Circuits

• Buffered Bipolar Power Supply
• $10 Car Stereo Alarm

Theory

• RC Circuits & Filters
• Transistors & Diodes
• Digital Logic

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Buffered Bipolar Power Supply
The 10Kohm resistors form a voltage divider to split the 12V DC (supplied by a wall pack) in half. The 100uF capacitors act as a filter on the input to the two 741 opamps (or a dual opamp). These opamps are the buffering component of the supply. They allow up to about 30mA difference in current between the positive and negative supplies. The 1000uF capacitors are output filters to smooth the output voltage. The 7805 and 7905 voltage regulators are optional - they are used to regulate the output at +5V and -5V respectively. The circuit works fine without the regulators if you want a +/- 6V supply.

$10 Car Stereo Alarm
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This simple circuit draws little current (about 12mA) and is a nice little security device I cooked up after my truck was broken into and my CD player and CD's were stolen. Its operation is simple: As long as the trip wire is connected to ground, the TIP120 darlington pair (a special high-gain amplifer transitor circuit) is off and no current can flow into the DPDT relay. Whenever the trip wire grounding is broken, the darlington pair is turned on through the 1Kohm resistor allowing current to flow through the relay coil. The relay engages and is latched so that the circuit can not be turned off by regrounding the trip wire. The other relay switch honks the car's horn. When tripped, the horn will sound continuously until power is removed from the circuit. The trip wire is grounded through the stereo chassis, preferably as shown in the diagram because it requires the grounding to be broken to remove the stereo. This circuit could be modified to include a coded transmiter/reciever remote with the trip wire replaced by a door switch to make a rudimentary car alarm. If you're wondering, I got the name from the fact that the components cost about $10 total.

RC Circuits & Filters
The resistor-capacitor (RC) circuit is a very basic building block of analog systems. As its name implies, it is made up of a resistor and a capacitor in series. The output of a RC circuit is exponentially decaying or growing with a specific rate called the time constant. The time constant is equal to the product of the resistance and capacitance of the circuit. Two types of circuits can be formed: low pass and high pass filters. A low pass filter allows only frequencies lower than its critical frequency to pass through the circuit. It is constructed by taking the output to be the voltage across the capacitor (FIG1). A high pass filter allows only frequencies higher than its critical frequency to pass. It is canstructed by taking the output as the voltage across the resistor (FIG2).
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Transistors and Diodes
Transitors and diodes are the basic active components in circuits. They act as switches and control the flow of currents in the circuit. Diodes are quite simple devices created by the junction of N-type and P-type materials. These materials are specially doped semiconductors. N-type has impurities added (doping) to make it have loosely bound electrons in its crystal structure (FIG3a). P-type has impurities added which create gaps or electron "holes" in its crystal structure (FIG3b).Because of the structure of the materials, electrons can only move in one direction - from the P-type side to the N-type side of the diode (FIG4). This means that current can only flow in one direction across a diode. This property is very useful to control the flow of currents through a circuit.
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Transistors come in two main varieties - bipolar junction (BJT) and field-effect (FET). BJT's are quite similar to diodes in construction. In fact, a BJT is basically a double ended diode with a few differences. They are composed of three layers to form a sandwich. These three layers have special names: base, collector, and emitter (FIG5). As Figure 5 shows, there are two flavors of BJT: NPN or PNP. A NPN BJT has a P-type layer (base) between a two N-type layers (collector and emitter) and vice versa for the PNP BJT. The current in the BJT base controls the current flowing from the collector to the emitter. The BJT has three operating modes depending on this current: cutoff, active, saturation. When used as a switch, the BJT is in cutoff mode for the "off" position, and in saturation mode for the "on" position. The active mode is very useful because it is the basis of transistor amplifer circuits.
Field effect transistors (FET's) have a quite different structure from BJT's (FIG6). There are several different technologies used to create FET's, but the metal oxide semiconductor FET (MOSFET) is the most common. This FET is composed of a metal electrode, two N-type or P-type wells, a N or P-type body. The electrode is insulated from the body by a thin layer of oxide (usually silicon dioxide). The electrode is called the gate and the two wells are called the source and the drain. The FET's gate, source, and drain are analogous to the BJT's base, collector, and emitter. As with BJT's, MOSFET's come in two flavors: NMOS and PMOS. NMOS FET's have a P-type body and N-type source and drain wells. The current flowing from the drain to source is controlled by the voltage applied to the gate. This gate voltage creates a channel in the body by attracting (NMOS) or repelling (PMOS) electrons. This channel is formed right below the insulating oxide layer and as the gate voltage is varied, the channel's depth and shape change. The MOSFET also has three modes of operation: cutoff, saturations, and triode. When used as a switch, the MOSFET is in the cutoff mode for the "off" position and in the triode mode for the "on" position. The saturatuion mode is used in MOSFET amplifers. Perhaps the most common use of MOSFETS is in integrated circuits (IC's). Most IC's are CMOS, which stands for complimentary metal oxide semiconductor. These IC's use both NMOS and PMOS FET's because it results in very efficient and symmetric circuits (especially digital logic circuits).
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Digital Logic
Digital logic circuits are the basis of the computer revolution. These circuits implement Boolean logic, a form of logic using only two states or values. These two states are "on" and "off" (also called "high" and "low") and represented symbolically by "1" for on and "0" for off. In circuits these values are represented by specific voltages, usualy +5V for a 1 ("on") and 0V for a 0 ("off"). There are three basic gates (circuits) of which all digital logic is created:NOT, OR, and AND. The NOT gate is the simplest logic gate. It takes one input and outputs the opposite value. For example, if a 1 is input into a NOT gate, the gate will output a 0. If a 0 is input into a NOT gate, then a 1 is output. Symbol:
The OR gate takes two inputs and outputs a 1 if at least one of the inputs is a 1. The truth table summarizes the OR gate's function.	Input 1 Input 2 Output 0 0 0 0 1 1 1 0 1 1 1 1
Symbol:
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The AND gate takes two inputs and outputs a 1 if both of the inputs are a 1. The truth table summarizes the AND gate's function.	Input 1 Input 2 Output 0 0 0 0 1 0 1 0 0 1 1 1
Symbol:
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These three gates can be combined into composite gates such as NAND, NOR, and/or XOR. A NAND gate is simply a AND gate with a NOT gate attatched to its output. The NOR gate is a OR gate with a NOT gate on its output. The XOR gate is an exclusive OR gate: it outputs a 1 only if exactly one of the inputs is a 1. Another important digital logic circuit is the flip-flop. The flip-flop acts as a memory cell to store a value. Using flip-flops and the gates mentioned earlier, counters, registers, and other circuts can be built - all the way up to the most advanced computer.
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