Difference between revisions of "Basic Circuit Building Blocks"

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== Filter Capacitor / Decoupling Capacitor / Low Pass Filter ==   
 
== Filter Capacitor / Decoupling Capacitor / Low Pass Filter ==   
  
== Pull Up and Switch ==  
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== Pull Up and Switch ==
 +
 
 +
Use this circuit when you want to feed a user input to a digital circuit, for example a PIC input pin.
 +
 
 +
A pull up is a fairly high value resistor ( say 1 to  100 K ohms ) that is connected to the positive side of the power supply.  This makes the other end of the resistor the same voltage as the power supply ( as long as it is connected to a high impedance.  The other end of the resistor is connected to a switch that is then connected to ground.  When the switch is connected current flow through the resistor dropps the entire power supply voltage and the input voltage for the circuit is now 0  (sometimes called active low, since when the switch is active the output is low).  Pull up is sometimes used without the switch to keep a signal high all the time.
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Circuit:
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[[Image:pus.png | Pull Up and Switch ]]
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Where
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*PUSH_BUTTON_SWITCH is a push button switch
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*R_LED is a current limiting resistor for the LE
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*R_PULLUP is the pull up resistor
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*VPLUS_VDD  is the power supply voltage
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More information: 
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[http://www.seattlerobotics.org/encoder/mar97/basics.html  Very Basic Circuits]
  
 
== Pull Down and Switch ==  
 
== Pull Down and Switch ==  

Revision as of 17:38, 15 January 2008

These are circuits and parts of circuits that we see over and over again in larger projects. Understanding a complex circuit is much easier if you understand these building blocks.

Series Circuit

Parallel Circuit

Current Sense Resistor ( Shunt Resistance )

A current Sense Resistor is a low value of resistor that is placed in series with some other circuit. We can then measure the voltage across the resistor to compute the current. If the resistor has a low value compared to other components we can ignore the effect on the circuit. We use the word shunt when the voltage is measured by a device that has a fairly low resistance itself. We then have to do a more careful calculation of how the current is shared by the two devices.

More information:

  1. Scienceshareware.com's How A Precision Resistor Is Used to Measure / Calculate Current and Power in an Electrical Circuit.
  2. High-Side Current-Sense Measurement: Circuits and Principles
  3. Shunt (electrical) From Wikipedia, the free encyclopedia

Filter Capacitor / Decoupling Capacitor / Low Pass Filter

Pull Up and Switch

Use this circuit when you want to feed a user input to a digital circuit, for example a PIC input pin.

A pull up is a fairly high value resistor ( say 1 to 100 K ohms ) that is connected to the positive side of the power supply. This makes the other end of the resistor the same voltage as the power supply ( as long as it is connected to a high impedance. The other end of the resistor is connected to a switch that is then connected to ground. When the switch is connected current flow through the resistor dropps the entire power supply voltage and the input voltage for the circuit is now 0 (sometimes called active low, since when the switch is active the output is low). Pull up is sometimes used without the switch to keep a signal high all the time.

Circuit:

Pull Up and Switch

Where

  • PUSH_BUTTON_SWITCH is a push button switch
  • R_LED is a current limiting resistor for the LE
  • R_PULLUP is the pull up resistor
  • VPLUS_VDD is the power supply voltage

More information: Very Basic Circuits

Pull Down and Switch

Op amp Non Inverting Amplifier

Op amp Unity Gain Buffer

Light Emitting Diode ( with current limiting resistor )

Three Terminal Regulator

Transistor Low Side Switch

Use this circuit when you wish to turn a load on and off with both a low voltage and a low current. Note that neither side of the load is grounded.

A low side switch is one which switches a circuit on and off at the ground or low side of the circuit. The advantage of a low side switch is that when using a transistor as the switch the voltage to drive the transistor is itself a low voltage. It is often the easy way to drive leds motors and other high current devices from such low power devices as PIC output ports. Low side switche are popular and there are many integrated circuits for them as well as this circuit.

Circuit:

Transistor Low Side Switch

Where

  • LED is a low power LED
  • R_LED is a current limiting resistor for the LED
  • Q is a bipolar transistor
  • R_1 is a current limiting resistor transistor base current
  • VPLUS_VDD is the power supply for the LED


The voltage at the collector of the circuit should fall to a fraction of a volt when the input is high. To acomplish this:

  • Compute the value of R_LED using ohms law and the specifications for the LED.
  • Compute the current through the LED.
  • The transistor must supply the current, it should be equal approxtely to the input voltage divided by R1 times the beta of the transistor.

An example calculation would be nice, and will appear later.

Transistor Emitter Follower

Voltage Divider

Light Emitting Diode ( with current limiting resistor )

Push Pull Circuit ( with transistor emitter follower )

Darlington connected Transistors

Schmitt Trigger