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Electronic Circuits For Projects Kits
Touch Sensitive Alarm
This simple project can be used in a variety of applications. But the most practical application is for intruder detector. As a potential thief touches the doorknob on the other side of the door, the alarm will briefly raised to warn the user. The circuit uses schmitt trigger inverter IC1, together with a self -oscillating piezo-electric buzzer and handful of other components.
About the Circuit:
IC1 consists of six inverters, out of which five are used here. One of the inverter forms an oscillator with frequency around 1MHz. The frequency is determined by R1/C2. The oscillator output is given to second inverter, which in turn is AC coupled to a full wave rectifier circuit. As long as the door handle (or a metal plate ) is left untouched, the oscillator output will alternately switch the input above and below its two trigger voltages. Output will thus switch alternately low and high at clock rate to drive the rectifier circuit.
When metal plate is touched, most of the signal from the oscillator will be capacitively shunted to the shield ground, which forms a pseudo earth. As a result output of at pin6 becomes high, transistor T1 becomes on and buzzer sounds an alarm. It remains on as long as contact is made with the door handle. Also LED D4 lights, which gives visual indication. Simple delay network consisting of P1, C1 and D3 provides minimum delay when momentary contact is made with the touch plate. The delay is varied from 2 to 20 seconds with the aid of P1.
Mail Indicator With Warning
The project is specially designed for people, who make several futile trips to their mailboxes before their mail finally arrives. The idea is typically based on a sensor placed in the mailbox and the sensing circuit using IC40106 (IC1).
The sensor consists of an LDR i.e. Light Dependent Resistor. When the letter is dropped in the mailbox, light falling on LDR gets interrupted, its resistance rises and triggers the sensing circuit. The circuit will generate a noticeable alarm tone and simultaneously begins to flash two visual indicators.
About the Circuit:
The heart of the circuit is IC1, which consists of six schmitt trigger inverter gates. The sensor assembly consists of an LDR and a light source. The light falling on LDR keeps the output at pin2 is at logic low level. Diodes D1 and D2 remains conducting and so the red ‘NO’ mail indicator LED D5 remains lit and piezoalarm tone oscillator remains deactivated.
But the moment a letter or a packet is dropped in the mailbox, the red ‘NO’ mail steady indication ceases. In turn oscillator designed around one of the inverter using R2/C1 operating at a visually attractive frequency of about 2 Hz and the set of green LEDs D3 and D4 indicating the arrival of ‘MAIL’, begin to flash. Simultaneously as D2 ceases to conduct, two inverters whose inputs are pin5 and 9 and outputs are pin6 and 8 simultaneously, both of which are wired in parallel to provide current drive boost, begin to oscillate at about 2KHz.. This frequency is set by capacitor C2 and resistor R5.
The sensitivity of the circuit is set by VR1. Adjust VR1 such that the alarm tone just goes off in the normal ambient light conditions. Next try putting a letter/envelope and ensure that it does set off the alarm.
Liquid Level Indicator With Alarm
The name of the project itself suggests its application. It can be used for level control in hydroculture project. Also for kitchen purpose like detecting the level of water in washing machine. The main feature of this circuit is that it shows each level in meaningful English letters. It displays letter ‘E’ for Empty, ’L’ for Low, ’H’ for Half, ’A’ for Above Average and ‘F’ for Full Tank. The sensors for each level are immersed in tank. Their other ends get interconnected through four NOR gates N1 to N4 which uses IC 4001(IC1). The output is then given to IC 4055 (IC2) which is BCD to seven segment display driver IC. The level of fluid is indicated on 7- segment display. Also when the tank is full, the piezobuzzer raises a noticeable alarm.
About the Circuit:
As shown in the circuit diagram, when tank is low, N1 of IC1 gives output high, whereas N2, N3 are at logic zero. Their logics are applied to BCD inputs of IC2. Hence IC2 drives 7- segment display according to the BCD Code present at inputs. For low level BCD logic is ‘1010’. For this logic, display will indicate ‘L’. As the level increases, BCD logic changes, and display will indicate accordingly. That is ‘H’ for half and ‘A’ for above average. You can see the code generation table or logic table.Note that there is no display pattern like ‘E’ or ‘F’ available from IC2. Therefore to obtain these pattern transistors T1 and T2 are used. These transistors blank out the unnecessary segments from 7- segment display. It can be seen that the letter ’E’ is generated by blanking ‘b’ and ‘c’ segments while it decodes digit ‘8’. Letter ‘F’ is obtained by blanking segment ‘b’ while it decodes letter ‘P’.
