate the crowbar directly into...
This circuit uses a type 741 op amp as a voltage comparator. One input of the 7.41 is connected to a reference voltage (a 12-V battery) through a resistor voltage divider. The potential at the noninverting input of the 741 is approximately 3 V. The inverting input of the op amp comparator is connected to the output of a line-operated 8-V...
The adjustable circuit breaker responds in 0.02 sunder all conditions-provided you select a fast relay for Kl. For moderate overload conditions, it"s preferable to use the fuse or the fast-acting breaker. Toroid transformer Tl senses ac load current and produces an ac signal at the wiper of Rl, when switch S3 is closed. Diode Dl rectifies...
This circuit protects expensive portable equipment against all types of improper hookups and environmental hazards that could cause an overvoltage condition. It operates very quickly and does not latchup, that is, it recovers when the overvoltage condition is removed. In contrast, SCR overvoltage circuits can latch and do not recover, unless...
This circuit mimics the dual-tone drone sound that"s produced by the unusual wind instrument. Urnjunction transistors Ql and Q2 are connected in similar audio-oscillator circuits. Each of the oscillator frequencies is determined by one of the two resistors selected by one of the pushbutton switches, S4 through Sll. The odd-numbered resistors...
This siren provides a constant audio output, but alternates between two separate tones. The LM13080 is set to oscillate at one basic frequency;
Unijunction transistors Ql and Q2 are both connected as relaxation-type, sawtooth oscillators. Transistor Ql is the low-frequency control oscillator and Q2 is the tone generator. Sawtooth waveforms are produced at the emitter terminals. Without R4 connecting the two emitters, each oscillator operates independently, with its frequency...
Transistors Ql and Q2, with feedback provided via Cl from the collector of Ql to the base of Q2, forms a voltage-controlled oscillator (VCO). De- pending on the voltage applied to Q2 "s base, the VCO frequency ranges from around 60 Hz to 7.5 kHz. The instantaneous voltage applied to the base of Q2 is determined by the values of C2, R2, R3,...
A low-frequency, op-amp oscillator and a VCO, both configured from a single MC3405 ~dual op amp and dual comparator, are tbe major components in a siren circuit tbat can be made to produce various warbles and wails, or serve as an audio sweep generator.
Six preset controls and seven selector switches enable a vast range of different sounds to be produced and altered at will. Such sounds as steam trains chuffing, helicopters flying, bird chirping, and machine guns firing are possible, as well as the usual police sirens, phaser guns, and bomb explosions. The circuit incorporates an amplifier...
A pair of timer IC"s are the heart of a circuit that simulates the warbling bee-haw of a British police siren. One of the 555 timers, U2, is wired as an astable multivibrator operating at about 900 Hz. The other, Ul, operates at approximately 1 Hz. Its output at pin 3 is a square wave with a 50% duty cycle-on and off cycles of about 0.5...
The wailing sound of a siren is generated by a VFO consisting of Q1 and Q2. Capacitor C2 provides the feedback for the oscillator. The frequency of the oscillator is varied by the voltage applied to the base of Q1 through R3. When switch S1 is closed, capacitor Cl charges, thus increasing the oscillator frequency. When S1 is released,...
The simple circuit transforms the steady beep of an audible-signal device, such as a Mallory sonalert, into a distinctive warble or chirp. The value of R, C2 determines just what tone color you"ll get. With 1k the 1-I"F value shown, the circuit produces a warble similar to the ring tone of an inexpensive phone. A R, 10-I"F value produces a...
This circuit uses one of the LM389 transistors.to gate the power amplifier on and off by applying the muting technique. The other transistors form a cross-coupled multivibrator circuit that controls the rate of the square-wave oscillator.
Burglar Alarm With Timed Shutoff
When SI (sensor) is closed, power is applied to U2, a dual timer. After a time determined by C2, CI is ene
Home Security System
This alarm circuit activates when SI through S5 are activated. This lights LED1 and activates Ql via IC1C and IC1D. RY
Digital Entry Lock
A keypad enters a four-digit access code, which is programmed via jumpers on a 24-pin plug-in header and socket. Ul is a
The circuit uses a four-bit latch (Ul). What makes the circuit sequential is that the set input of the first bit latch is tied
These two circuits are SSB generators. One uses a crystal filter by KVG Electronics at 9 MHz, the other uses a 455-kHz mecha
"Hee-Haw" Electronic Siren
The oscillator based on IC2 is responsible for producing the sound. Its output is connected to the base of TRl,
6W Warble-Tone Siren
This circuit uses a CMOS chip and a VMOS FET amplifier for 6 W of audio output. 18 W of audio can be generated using
Warble Alarm Circuit
This circuit uses a 556 to first generate a low frequency square wave, that is modulated to produce two alternate tones of about 400 and 500 Hz. Circuit generates warble alarm of European emergency vehicles.
Audio Alarm Circuit
In the circuit, U1 amplifies the audio picked up by the condenser microphone. Resistor Rl limits current, while R2 and R3 center the output of the amplifier to %B+ to allow a single-ended supply to be used. Diodes D1 and D2 rectify the output of Ul, and C3 filters the resulting pulsing dc. Thus, a dc voltage that is proportional to the ambient...
Piezoelectric Alarm Circuit
The alarm uses a fixed-frequency piezoelectric buzzer in conjunction with the cadmium-sulfide (CDS) cell and the two-transistor circuit to provide a unique effect. Whenever light reaches the CDS photo-electric cell, the alarm is silent. But when no light strikes the cell, transistor Ql turns on, and the circuit emits a high-pitched tone.The...
