Friday, December 27, 2013

Intelligent Electronic Lock

This intelligent electronic lock circuit is built using transistors only. To open this electronic lock, one has to press tactile switches S1 through S4 sequentially. For deception you may annotate these switches with different numbers on the control panel/keypad. For example, if you want to use ten switches on the keypad marked ‘0’ through ‘9’, use any four arbitrary numbers out of these for switches S1 through S4, and the remaining six numbers may be annotated on the leftover six switches, which may be wired in parallel to disable switch S6 (shown in the figure).

Intelligent Electronic Lock circuit diagram

When four password digits in ‘0’ through ‘9’ are mixed with the remaining six digits connected across disable switch terminals, energisation of relay RL1 by unauthorised person is prevented. For authorized persons, a 4-digit password number is easy to remember. To energise relay RL1, one has to press switches S1 through S4 sequentially within six seconds, making sure that each of the switch is kept depressed for a duration of 0.75 second to 1.25 seconds.

The relay will not operate if ‘on’ time duration of each tactile switch (S1 through S4) is less than 0.75 second or more than 1.25 seconds. This would amount to rejection of the code. A special feature of this circuit is that pressing of any switch wired across disable switch (S6) will lead to disabling of the whole electronic lock circuit for about one minute. Even if one enters the correct 4-digit password number within one minute after a ‘disable’ operation, relay RL1 won’t get energized.

So if any unauthorized person keeps trying different permutations of numbers in quick successions for energization of relay RL1, he is not likely to succeed. To that extent, this electronic lock circuit is fool-proof. This electronic lock circuit comprises disabling, sequential switching, and relay latch-up sections. The disabling section comprises zener diode ZD5 and transistors T1 and T2. Its function is to cut off positive supply to sequential switching and relay latch-up sections for one minute when disable switch S6 (or any other switch shunted across its terminal) is momentarily pressed.

During idle state, capacitor C1 is in discharged condition and the voltage across it is less than 4.7 volts. Thus zener diode ZD5 and transistor T1 are in non-conduction state. As a result, the collector voltage of transistor T1 is sufficiently high to forward bias transistor T2. Consequently, +12V is extended to sequential switching and relay latch-up sections. When disable switch is momentarily depressed, capacitor C1 charges up through resistor R1 and the voltage available across C1 becomes greater than 4.7 volts.

Thus zener diode ZD5 and transistor T1 start conducting and the collector voltage of transistor T1 is pulled low. As a result, transistor T2 stops conducting and thus cuts off positive supply voltage to sequential switching and relay latch-up sections. Thereafter, capacitor C1 starts discharging slowly through zener diode D1 and transistor T1. It takes approximately one minute to discharge to a sufficiently low level to cut-off transistor T1, and switch on transistor T2, for resuming supply to sequential switching and relay latch-up sections; and until then the circuit does not accept any code.

The sequential switching section comprises transistors T3 through T5, zener diodes ZD1 through ZD3, tactile switches S1 through S4, and timing capacitors C2 through C4. In this three-stage electronic switch, the three transistors are connected in series to extend positive voltage available at the emitter of transistor T2 to the relay latch-up circuit for energising relay RL1. When tactile switches S1 through S3 are activated, timing capacitors C2, C3, and C4 are charged through resistors R3, R5, and R7, respectively.

Timing capacitor C2 is discharged through resistor R4, zener diode ZD1, and transistor T3; timing capacitor C3 through resistor R6, zener diode ZD2, and transistor T4; and timing capacitor C4 through zener diode ZD3 and transistor T5 only. The individual timing capacitors are chosen in such a way that the time taken to discharge capacitor C2 below 4.7 volts is 6 seconds, 3 seconds for C3, and 1.5 seconds for C4. Thus while activating tactile switches S1 through S3 sequentially, transistor T3 will be in conduction for 6 seconds, transistor T4 for 3 seconds, and transistor T5 for 1.5 seconds.

The positive voltage from the emitter of transistor T2 is extended to tactile switch S4 only for 1.5 seconds. Thus one has to activate S4 tactile switch within 1.5 seconds to energise relay RL1. The minimum time required to keep switch S4 depressed is around 1 second. For sequential switching transistors T3 through T5, the minimum time for which the corresponding switches (S1 through S3) are to be kept depressed is 0.75 seconds to 1.25 seconds.

