Blinking LED Circuit—Physics Lab with RC Timing Circuit
OBJECTIVE
To set the blinking rate of a LED light by changing resistances in a RC circuit
MATERIALS
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Breadboard
6-volt Battery
555 Timer chip
Capacitor (10 microF)
Resistor 1 (1 kOhm)
Resistor 2 (470 Ohm)
Potentiometer (variable resistor)
Wires
Multimeter
Stopwatch
BACKGROUND
Circuits that have a resistor and capacitor connected are called RC circuits. The flash in a disposable camera is part of an RC circuit. A battery provides a current
through a resistor to a capacitor and charges it to the same voltage as the battery. A 6-volt battery will charge the capacitor to 6 volts after a long enough time.
When taking a picture with the flash, the capacitor gives up all its voltage by discharging to generate light. To take another picture with flash, we will need to
charge and discharge the capacitor.
In this lab, we will use an RC circuit to make an LED blink by charging and discharging the capacitor. In disposable cameras, it takes a few seconds to charge and
discharge. We can quickly or slowly charge and discharge by choosing the right resistor and capacitor values. That way, we can make the LED blink quickly or
slowly.
BLINKS PER SECOND i.e. frequency
 blinks sec 

calculated
1
ln(2)C  R1  2 R pot 
Resistance, R
Ohm
Capacitance, C
Farad
+ -
A B C D E
F G H I J
+ -
1
CC+
LED Batt
- Blk
555
Wire 555
1
555
R2
wire
555
2
5
Batt
555 Wire R1 (Red)
2
Wire
555
3 R1
Wire
555 Wire
1
4
555
LED R2
+
Pot
15
Do not connect this leg.
Pot Wire
3
10
Pot
Wire
4
PROCEDURE
CREATE CIRCUIT
1. Measure resistance value for R1 in Ohm. Record in table.
2. Place 555 timer component on the breadboard over the center groove. Connect the left side legs in column E and the right side legs on column F. Be sure
the circle on the chip is at the upper left corner.
3. Connect the potentiometer (variable resistor). Be sure one of the two legs is NOT connected.
4. Connect Wire 1, Wire 2, Wire 3, and Wire 4.
5. Connect Resistor 1. Now you are done with the right side of the chip.
6. Connect Resistor 2. Ask the teacher the purpose of having this resistor in the circuit.
7. Connect LED +-. The longer leg is positive.
8. Measure capacitance of capacitor in Farad using multimeter and record its value in the table.
9. Connect the Capacitor +-. The longer leg is usually positive. The arrow on the capacitor points to the negative leg.
CHECKPOINT: Call teacher over before the next step.
10. Connect black wire of battery. Then connect red wire of battery. Your LED should now blink.
11. Vary the resistance by turning the potentiometer dial. Your LED blinking rate will change.
ANALYZE CIRCUIT
Carefully touch the 555 timer every now and then. If hot, immediately remove battery wires from breadboard and call over the teacher.
12. Disconnect the battery. Make the potentiometer 100 kOhm by turning dial. Measure its resistance from F14 to G10 using the multimeter.
13. Connect the battery. Count the number of blinks that occur in 20 seconds using stopwatch. Determine blinks per second.
14. Repeat steps 12-13 for different potentiometer resistances of approximately 75 kOhm, 50 kOhm, and 25 kOhm.
CALCULATIONS (show in boxes)
R1
(Ohm)
Rpot
(Ohm)
Rtotal
(Ohm)
Measured
Calculated
blinks/sec
blinks/sec
(Blinks/20) (freq)
Blinks
C(Farad)
(over 20s)
100,000
75,000
50,000
25,000
 blinks sec
error 

calculated
1
ln(2)C  R1  2 R pot 
calculated  measured
measured
100%
Error of
blinks/sec
QUESTIONS
1. What did we change in this lab to make the LED blink faster? Do we want to increase or decrease this?
2. What else can we change in this circuit to alter the blinking rate, but didn’t?
3. Why does the LED appear to stay on and stop blinking after a certain point? Go to the oscilloscope station and record the frequency when this occurs.
Why do you think our lights and TV screens do not have a frequency lower than this?
4. Go to the speaker station and connect your circuit. Explain why you are convinced that the LED is still blinking when it looks like it isn’t.
5. What are the possible causes for errors in today’s lab?