A potentiometer is a variable resistor that is used to control the amount of electrical current in a circuit. A common example is a volume control on a television. An analogue potentiometer has a knob or dial that varies the level of its resistance. A digital potentiometer is an Integrated circuit that has internal resistive "sections." Those sections are accessed and controlled by a digital signal of +5V or 0V. Often, pushbuttons are used to operate a digital potentiometer. Both types of potentiometers increase or decrease current by increasing or decreasing resistance. Replacing an analogue potentiometer with a digital requires several electrical connections. Building a prototype circuit is the best way to understand how the digital potentiometer functions as a replacement.
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Things you need
- AD5220 digital potentiometer (or equivalent)
- Electronics breadboard
- 7805 voltage regulator
- 7805 datasheet
- Jumper wires
- AD5220 datasheet
- 9-volt battery
- Two pushbuttons (two-pin type)
- Two 10K resistors
- 9-volt battery clip
- LED (general-purpose, low-voltage)
Insert the AD5220 digital potentiometer into the breadboard. The AD5220 should straddle the blank column that divides the rows into A-to-E and F-to-J sections.The dot on the surface of the AD5220 should be on top. The dot designates the location of Pin 1 of the AD5220.
Insert the 7805 voltage regulator into the breadboard so that each of the three pins are in separate rows. The 7805 will be vertical. Refer to the 7805 datasheet and note which pins are "Input," "Gnd" and "Output."
Connect the 7805 Input pin to a breadboard power bus. Connect the 7805 Gnd pin to a breadboard ground bus. Use jumper wires to make all connections. The power bus is a breadboard column labelled with a red "+" and the ground bus is a column labelled with a black or blue "-."
Locate the AD5220 pins labelled "VDD" and "GND." Refer to the AD5220 datasheet for the pin description. Connect the AD5220 VDD pin to the 7805 Output pin. Connect the AD5220 GND pin to the breadboard ground bus.
Insert the two pushbuttons into the breadboard. The two pins should straddle the blank divider column -- similar to the AD5220. Connect one pin of each pushbutton to the breadboard ground bus. Designate one pushbutton "PB1" and the other pushbutton "PB2."
Connect one leg/pin of a 10K resistor to the 7805 Output pin. Connect the other leg of the resistor to the open pin of PB1. In the same manner, connect the other 10K resistor to the 7805 and PB2.
Connect the AD5220 pin labelled "U/D" to the resistor-connected pin of a PB1. On the datasheet, the "D" has a line over it. This signifies that a 0V signal activates it. PB1 will control the Up/Down action of the potentiometer. This increases or decreases the resistance of the AD5220.
Connect the AD5220 pin labelled "CLK" to the resistor-connected pin of PB2. This provides the "Clock" signal that moves the potentiometer Wiper. This will control the amount of resistance that is increased or decreased in the AD5220.
Connect the AD5220 pin labelled "CS" to the breadboard ground bus. On the datasheet, "CS" has a line over it, signifying that it requires a 0V signal. Connecting it to ground keeps the potentiometer active, or "turned-on."
Connect the positive wire (red) of the 9-volt battery clip to the breadboard power bus. Connect the negative wire (black) of the clip to the breadboard ground bus.
Insert the LED into the breadboard with the cathode (shorter leg) connected to the breadboard ground bus.
Connect the AD5220 pin labelled "Wiper Terminal" to the anode (longer leg) of the LED. The Wiper is controlled by the CLK connection of PB2. The Wiper will move in an up or down direction, which is controlled by PB1.
Connect the AD5220 pin labelled "Terminal B1" to the breadboard ground bus. The B1 connection provides a reference point for the changing resistance. This connection is necessary for the potentiometer to function properly.
Connect the AD5220 pin labelled "Terminal A1" to the Output pin of the 7805. The 5V signal from the 7805 connects to A1. This provides the voltage and current which is needed to operate the LED. The AD5220 controls the amount of current that is passed through A1 and the Wiper.
Locate the same three terminals (A1, B1 and Wiper) on the 10K analogue potentiometer. The terminals may or may not be labelled on the analogue potentiometer. In general, the middle pin (pin 2) is the Wiper terminal. Note that the digital replacement should be the same resistance value as the analogue potentiometer.
Draw a diagram and label the terminal connections of the 10K analogue potentiometer. The A1, B1 and Wiper terminals of the AD5220 will be connected in the same manner.
Connect the 9-volt battery to the battery clip. When the AD5220 is turned on, the resistance is automatically set halfway. Therefore, the LED may be dim or may be off.
Press PB2 a few times. The LED should turn off completely. This is because the AD5220 U/D pin is automatically set to the Up position. It receives a steady 5-volt level from PB1. Pressing PB2 without pressing PB1 will increase the resistance of the AD5220 and will decrease current.
Press PB1 and keep it pressed.
Press PB2 repeatedly and the LED should turn on and become brighter. Pressing PB1 activates the Down position of U/D. It connects to ground, bringing that previous 5V level to 0V. Pressing PB2 while pressing PB1 causes resistance to decrease and current to increase. This test signifies that the digital potentiometer is functioning properly and the circuit is ready to replace the analogue potentiometer.
Tips and warnings
- Digital potentiometers with pushbutton interfaces usually have one button for going up and one for going down. The U/D pin can be wired so that it is controlled by a dedicated "down" pushbutton. Generally, Terminal A1 and Terminal B1 can be interchanged, as in a regular, fixed-value resistor. The most important connection is the Wiper terminal.
- Don't leave the LED on its brightest setting; it may become damaged from excessive current. Make sure the digital potentiometer circuit is properly grounded to the circuit where it replaces the analogue potentiometer. The digital potentiometer should be the same resistance level as the analogue. Replacing it with a higher or lower resistance value may damage the circuit or cause the circuit to malfunction.
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