| 1. |
Wait! Let the circuit sit for at least 2 minutes before you take
this measurement. This is because even when the LED is off, the capacitor
is still draining, and will continue to do so until it reaches 0.0 volts.
The "AT REST" voltage at Test Point 3 (TP3) is ideally 0.0 volts. |
|
2.
|
You have to keep the plunger down so that pin 11 provides a steady
High (V+) output. Your measure of voltage should be nearly
full voltage. |
|
3.
|
Specifically, the diode will use .8 volts. So if your
power supply is providing 9.5 volts, your reading will be 8.7 volts.
9.5 - .8 = 8.7
This goes way back to Lesson #3. The regular diode always uses about
.8 volts. |
|
4.
|
Have the DMM red probe touching TP3 when you release the plunger.
It will begin draining immediately. |
|
5.
|
The output LED should remain on for 8 to 10 seconds after you release
the plunger. |
|
6.
|
Watch carefully. The inputs to the second gate (TP3) will sense
the change from a High to a Low input as the capacitor's draining voltage
crosses the "half of V+" mark. |
|
7.
|
This may vary depending on a number of factors. Resistor-Capacitor
circuits are analog at heart. The timing does not always draw a perfectly
straight line. There is a decrease in the rate of draining as the
voltage approaches zero. |
|
8 & 9
|
The capacitor is similar to a sink that gets filled with water.
The resistor acts like a drain. |
|
10.
|
Consider that you originally had a 10MW
resistro acting as a drain. By doubling the resistance to 20MW
you have effectively decreased the size of the drain by 1/2 (half).
Now think of your kitchen sink. You decrease the size of the
hole in the drain. The size of the sink is the same.
Logically then, it follows that the capacitor will hold the same amount
but drains at half the speed as before. |
|
11.
|
Think about it!! |
|
12.
|
Think here too. There should be a pattern emerging. |
|
13.
|
Make sure you changed R2 back to 10MW and
changed the capacitor to 10 mF.
If your original LED's output was timed at 10 seconds, it should be
almost exactly 100 seconds now.
.Most capacitors have a tolerence (range of accuracy) of plus or minus
10%. So a 10 uF Capacitor might be as small as 9 mF
or as big as 11 mF.
A sink 10 times larger will naturally take 10 times longer to drain. |
|
14.
|
Why? There are a few reasons why I emphasize this is a rough
formula.
a. Because the inputs here are analog.
b. Because the diode between TP2 and TP3 effects
the voltage provided to the RC timer.
But all in all, the pattern is predictable.
|
1 mF
|
10 MW
|
10 seconds
|
8 seconds
|
|
1 mF
|
20 MW
|
20 seconds
|
16 seconds
|
|
10 mF
|
1 MW
|
10 seconds
|
8 seconds
|
|
10 mF
|
2.2 MW
|
22 seconds
|
18 seconds
|
|
10 mF
|
4.7 MW
|
47 seconds
|
38 seconds
|
|
|
16.
|
OK. What's the pattern. Look at questions 10 and 13 if
you are still having trouble explaining. |
