#### Introduction to Potentiometers (variable resistors, or trimmers)

Potentiometers are variable resistors. The resistance changes when the wiper is adjusted. The variable resistance is measured from one of the outside leads to the center (wiper) lead. The small variable resistors are generally called trimmers.

#### Potentiometers (variable resistors, trimmers or trimpots) Explained and Schematic Symbol

The potentiometer is a device that increases or decreases resistance as a variable resistor. Potentiometers can also be used as a voltage divider. If the potentiometer is to be used as a variable resistor, the potentiometer is only wired using two of the leads, the wiper (usually the middle lead) and one of the other leads (depending on which direction you would like the resistance to increase or decrease).

As a voltage divider, the two outer leads are aired to each pole and the center lead would be the output of the new divided voltage.

#### On Paper: Intro to Potentiometers and Schematic Symbol

The potentiometer generally have three leads, two outer leads that hold the total resistance, and the middle lead as the wiper.

#### On Paper: Understanding Voltage Dividing using Resistors or a Potentiometer

A voltage divider is a way to decrease a voltage to a proportion of the circuit voltage using two resistors or a potentiometer. The proportion of two resistors will create that proportion output of voltage between the resistors (or the wiper in the case of a potentiometer).

#### Result: ADC (Analog to Digital Conversion) using a Potentiometer as the ADC Input

A potentiometer is connected to the ADC using voltage divider connection to vary the voltage to the ADC pin. The output of the voltage to digital number is displayed on the LCD.

The center lead (Vout) of the potentiometer is connected to the ADC pin 0. The potentiometer's outer leads are connected to ground (GND) and 5V (VCC). These connections create the voltage divider. Optionally you can used resistors rather than wires for the two outer lead connections to minimize the possibility of a short where the resistance goes very low across the center lead to one of the outer leads.

The ADC needs to be powered. The ADC has its own power pins for AVCC and GND. the AVCC is connected directly to VCC (the 5V rail) and the GND is simply connected to GND on the - rail. Across these two power pins should reside a 100nf (nanofarad) or .1uf (microfarad) capacitor just like on the main power pins.

Another important pin for ADC is the voltage reference pin. This pin will receive the top voltage in our range of voltages we need to consider in the ADC input. Say, for instance, you don't want the 5v to be your voltage reference, because your device only has a range of 0v to 3.3v that will be delivered to the ADC. The top voltage in this range, 3.3v, should be connected to the ADC voltage reference Vref pin. If you had 5v connected to this Vref pin, but the device only gave you 0v to 3.3v, then your precision will be reduced.

The Vref pin can be set in programming, which is the case in this video clip.

#### Programming and Result: ACD (Analog to Digital Converter) Using Interrupts

The ADC (Analog to Digital Converter) converts an analog voltage between a range of voltages and provides an 8 to 10 bit number in proportion to the voltage sensed by the ADC.

In this video, the ADC automatically notifies of it's conversion complete using an interrupt. A potentiometer is used as a voltage divider to provide variable voltages to the ADC pin.

#### Introduction to Servos and Servomechanisms

Servos are motors that use feedback to control the motor's position. This type of control is called closed loop control. Servos use a special electromechanical device to provide this feedback. This electromechanical device can be a potentiometer or encoder. Encoders come in various shapes, sizes and resolutions.

Encoders provide a pulse (usually caused by an LED and sensor) that determines the position of the shaft of the motor. When the controller counts to the commanded number of pulses, the motor is stopped, or put into reverse if the motor over ran the position. The motor and controller will constantly provide torque in the direction that is needed to keep the position and is always correcting.

If a potentiometer is used, the motor shaft is connected to the potentiometer in some way (hobby servos connect the potentiometer with gears).

#### Hitec HS-322HD Disassembly (teardown)

The Hitec HS-322HD is a hobby servo. This servo uses a potentiometer to determine the position of the horn. The potentiometer is connected directly to the rotation of the horn. The horn and potentiometer is connected to the motor shaft by a series of gears giving the motor a very high mechanical advantage.

The potentiometer is connected to the motor via a circuit that is used to control the motor.

This disassembly is not advised. The motor housing is glued to the inside housing of the servo at a location that is very difficult to reach.

#### Controlling Motor Speed with a Potentiometer

Using a potentiometer, the speed of a motor can be controlled. The potentiometer is connected in a way that it functions as a voltage divider. The motor is connected to the potentiometer's middle lead and GND and the potentiometer is connected to the battery using its outer leads.

#### Controlling LED Dimming with a Potentiometer

Using a potentiometer, the dimming of an LED can be controlled. The potentiometer is connected in a way that it functions as a voltage divider. The LED and a resistor in series with the LED to limit the current to the LED is connected to the potentiometer's middle lead and GND and the potentiometer is connected to the battery using its outer leads.

#### A General Description of a System of Parts for the Constant Vacuum Source

The constant vacuum source is a system that maintains a vacuum within a tank. The vacuum is created by a motor and pump (vacuum pump) that is connected to the tank. The vacuum pump is actuated by a microcontroller and relay that uses a pressure sensor to sense the amount of vacuum in the tank. When the tank looses vacuum of 15 inches of Hg (Mercury) or more, then the motor will turn on and will climb to 25 or so inches of Hg.

The value of the low andn high thresholds for inches of Hg that is desired inthe tank and the amount of vacuum in the tank is shown on an LCD. Two potentiometers are used to adjust the low and high vacuum thresholds.

#### Breadboarding: Connecting an LCD Display to the Microcontroller Using a DIY Interface for the LCD

An interface (breakout) was created to make the connection from the LCD to the microcontroller easier. The DIY interface contains a trimmer potentiometer to adjust the contrast of the LCD. The interface has ribbon cable soldered and connected to the data lines of the LCD. The power, RS, R/W and Enable share another cable assembly that go to another part of the microcontroller.

This reduces the amount of space that the LCD uses on the breadboard.

In this case, the LCD data lines are connected to the PORT B of the AVR microcontroller. The E (enable), RW (Read Write) and RS (Register Select) are all plugged into PORT D. The power is plugged into the power rail.

Trimmer Potentiometers are small potentiometers intended only to be adjusted on the circuit and these are generally not able to be accessed. For example, a radio has a volume control that my be a standard potentiometer, but if you break open the case and pull out the circuit board, there may be small potentiometers (trimmers) on the board that cam be adjusted with a screwdriver.

The two trimmers are connected to the ADC pins 1 and 2 to adjust a low and high threshold.

#### Using the Slope Intercept Formula to Adjust the ADC Value for Trimmer Potentiometers Showing on the LCD

The trimmer potentiometers are used to adjust the high and low thresholds for the pressure sensor circuit. The adjustment needs to read between 0 and 33 rather than the ADC result of 0 and 1023, so the slope intercept formula is a good formula to use in this case.

#### Programming: Applying the Slope Intercept Formula to Adjust the ADC Value for Trimmer Potentiometers Showing on the LCD

The code in the ADC channels are updated to reflect the slope intercept formula so the trimmer potentiometers show a range or 0 - 33 rather than the raw ADC result of 0 - 1023.

#### Result: Applying the Slope Intercept Formula to Adjust the ADC Value for Trimmer Potentiometers Showing on the LCD

The code in the ADC channels are updated to reflect the slope intercept formula so the trimmer potentiometers show a range or 0 - 33 rather than the raw ADC result of 0 - 1023.