PC/CP200 Electronics Laboratory I
The total resistance of the potentiometer, R1 + R2, is constant. The arrow represents a slider or knob which will vary the values of R1 and R2 subject to the constraint that the total resistance does not change. Use an ohmmeter to study the resistance of the (unconnected) potentiometer. Correlate your observations with the diagram above. Determine the minimum and maximum values of R1, R2, and R1+R2.
Modify your circuit to add a "load" (e.g. 10 kOhms) to the center tap of the potentiometer. Repeat you measurements.
Demonstrate and explain your results to the lab instructor.
Potentiometers can be linear, i.e. R2 varies in proportion to the wiper's displacement. Potentiometers can also be logarithmic, i.e. logR2 varies in proportion to the wiper's displacement. The logarithmic potentiometers are popular in volume control circuits because the perception of loudness follows a logarithmic law.
Consider the following circuits:
In (a) the input to the buffer gate is directly connected to the switch. If the switch is closed, the input is connected to ground and the input is a stable low. However, when the switch is open, the input floats as it is not attached to any stable source. Although the input will tend to float high, any electrical noise could cause the input to go low. This is not a good circuit.
In (b) the input to the buffer is directly connected to the supply voltage, Vs at +5V, to ensure that the input will not float when the switch is open. However, when the switch is closed, there will be a direct connection between power and ground or, in other words, a short circuit. The circuit will not work. There will be too much current, too much heat, goodbye wire! This is not a good circuit.
In (c) the input to the buffer is connected to the supply voltage, Vs at +5V, through a resistor. This is called a pull-up resistor because it "pulls up" the input and ensures it does not float. When the switch is closed, the input to the buffer will be pulled low. Although current does flow from power to ground, it is severely limited by the resistor.
In a typical digital circuit, Vs is +5V and the pull-up resistor is 10k or 47k Ohms. Construct a pull-up resistor circuit for a switch. The circuit does not have to feed any gate, simply measure the voltage Vout at the point the gate would be connected. When the switch is open, what is the voltage Vout? When the switch is closed, what is the voltage Vout? Calculate the current when the switch is closed.
Points to ponder after the lab (include in post lab writeup)
LEDs are directional (have a positive and negative leg) and are forward biased because the current must flow from the positive to the negative. Directional components must be inserted in the correct orientation for the component to work. The symbol for the LED, a triangle with a bar at the negative end, may also include arrows going outward from the object to symbolize the light.
The circuit would be set up as in figure (a). The circuit in figure (b) will not work because there is no current. (Do you understand why there is no current in this circuit? Explain.)
Construct a test circuit to light up an LED. Assume the LED current rating is 10mA unless told otherwise by the lab supervisor. Show your calculations for determining the value of the current limiting resistor.
Demonstrate and explain your circuit to the lab instructor.
Before you leave the lab, have the lab instructor sign your lab notebook immediately after your last entry.