If one operates these switches for less than 0.75 seconds, timing capacitors C2 through C4 may not get charged sufficiently. As a consequence, these capacitors will discharge earlier and any one of transistors T3 through T5 may fail to conduct before activating tactile switch S4. Thus sequential switching of the three transistors will not be achieved and hence it will not be possible to energise relay RL1 in such a situation. A similar situation arises if one keeps each of the mentioned tactile switches de-pressed for more than 1.5 seconds.

When the total time taken to activate switches S1 through S4 is greater than six seconds, transistor T3 stops conducting due to time lapse. Sequential switching is thus not achieved and it is not possible to energise relay RL1. The latch-up relay circuit is built around transistors T6 through T8, zener diode ZD4, and capacitor C5. In idle state, with relay RL1 in de-energised condition, capacitor C5 is in discharged condition and zener diode ZD4 and transistors T7, T8, and T6 in non-conduction state.

However, on correct operation of sequential switches S1 through S4, capacitor C5 is charged through resistor R9 and the voltage across it rises above 4.7 volts. Now zener diode ZD4 as well as transistors T7, T8, and T6 start conducting and relay RL1 is energised. Due to conduction of transistor T6, capacitor C5 remains in charged condition and the relay is in continuously energised condition. Now if you activate reset switch S5 momentarily, capacitor C5 is immediately discharged through resistor R8 and the voltage across it falls below 4.7 volts. Thus zener diode ZD4 and transistors T7, T8, and T6 stop conducting again and relay RL1 de-energises.
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Wednesday, December 25, 2013

Simple Microprocessor power supply watchdog circuit Diagram

The Simple Microprocessor power supply watchdog circuit Diagram monitors the input to the microprocessor 5 V regulated supply for voltage drops and initiates a reset sequence before supply regulation is lost. In operation, the resistor capacitor combination Rs and Cj form a short time constant smoothing network for the output of the fullwave bridge rectifier. 

An approximately triangular, voltage waveform appears across C and Rs and it is the minimum excursion of this that initiates the reset. Diode Dg prevents charge sharing between capacitors Cj and Ck. Resistors Rn and Rm form a feedback network around the voltage reference section of the LM10C, setting a threshold voltage of 3.4 volts. 

 Microprocessor power supply watchdog circuit Diagram

Simple Microprocessor power supply watchdog circuit Diagram

The threshold voltage is set at 90% of the minimum voltage of the triangular waveform. When the triangular wave trough, at the comparators non-inverting input, dips below the threshold, the comparator output is driven low. This presents a reset to the microprocessor. Capacitor Ch is charged slowly through resistor Rk and discharged rapidly through diode De.
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Monday, December 23, 2013

Build a Remotely Adjustable Solid State High voltage Supply Circuit Diagram

How to build a remotely adjustable solid state high-voltage supply Circuit Diagram. The output voltage changes approximately linearly up to 20 KV as the input voltage is varied from 0 to 5 V. The oscillator is tuned by a 5-0 potentiometer to peak the output voltage at the frequency of maximum transformer response between 45 and 55 kHz. 

The feedback voltage is applied through a 100-KO resistor, an op amp, and a comparator to a high-voltage amplifier. A diode and varistors on the primary side of the transformer protect the output transistor. The transformer is a flyback-type used in color-television sets. A feedback loop balances between the high-voltage output and the low-voltage input.

Remotely Adjustable Solid State High-voltage Supply Circuit Diagram

Remotely Adjustable Solid State High-voltage Supply Circuit Diagram
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Saturday, December 21, 2013

Battery Powered High voltage Generator Circuit Diagram

This is the battery powered high-voltage generator circuit diagram. Output voltage great enough to jump a l-inch gap can be obtained from a 12-V power source. A 555 timer IC is connected as an stable multi vibrator that produces a narrow negative pulse at pin 3. The pulse turns Ql on for the duration of the time period. The collector of Ql is direct-coupled to tbe base of tbe power transistor Q2, turning it on during the same time period. 

The emitter of Q2 is direct -coupled through current limiting resistor R5 to the base of the power transistor. Q3 switches on, producing a minimum resistance between the collector and emitter. The high-current pulse going through tbe primary of high-voltage transformer Tl generates a very high pulse voltage at its secondary output terminal (labeled X). The pulse frequency is determined by tbe values of Rl, R2, and C2. The values given in the parts list were chosen to give the best possible performance when an auto-ignition coil is used for Tl. 

