Whats A Microcontroller v3

Measuring Rotation · Page 141

ACTIVITY #1: BUILDING AND TESTING THE POTENTIOMETER CIRCUIT

Placing different size resistors in series with an LED causes different amounts of current to flow through the circuit. Large resistance in the LED circuit causes small amounts of current to flow through the circuit, and the LED glows dimly. Small resistances in the LED circuit causes more current to flow through the circuit, and the LED glows more brightly. By connecting the W and A terminals of the potentiometer, in series with an LED circuit, you can use it to adjust the resistance in the circuit. This in turn adjusts the brightness of the LED. In this activity, you will use the potentiometer as a variable resistor and use it to change the brightness of the LED.

Dial Circuit Parts

(1) Potentiometer – 10 kΩ

(1) Resistor – 220 Ω (red-red-brown)

(1) LED – red

(1) Jumper wire

Building the Potentiometer Test Circuit

Figure 5-5 shows a circuit that can be used for adjusting the LED’s brightness with a potentiometer.

9 Build the circuit shown in Figure 5-5.

Tip: If you have trouble keeping the potentiometer seated in the breadboard sockets, check its legs. If each one has a small bend, use a needle-nose pliers to straighten them out and then try plugging the pot into the breadboard again. When the pot’s legs are straight, they may maintain better contact with the breadboard sockets.

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Figure 5-5

Potentiometer-LED

Test Circuit

Testing the Potentiometer Circuit

9Turn the potentiometer clockwise until it reaches its mechanical limit shown in Figure 5-6 (a).

Press the pot against the breadboard a little as you turn its knob. For these activities, the potentiometer needs to be firmly seated in the breadboard sockets. If you’re not careful when you turn the knob, the pot can become disconnected from the breadboard sockets, and that can lead to incorrect measurements. So, apply a little downward pressure as you turn the potentiometer’s knob to keep it seated in the breadboard.

Handle with care: If your potentiometer will not turn this far, do not try to force it. Just turn it until it reaches its mechanical limit; otherwise, it might break.

9Gradually rotate the potentiometer counterclockwise to the positions shown in Figure 5-6 (b), (c), (d), (e), and (f) noting the how brightly the LED glows at each position.

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positions.

How the Potentiometer Circuit Works

The total resistance in your test circuit is 220 Ω plus the resistance between the A and W terminals of the potentiometer. The resistance between the A and W terminals increases as the knob is adjusted further clockwise, which in turn reduces the current through the LED, making it dimmer.

ACTIVITY #2: MEASURING RESISTANCE BY MEASURING TIME

This activity introduces a new part called a capacitor. A capacitor behaves like a rechargeable battery that only holds its charge for short durations of time. This activity also introduces RC-time, which is an abbreviation for resistor-capacitor time. RC-time is a measurement of how long it takes for a capacitor to lose a certain amount of its stored charge as it supplies current to a resistor. By measuring the time it takes for the capacitor to discharge with different size resistors and capacitors, you will become more familiar with RC-time. In this activity, you will program the BASIC Stamp to charge a capacitor and then measure the time it takes the capacitor to discharge through a resistor.

Introducing the Capacitor

Figure 5-7 shows the schematic symbol and part drawing for the type of capacitor used in this activity. Capacitance value is measured in microfarads (µF), and the measurement is typically printed on the capacitors.

The cylindrical case of this particular capacitor is called a canister. This type of capacitor, called an electrolytic capacitor, must be handled carefully.

9 Read the CAUTION box on the next page.

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9 Repeat these steps for each resistor value in Table 5-1.

Table 5-1: Resistance and RC-time for C = 3300 μF

470

1k

2k 10 k

'What's a Microcontroller - PolledRcTimer.bs2

'Reaction timer program modified to track an RC-time voltage decay.

'{$STAMP BS2}

'{$PBASIC 2.5}

HIGH 7

DEBUG "Capacitor Charging...", CR

FOR counter = 5 TO 0

PAUSE 1000

DEBUG DEC2 counter, CR, CRSRUP

NEXT

DEBUG CR, CR, "Measure decay time now!", CR, CR

INPUT 7

DO

PAUSE 100

timeCounter = timeCounter + 1

DEBUG ? IN7

DEBUG DEC5 timeCounter, CR, CRSRUP, CRSRUP

LOOP UNTIL IN7 = 0

DEBUG CR, CR, CR, "The RC decay time was ", DEC timeCounter, CR,

"tenths of a second.", CR, CR

END

Page 148 · What’s a Microcontroller?

How PolledRcTimer.bs2 Works

Two variables are declared. The timeCounter variable is used to track how long it takes the capacitor to discharge through Ri. The counter variable is used to count down while the capacitor is charging.

The command DEBUG CLS clears the Debug Terminal so that it doesn’t get cluttered with successive measurements. HIGH 7 sets P7 high and starts charging the capacitor, then a “Capacitor charging…” message is displayed. After that, a FOR...NEXT loop counts down while the capacitor is charging. As the capacitor charges, the voltage across its terminals increases toward anywhere between 3.4 and 4.9 V (depending on the value of Ri).

DEBUG CLS

HIGH 7

DEBUG "Capacitor Charging...", CR

FOR counter = 5 TO 0

PAUSE 1000

DEBUG DEC2 counter, CR, CRSRUP

NEXT

A message announces when the decay starts getting polled.

DEBUG CR, CR, "Measure decay time now!", CR, CR

In order to let the capacitor discharge itself through the Ri resistor, the I/O pin is changed from HIGH to INPUT. As an input, the I/O pin, has no effect on the circuit, but it can sense high or low signals. As soon as the I/O pin releases the circuit, the capacitor discharges as it feeds current through the resistor. As the capacitor discharges, the voltage across its terminals gets lower and lower (decays).

INPUT 7

Back in the pushbutton chapter, you used the BASIC Stamp to detect a high or low signal using the variables IN3 and IN4. At that time, a high signal was considered Vdd, and a low signal was considered Vss. To the BASIC Stamp, actually a high signal is any voltage above about 1.4 V. Of course, it could be up to 5 V. Likewise, a low signal is

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anything between 1.4 V and 0 V. This DO...LOOP checks P7 every 100 ms until the value of IN7 changes from 1 to 0, which indicates that the capacitor voltage decayed to 1.4 V.

DO

PAUSE 100

timeCounter = timeCounter + 1

DEBUG ? IN7

DEBUG DEC5 timeCounter, CR, CRSRUP, CRSRUP

LOOP UNTIL IN7 = 0

The result is then displayed and the program ends.

DEBUG CR, CR, CR, "The RC decay time was ",

DEC timeCounter, CR,

"tenths of a second.", CR, CR

END

Your Turn – A Faster Circuit

By using a capacitor that has roughly 1/3 the capacity to hold charge, the time measurement for each resistor value that is used in the circuit will be reduced by 1/3. Later on in the next activity, you will use a capacitor that has 1/33,000 the capacity! The BASIC Stamp will still take the time measurements for you, using a command called

RCTIME.

9Disconnect power to your Board of Education or HomeWork Board.

9Replace the 3300 µF capacitor with a 1000 µF capacitor.

9Confirm that the polarity of your capacitor is correct. The negative terminal should be connected to Vss.

9Reconnect power.

9Repeat the steps in the Example Program: PolledRcTimer.bs2 section, and record your time measurements in Table 5-2.

9Compare your time measurements to the ones you took earlier in Table 5-1. How close are they to 1/3 the value of the measurements taken with the 3300 µF capacitor?