Overview

A LED flashing briefly once every 5 seconds to imitate the indicator of a real alarm is the function of this dummy alarm project with the use of a 7555 timer IC.

Explanation

To prolong battery life, the circuit was designed to use very little current in order to be left ON permanently. Optionally, an ON/OFF switch may be included if desired. The circuit uses a low power version of the standard 555 timer, the 7555 timer IC. A bright flash with a low current is provided by a super bright red LED. The average total current for the circuit is less than 0.2mA since the LED is OFF for most of the time. A set of 3 alkaline AA cells should last for several months or a year with this very low current.

A standard 555 timer IC can also be used in the circuit which will increase the average current to about 2mA while the battery life will be much shorter. If a greater power supply voltage at 15V maximum is used in this alarm, the 1K resistor for the LED should be increased to keep the LED current low around 3mA.

Overview

The circuit was constructed to provide a simple electronic lock system based on a single integrated circuit and will require a code of seven digits.

Terminology

4022 – a 4-stage CMOS counter with 8 decoded outputs used for binary counter/decoder, divide-by-N counting, frequency division, decade counter/decimal decode display, and counter control/timers due to its features such as standardized symmetrical output characteristics, medium speed operation, parametric ratings (5V, 10V, 15V), 100% tested for quiescent current at 20V, and fully static operation

BS170 – an N-channel enhancement mode field effect transistor designed to minimize on-state resistance while providing reliable, rugged, and fast switching performance and particularly suited for low voltage, low current application such as power MOSFET gate drivers and small servo motor control due to its features

of high saturation current capability, voltage controlled small signal switch, and high density cell design

BD679 – a monolithic NPN Silicon epibase power Darlington transistor with resistors and diode in a TO 126 plastic package and is typically used for AF applications due to its high current gain

Circuit Explanation

The construction of the circuit is relatively simple since it utilizes a very few components. One important thing to be considered in doing this circuit is the time it will take, after the push button switches are pressed, for the code to reach the main IC. There will be a delay unless all the keys were entered. When the right sequence of code was entered correctly, the output of Q7 will be activated for approximately 4 seconds. This will drive the transistor Q2 which in turn would drive one relay that will open the door or any other circuit attached.

The use of red LED D3 provides a visual indication of the activation of Q2. The code that has been set for the circuit is 1704570, as seen in the diagram. This was made possible by the arrangement of the resistors with their corresponding connections to the switches. The resistors are arranged from points A to G order while the switches are connected to the corresponding resistors in random order. The security code can be changed by altering the connections between the switches and the outputs of the IC1.

Part List

R1-7=4.7KohmR8=15KohmR9=1MohmR10-13=10KohmR11=100ohm

R12=220KohmR14=1.2KohmC1-3=100nF 100VC2=4.7uF 25VD1-2=1N4148

D3=RED LED 3mmIC1=4022Q1=BS170Q2=BD679S1-10=Push button or keyboard

Application

In most secured places with the capability of the company to purchase high end equipments, an electronic key operated door is usually installed. This will prohibit the entry of unauthorized persons or verify the entrance of a person by supplying the correct key code. This electronic security door key may also be found on vehicle doors for locking and unlocking purposes. Other establishments that utilize this are jewelry shops, banks, apartments, factories, hotels, prisons, apartments, and homes. They are very reliable since they are convenient to use and can be integrated easily with other circuitries.

Overview

The circuit was designed to provide a prototype of a metronome circuit utilizing the functions of CA741 operational amplifiers.

Terminology

Metronome – a device or apparatus used to indicate or establish the exact speed or tempo of music; physically, it resembles the function of a clock but using an inverted pendulum that regularly ticks or beats at an adjustable rate or intervals due to the position of the sliding weight on the pendulum

Op-Amp – a differential amplifier having a large voltage gain, very high input impedance and low output impedance

CA741 – a high gain operational amplifier, functioning as single or dual at 0.9 MHz frequency, used for commercial, industrial, and military applications

Circuit Explanation

The operation is of circuit can be considered as astable where it continuously oscillates to different states. To produce better sound effect, two op-amps were used wherein the left side op-amp will function as a standard astable while the right side op-amp as an integrator. Other components can also be modified like the C3, C4, R7, R8, and R9 to achieve the same effect on the sound.

The astable mode is being managed by the VR1, R2 and C1 in which timing pulses are produced. The output from the first op-amp is supplied through the capacitor C2 which has direct coupling with the zener diode ZD1 going to Q1. The conduction of ZD1 and Q1 occurs for every positive change on the output pulses.