Battery Powered High-voltage Generator Circuit Diagram

Battery Powered High-voltage Generator Circuit Diagram
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Thursday, December 19, 2013

Video Tracer Circuit Diagram

This circuit was designed as an aid to installers and maintainers of video systems. It is basically a video sync separator (IC1) followed by a LED and buzzer driver (IC2, Q1 & Q2). In use, the device is connected to a video cable and if there is video present, the LED will flash at about 10Hz. If there is no video, the LED flashes briefly every couple of seconds. A buzzer can also be switched in to provide an audible indication. The buzzer is particularly useful when tracing cabling faults or trying to find a correct cable amongst many, where it is difficult to keep an eye on the LED.

Another use for the buzzer option is to provide a video fault indication. For example, it could be inserted in bridging mode, with switch S1 in high impedance mode (position 2) across a video line and set to alarm when there is no video present. If someone pulls out a cable or the video source is powered off, the alarm would sound. IC1 is a standard LM1881 video sync separator circuit and 75Ω termination can be switched in or out with switch S1a. The other pole of the switch, S1b, turns on the power. The composite sync output at pin 1 is low with no video input and it pulses high when composite sync is detected.

Video Tracer Circuit diagram:


These pulses charge a 100nF capacitor via diode D1. When there is no video at the input, oscillator IC2b is enabled and provides a short pulse every couple of seconds to flash the LED. The duty cycle is altered by including D2, so that the discharge time for the 10μF capacitor is much shorter than the charge time. The short LED pulse is used as a power-on indicator drawing minimal average current. When video is present at the input, IC2b is disabled and IC2d is enabled. The output of IC2d provides a 10Hz square wave signal to flash the LED. The buzzer is controlled by switch S2. In position 2 the buzzer will sound when there is video at the input and in position 1 the buzzer will sound when there is no video at the input.

Source :
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Tuesday, December 17, 2013


     Here is a simple alarm circuit to thwart snatching of your valuables while travelling. The circuit kept in your bag or suitcase sounds a loud alarm, simulating a p attempts to snatch your bag or suitcase. This will draw the attention of other  passengers and the burglar can be caught red handed.

     In the standby mode, the circuit is locked by a plug and socket arrangement (a mono plug with shorted leads plugged into the mono-jack socket of the unit). When the burglar tries to snatch the bag, the plug detaches from the unit’s socket to activate the alarm.

     The circuit is designed around op-amp IC CA3140 (IC1), which is configured as a comparator. The non-inverting input (pin 3) of IC1 is kept at half the supply voltage (around 4.5V) by the potential divider comprising resistors R2 and R3 of 100 kilo-ohms each. The inverting input (pin 2) of IC1 is kept low through the shorted plug at the socket. As a result,olice horn, if someone the voltage at the non-inverting input is higher than at the inverting input and the output of IC1 is high.

     The output from pin 6 of IC1 is fed to trigger pin 2 of IC NE555 (IC2) via coupling capacitor C1 (0.0047 μF). IC2 is configured as a monostable. Its trigger pin 2 is held high by resistor R4 (10 kilo-ohms). Normally, the output of IC2 remains low and the alarm is off. Resistor R6, along with capacitor C3 connected to reset pin 4 of IC2, prevents any false triggering. Resistor R5 (10 mega-ohms), preset VR (10 megaohms) and capacitor C2 (4.7 μF, 16V) are timing components. With these values, the output at pin 3 of IC2 is about one minute, which can be increased by increasing either the value of capacitor C2 or preset VR.

     When there is an attempt at snatching, the plug connected to the circuit detaches. At that moment, the voltage at the inverting input of IC1 exceeds the voltage at the non-inverting input and subsequently its output goes low. This sends a low pulse to trigger pin 2 of IC2 to make its output pin 3 high. Consequently, the alarm circuit built around IC UM3561 (IC3) gets the supply voltage at its pin 5.

     IC UM3561 is a complex ROM with an inbuilt oscillator. Resistor R8 forms the oscillator component. Its output is fed to the base of single-stage transistor amplifier BD139 (T1) through resistor R9 (1 kilo-ohm)

      The alarm tone generated from IC3 is amplified by transistor T1. A loudspeaker is connected to the collector of T1 to produce the alarm. The alarm can be put off if the plug is inserted into the socket again. Transistor T1 requires a heat-sink.

     Resistor R7 (330 ohms) limits the current to IC3 and zener diode ZD1 limits the supply voltage to IC3 to a safe level of 3.3 volts. Resistor R9 limits the current to the base of T1.

     The circuit can be easily constructed on a vero board or general-purpose PCB. Use a small case for housing the circuit and 9V battery. The speaker should be small so as to make the gadget handy. Connect a thin plastic wire to the plug and secure it in your hand or tie up somewhere else so that when the bag is pulled, the plug detaches from the socket easily.
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