In addition the piezobuzzer connected to transistor T3 raises alarm when tank is full. In this case, N4 of IC1 gives logic high and makes T3 to conduct.
Note:The circuit should not be used with inflammable or highly reactive fluids.
Earthquake Vibration Alarm
The Piezo electric sensor is used to sense any kind of vibration /jerk. It produces small ac signal when activated by a jerk or vibration. Since it is very sensitive to any kind of vibration it can serve as earthquake alarm also.
A piezo electrical sensor is available in market in the form of a round disc. It has two terminals for output. When we want to use it, it can be placed on the surface or can be adhered to the surface with the help of cello tape or a general purpose chemical adhesive.When the surface encounters any vibrations, these are transferred to this piezo sensor and it produces a signal.
About the circuit :
A simple circuit is built around a 555 timer IC. The output from sensor is fed to a transistor T1. Capacitor C1 and Resistor R1 act as fixed bias to the transistor T1.
T1 amplifies the signal and delivers amplified output at the collector. This out put is again fed to another transistor through ac coupling in order to further amplify the signal. Since T1 is weekly biased, it automatically rectifies the ac signal and only pulsating dc waveform is amplified and delivered by T2. Output of this T2 is fed to trigger input of IC555 through capacitor C3. Capacitor C3 differentiates the rising and falling edge of the signal produced at the collector of T2. The positive going pulse will not have any effect on trigger input. However negative going pulse will trigger the timer and produce HI state at its output. This HI state will glow LED D1. Also buzzer starts sounding when the sensor signal is detected. The 555 configuration is a monostable flipflop and the signal goes on retriggering this monostable. Therefore output is contuously available as long as signal is present. VR1 sets the sensitivity of the circuit and VR2 sets the timing of the monostable. If the trigger pulses which triggers the mono are generated at delayed intervals, the output may not be continuos. TO make it continuous at even the lowest frequency of signal, VR2 should be set.The circuit an be powered either from 9 v battery or from mains through eliminator.
Infrared Obstacle Detector
Obstacle detector is useful in many applications. One of the most demanding applications can be a tool for blind person. Latest technologies like Automatic (driverless automobiles, train anti-collision, electronic toys like car, train and robotic gadgets etc.)
The principle involved is infrared (IR) emitter and receiver. When IR emitter emits light and it becomes incident on an obstructing object, it gets reflected. This reflected light is picked up by a sensitive receiver and can be converted to suitable signal to alarm the user regarding the obstacle. Absence or reduction of received light decides the presence or absence of the obstacle.
Infrared actually is normal light with a particular colour. We humans can't see this colour because its wavelength of 950nm and is below the visible spectrum. That's one of the reasons why IR is chosen for sensing purposes in many applications. We want to use it but we're not interested in seeing it. It avoids interference of ambient light in sensing. Another reason is because IR LEDs are quite easy to make, and therefore can be very cheap.
Unfortunately for us there are many more sources of Infrared light. The sun is the brightest source of all, but there are many others, like: light bulbs, candles, central heating system, and even our body radiates Infra-Red light. In fact everything that radiates heat, also radiates Infrared light.
Therefore we have to take some precautions to guarantee that our IR message/signal gets across to the receiver without errors being caused by ambient sources around us.
Modulation is the answer to make our signal stand out above the noise. With modulation we make the IR light source blink in a particular frequency.
Another purpose this modulation solves is to make the circuit more sensitive and able to work with higher range i. e. to detect obstacles from more distance, the light emitter is modulated at about 38 KHz.
About the circuit :
The Emitter is fabricated from IR diodes D1, D2. These diodes are driven by IC2- 555, used as astable multivibrator running at about 38 KHz. The frequency can be tuned with VR1. The transistor T1 driven from 555 outputs, drives two IR LEDs connected in series. Two LEDs are used to improve upon the range and sensitivity of the circuit.
The receiver is an integrated circuit TSOP 1738. TSOP 1738 is the standard IR remote control receiver series, supporting all major transmission codes.