Since the second op-amp functions as integrator, the output from the left side op-amp is distorted. It functions as low pass filter where it amplifies low frequency signals passing through it. Because of this, the circuit yields to a ticking reverberation state or sound where the output pulses are represented in ringing manner. To provide a low impedance input to the integrator, resistor R8 is used which produces a tock reverberation state or sound.

Application

Since CA741 op-amps have input bias current of 500 nA and input offset current of 200 nA, they are mainly used for applications such as narrow band or band pass filter, integrator or differentiator, summing amplifier, DC amplifier, multivibrator, and comparator.

Metronomes were mainly utilized by musicians for the purpose of sustaining a steady rhythm by improving the timing, by synchronizing with the beat, and by tracing the progress of learning using any musical equipment. They can be of different types such as software metronomes used as stand alone for multitracking of audio and sequencing of music, as electronic metronomes as compared to wristwatches having a constant accuracy, and as mechanical metronomes which controls the music tempo by means of an inverted pendulum rod.

Overview

The circuit was designed sensitive enough to trigger and activate the alarm upon sensing wetness, moisture or dampness on its probes.

Terminology

555 Timer – an 8-pin electronic device used in several mixtures of applications involving multivibration and timingOperating Modes:

BC109C – an epitaxial NPN silicon planar transistor having the collector electrically connected to the TO 18 metal case

Circuit Explanation

The operation of the circuit depends on the astable mode of the 555 timer, which is driven by the emitter current coming from BC109C transistor, since this transistor has high gain. Being in astable mode, the IC functions as an oscillator or as a free running mode. To have the 555 oscillator in full operation of sounding, it needs to be triggered or enabled by a larger current that flows in the collector circuit. This is made possible when the transistor is switched on by the small current that flows between the base and emitter because of the moisture or wetness sensed by the probes. On the other hand, the transistor will remain OFF and without any bias during dry conditions.

To control the conduction of the circuit, a switch for ON/OFF state is supplied. The probe contacts should be made of non-reactive metal so that there will be no capacitance or impedance present. It can be achieved by salvaging silver or gold plated contacts from old relays. A cheaper alternative is wiring alternate copper wires. Since the transistor is in emitter follower mode, the impedance at the emitter or the oscillator circuit acts as the current limit so placing a base resistor is no longer required.

When choosing the resistance rating of the loudspeaker, it is important to consider that an 8 ohm speaker can damage the 555 IC if directly driven by the IC with a 9 V supply because the maximum allowed output current is 200 mA. To avoid this, a 10 ohm resistor can be connected in series with the 8 ohm speaker or might as well use an 18 ohm loudspeaker.

Application

In the market, one of the most effective electronic devices is the 555 timer. When it operates at monostable mode, it may be utilized in pulse width modulation (PWM), capacitance measurement, frequency divider, touch switches, bounce free switches, missing pulse detection, and timers. When operating in astable mode, it may be utilized in pulse position modulation, security alarms, tone generation, logic clocks, pulse generation, lamp flashers, and light emitting diode. When operating in bistable mode, it may be utilized in bounce free latched switches, pulse-type waveform processing, and railroad track designing.

Led Flasher Circuit Using 555 IC

The flashing frequency is determined by C1, R1, and R2. You can try values around 100k for R1 and R2 to give around 1.4 s flashing period. Variable flashing rate can be implemented by replacing the R1 with 1Mega ohm potentiometer in series with 22k resistor. Lower power consumption can be achieved  if we  remove LED 2 and R4 and use only LED 1. When the R2 value is much lower than R1, then the LED 1 wil flash

only for fast period, gives a strobe efffect. I recommend 9V for this flasher circuit since higher voltage will cause exessive heat on 555 IC.

car, batteries, etc. It...

Description.

A lot of electronic circuits using NE555 timer IC are already published here and this is just another one.Here is the circuit diagram of a police siren based on NE55 timer IC. The circuit uses two NE555 timers ICs and each of them are wired as astable multivibrators.The circuit can be powered from anything between 6 to 15V DC and is fairly loud.By connecting an additional power amplifier at the output you can further increase the loudness.

IC1 is wired as a slow astable multivibrator operating at around 20Hz @ 50% duty cycle and IC2 is wired as fast astable multivibrator operating at around 600Hz.The output of first astable mutivibrator is connected to the control voltage input (pin5) of IC2. This makes the output of IC2 modulated by the output frequency of IC1, giving a siren effect. In simple words, the output frequency of IC2 is controlled by the output of IC1.