Burglar Chaser Circuit
The burglar chaser makes a great accessory for any alarm system. It creates brilliant flashes of white light and a loud, irritating sound from a metal horn buzzer. Transformer T1 is connected to Ql, Rl, and R2 to form a blocking oscillator. This creates a 6-Vac signal on the primary of Tl. Because of Tl`s large ratio of turns from primary to...
Carrier-Current Baby-Alert Transmitter Circuit
The baby-alert transmitter is built around an LM324 quad op amp (Ul), two LMC555CM CMOS oscillator/timers (U2 and U3), and a few support components. The transmitter sends a signal on receipt of a sound at MIC1.
Carrier-Current Baby-Alert Receiver Circuit
The baby-alert receiver is comprised of three transistors: Q2, which is configured as a high-gain linear amplifier; Q3, which serves as both an amplifier and detector; and Q4. which is essentially used as a switch; and a few additional components.
Digital Entry Lock Circuit
A block pinout diagram of the LS7220 keyless-lock IC is shown. The keypad must provide each key with a contact to a common connection. In this case, the common connection goes to the positive supply rail so that when a key is pressed, a positive voltage is passed through to the wire associated with that key. Each of the 12 keys are brought out...
Flash Signal Alarm Circuit
This circuit is useful if you need a low-energy flashing alarm. The 200 to 400-dc supply should have enough internal resistance to charge the 0.5 capacitor between flashes, about 2 or 3 time constants, which means about 500 kQ to 1 for a 1-s rate.
Speaker Protector Circuit
Most of the transistors in this speaker protector function as switches. Normally, Q4, Q5, and K1 are on and the speakers are connected to the amplifier. However, if a large dc voltage appears at an amplifier output, either Q3, or Ql and Q2 turn on, biasing Q4 off. That action turns Q5 off, de-energizes the relay, and disconnects the speakers...
Electronic Fuse Circuit
Basically, this circuit is an adjustable electronic circuit breaker, containing a toroidal transformer that senses 60-Hz load current. Tl has a two-turn winding for primary, and 100 turns of #30 gauge wire for the secondary. A high-low range switch selects 0.1 to 6 A or 1 to 12 A. The primary winding of Tl carries full load current and voltage;
Speaker Protection Circuit Circuit
A speaker system can be protected against amplifier failure when dc voltages (on speaker line a-b) arc sensed by the circuit. Either positive or negative dc voltages are sensed. A relay opens in this case, removing the dc from the speakers.
Ultrasonic distance finder circuit
The circuit described here uses ultrasonic oscillations and operates based on the propagation velocity of these oscillations in the air. Thus, we can easily determine the distance of two points if the time within which the wave travels this distance is measured. There are three main categories of distance measurement methods in use: a) By...
Simple Metal detector circuit with CD4030
Sometimes the precious metals are hidden too deep and are not detected except with complicated devices. In many cases, however, small pieces of precious metal buried near the surface can be detected by relatively simple means. Everyone is very attractive to find something potentially valuable, hidden a few centimeters below the soil. So many...
Fully automatic watering circuit for flower pots
Many times for various reasons we forget or can not water the plants that we have in our homes. And many humidity sensors units just notify us with a beeping sound or with a flashing light, that the pot needs watering. But what if we are away from home? This circuit comes as a solution to this problem. Depending on humidity of the pot at...
Simple deep searching metal detector circuit
The principle behind a metal detector is really very simple. Proof of this is the circuit that follows, in which it proves that the construction of a metal detector can be done in the minute, with few components that we find very easily everywhere. With the circuit of this metal detector, it is possible to detect a metal coin in depth of 90 to...
Sub-Harmonic Bass Converter for Electric Guitars
The sub-harmonic bass generator is a sound producing unit for guitars. The sound it produces looks very much like that of the bass guitar. The octave generator is sometimes quite unusual since it does not produce any of the usual ways of coloring the sound, such as filtering or distortion. Instead, a frequency division system produces outputs...
Echo Depth Sounding Sonar for Boats
In the past the sounding (sea bottom measurement) was done with the "bullet", that is, with a heavy lead object that the seamen plucked into the sea hung from a calibrated rope. As soon as the "bullet" reached the bottom, the depth appeared directly from the calibration of the rope. This arrangement still exists in some yachts. The big...
100W HiFi Audio Amplifier Circuit
This Amplifier was designed to have the following specifications: Distortion less than 0.1% at full power of 100W even at 20KHz. Power has to be attributed to an extended bandwidth. The output transistors must be protected against short circuits. The power supply must be symmetrical so that no electrolytic capacitors are needed at the outlet....
Plant-Pot Water Level Indicator Circuit
A series of LEDs serve to alert the gardener when plants need water. Using two conventional digital integrated LEDs and a series of LEDs, we make a very useful device for gardening. The device detects the amount of water in the pot and alerts the grower. The circuit of the device is shown in Figure 2. The water detection is made from the bottom...
Analogue Sound Preasure dB-Meter Circuit
The best human ear can capture sounds from 20Hz to 20KHz. These limits are known to be the broadest that can exist. Normally the normal limits range from 100Hz to about 13KHz and depend on the age and health of the individual. We for our measurements and to have a good accuracy we will get the limits of 20Hz to 20KHz. A sound to be heard does...
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