It has Photo detector and preamplifier in one package. Also it has internal filter for PCM(pulse code modulation) frequency. Its output is active low, and gives active low pulses when it receives IR signal from transmitter after reflection from the obstacle. Normally when there is no obstacle, the transmitter signal will not be received by receiver since both are arranged in parallel on the same end of a PCB. This output is connected to trigger input of IC1 which triggers when IR light is obstructed. IC1 is configured in mono-stable mode and generates a pulse of 1.1 R3xC3 time each time it is triggered. With no obstruction present, the 555 (IC1) output is LO.
The relay is driven by the pulse produced by IC1. Simultaneously the buzzer will also sound alarm for the duration of the pulse produced and LED D4 gives visual indication. Relay contacts can be used to actuate any other mechanism to suit an application.
Four Channel On Off Control
This four-channel control uses only one D type latch IC1, and some associated circuitry to control four different gadgets. To control more devices, identical circuits, in multiples, can be used. IC1 consists of eight latches, out of which four are used here.
The circuit incorporates the following features:
- Individual ‘on/off’ control for all channels.
- Emergency ‘off’ control for all channels.
- Immediate ‘on’ control for all channels
- LED indication for ‘on’ channels
- Optional push-to-on buttons or tectile switches.
Two Tank Auto On Off Controller
This circuit is specially designed for application such as hostels, apartments, hospitals etc where controlling the water tank is very essential. The specialty of this circuit is that it controls upper as well as lowers tanks. Motor becomes on when upper tank is empty and lower tank is at predetermined low level. Also it becomes off when water level in the upper tank reaches to high level or it is full. The sensors for lower and upper tank are shown in circuit diagram.
About the Circuit:
The circuit uses IC1, which consists of four AND gates. The Low and High sensors of upper tanks are connected to one of the inputs of two AND gates in IC1. The low- level sensor of lower tank is connected to one of the input of NAND gate in IC4 and IC3. IC2, which is wired as bistable multivibrator gets triggered according to the logic level present at the threshold input pin6 and trigger pin2. IC3, which is a four input AND gate gets triggered when all its input are logic high which in turn drives transistor T1. Also there is an audible warning when lower tank is below the low level.
The basic operation of the circuit is as follows: Suppose lower tank is at low level and upper tank is empty. In this case output of IC1 at pin4 is low. As a result IC2 gets triggered and gives logic high pulse at output pin3. As all the inputs of IC3 are high, it gives output high. Transistor T1 conducts and relay becomes on. LED D2 glows, which shows that motor is on. It remains on as long as water level in upper tank reaches to High level.
When upper tank reaches to High level, the output of IC1 at pin4 becomes high, IC2 gets reset and generate low pulse at pin3. As one of the input of IC3 receives logic low value, its output becomes low. T1 turns off and relay connected to the collector of T1 becomes off. Also LED D2 goes off and it remains off until upper tank becomes empty.
Also there is a provision for audible warning. When lower tank is below low level, the output of IC3 is low and motor is off as this water is not good for drinking. At the same time IC4 gives high output. As a result Transistor T2 conducts and buzzer connected at the collector terminal raises alarm which gives an indication that the lower tank is empty. Also LED D3 glows, which gives visual indication.
Fire alarm circuit is used to sense the fire as well as temperature. Here reverse bias Germanium (Ge) diode is used as a heat sensor. At temperature about 60°C and above, reverse resistance of Ge diode drops and makes pin4 (reset) of IC 555 at logic high level. Hence IC generates output pulse and raises the alarm. It has household as well as industrial applications.
Circuit operation: At room temperature reverse resistance of diode D1 (the sensor) being very high (over 10Kohms) it produces no effects on transistor T1 that conducts and keeps the reset pin4 of IC555 at ground level, so the alarm does not sound.
When temperature in the vicinity of diode D1 increases in case of a fire the reverse resistance drops to a value below 1KW. This stops T1 conduction and reset pin 4 becomes positive through resistor R2. Hence 555 give output pulses, which sounds the alarm. Alarm can be heard from 8W speaker.
For installation of alarm two or three reverse biased Ge diodes connected in parallel can be kept at different locations. In case of a fire, any of the diodes can sense the heat and raise the alarm. Here diode D2 is used as a rectifier diode. Capacitor C4 is used as a filter capacitor, C3 as a coupling capacitor.
Calibration : VR1 is a preset used for temperature sensitivity.