Here is an electronic code lock, which can be used as a door latch or key for ignition, etc. Operation is fairly tricky and there lies the beauty of the circuit. There are a number of switches, actually nine, which should be operated in a particular and owner – configured -sequence, or else the circuit will not drive the final SCR. The switches can of course be reconfigured as desired. SCR Output should be used with a suitable driver such as a solenoid for the door latch or a relay for the key.

The circuit is shown in Schematic 29. CD 4066 quad bilateral switch is made use of here. 12 V DC powers the circuit through SI. External switches S2—S9 are not connected in the same order as their number and that is part of the trick.

52 is a dummy switch, when pressed, LED D2 lights up only to fool the intruder. It is not connected to the rest of the circuit.

53 is the next switch. This operates internal switch 1 of CD 4066. When this switch is pushed, it pulls up trigger terminal (Pinl), and switch across 13 and 2 (SW1) is closed. It stays closed because of the feedback action of 3.3M resistance (Rl). Dl lights up indicating the closure of one switch in the sequence.

This powers the second internal switch (SW2) consisting of 5, 4, 3 pins. Power reaches Pin 5 and Pin 4 is the trigger terminal. When S5 switch is pushed on internal switch across 5 and 3 (SW2) closes. It charges CI capacitor 47uf through 100K resistance (R3). It can now feed the next switch as long as the capacitor can hold charge. CI is discharged through D3 and R5, which mean that next switch should be operated before this charge finishes.

To add to the confusion, the next switch is actually two switches in series comprising of S4 and S7 with trigger terminal at Pin 6. If they are pressed simultaneously, only if they are pressed simultaneously, internal switch across pins 8 and 9 (SW 3) closes. This charges 47uF capacitor (C2) through 100 k resistor (R6) which discharges through D4 and R7. Hence one has to press the next switch S8 before this charge is completed.

When S8 with trigger terminal at Pin 12 is operated in time, internal switch across pins 11 and 12 (SW4) closes.

SCR is fired now through R9. SCR drives a solenoid or a coil or any other drive mechanism of the lock. Final LED (D6) also lights up.

S9 is a blind switch only to fool the inadvertent user. S6 is another clever switch. This lights up LED D5 but also starts a piezo buzzer warning that somebody is fiddling with the lock. A 2200 uF capacitor charges and keeps the buzzer for some time. Use of capacitor is deliberate. It also makes the rogue user take a quick run.

Construction with CMOS IC is simple and straight. The trick here is to lay out the switches in a haphazard sequence, known only to the authorized user. Provision must also be made for easy change of code. With nine switches available, permutations are really many. Wiring must be carefully done to avoid false triggering.

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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. It is not as sophisticated as the ones the professionals use, but it works. It works by measuring skin resistance, which goes down when you lie.

Here are the details of the specific parts you will need

Part Total Qty. Description Substitutions

R1 1 33K 1/4W Resistor
R2 1 5K Pot
R3 1 1.5K 1/4W Resistor
C1 1 1uF 16V Electrolytic Capacitor
Q1 1 2N3565 NPN Transistor
M1 1 0-1 mA Analog Meter
MISC 1 Case, Wire, Electrodes (See Nots)

Notes
1. The electrodes can be alligator clips (although they can be painful), electrode pads (like the type they use in the hospital), or just wires and tape.

2. To use the circuit, attach the electrodes to the back of the subjects hand, about 1 inch apart. Then, adjust the meter for a reading of 0. Ask the questions. You know the subject is lying when the meter changes.

http://www.aaroncake.net/circuits/lie.htm

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Capacitors store a bit of electrical energy and come in many types and varieties. Capacity is measured in terms of Farads named after the great scientist Michael Faraday. As farad is too large a value for most general applications, microfarads (mfd) and nanofarads (nf) are normally used in hobby and consumer applications.

Ceramic, polyester, paper, mica, and electrolytic capacitors are some of the types. A detailed discussion is left out here. Each type has distinctive features and qualities and capacities. Capacitors are marked with voltages at the maximum voltage they can operate. It is a good practice to use a capacitor rated at twice the operating the voltage. Capacitors above the range ofl mfd generally are electrolytics and are used such in these circuits. Electrolytic capacitors are polarized and marked on the can. They should be connected as per the polarity. Electrolytic capacitors of 25 V rating are used in the following circuits unless otherwise specified. Capacitors rated at higher voltages can be used in any of the circuits. Capacitors with higher capacity should not be used unless the implications are understood. Other capacitors normally rated at 30V are used in the following circuits unless otherwise specified.

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Half wave rectification is simplest, as it requires only one transformer, one diode and one capacitor, but we have added a LED to give power supply indication. A simple half wave rectifier for 12V is shown below

500mA transformers are generally used for these circuits. Connect the primary wires to the mains chord after carefully insulating the joints. This transformer has three wires on the secondary side. AC voltage across both end wires is 12 and voltage across any one end wire and center wire is 6. So if both end wires are connected as shown in the present schematic the output DC voltage will be 12 and if any one of the end wires and center tap are connected the voltage will be 6.

Take a Veroboard and solder all components on it except mains power supply. Solder IN4003 diode to one end of secondary winding making note of the cathode. Solder lOOOuf /25V capacitor. Please note the capacitor is polarized, which means that you should connect it one way only. Negative side of the pin is marked on the can. Add a light emitting diode to know that the power supply is on. Now use a 1K-1A watt resistor in series with it.

LEDs are also polarized and marked. LEDs will not light up if they are connected in reverse. Take two pen light cells and connect the ends of a Red LED to positive and negative and then turn the leads over and try. (Forget the cathodes and anodes for the time being.) You will know that it works only one way. That also explains how a diode works. Do not connect them directly to 12 V  LEDs cannot work beyond 5 V and their current capability is extremely limited. Although they are very rugged devices, they must always be used with a current limiting resistor.

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