Lab Partner(s):
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Introduction
As per ohm’s law V= IR is the basic relationship between current, voltage and resistance. In case of a circuit having very high resistance the current flow is very low. In such cases, voltage drop across resistance is proportional to the value of the resistances. For example, if the total voltage is V and there are two resistances R1 and R2 connected in parallel in the circuit across the voltage and if the voltage measured across each resistors are V1 and V2 then V1/ V2 = R2/R1.
Data Table
Sample Calculations –
There are different types of resistances. Resistances with a gold band has a tolerance of +/ 5%.
The resistances used in the Lab are as below:
Resistance 1:
As per the color band value of the resistance is = 6800 Ohm or 6.8 K Ohm.
Calculation: (6(blue) 8(Grey) *10^ 2(red) = 6800 Ohm with +/- 5% (Gold Band).
Resistance 2:
Similarly the above resistance value as per color code is 15000 ohm +/- 5%.
Resistance 3:
Similarly the above resistance value as per color code is 220 ohm +/- 5%.
In the 555 circuit, in the up cycle when the LED is on then the voltage across resistor 6.8 KOhm should be 3.00 Volts and the voltage across 15 KOhm resistor should be 2 Volt theoretically. The voltage across 220 Ohm resistance should be very small.
However, using the Ohmmeter the measured resistances are 6.6 KOhm, 14.2 KOhm and 219.3 Ohm respectively. The percent deviation from the actual vs. the measured is shown in the above table. The calculation for the calculation of deviation is as below:
% Deviation = (Actual- Measured)/ (Actual + Measured)/2 *100
= (6800-6600)/(6800+6600)/2 *100
=0.75%
Questions
The differences between measured vs. actual resistances are less than the specified deviation (+/- 5%). The maximum deviation measured for the resistances is 1.37%. The possible reason can be, 1) the way slight deviation in the materials used in creating the resistances 2) Ohmmeter error in measuring the resistance and 3) wire resistances.
If the 220 Ohm resistor from the circuit is directly connected to the main voltage rail of 9V then the output PIN 3 will be forced to stay at 9V causing no change of the output. The LED bulb will switch on and off. The state of the Led will remain same.
The voltage output from the PIN 3 of 555 is an output PIN of a square wave. It depends on the input from PIN 2. When both PIN 2 and PIN 6 are low the output of PIN 3 goes high. That means the voltage goes up from PIN 3. This causes the voltage drop across the top resistance and the current flow direction changes and the Voltage Discharge cycle starts. This causes the voltage in PIN2 and PIN 6 go high making the voltage through PIN 3 go low. It then starts the charging of the capacitor through PIN 7 and the resistors. When charged again, PIN6 and PIN2 go low and trigger PIN 3 again to go high. The above cycle starts again. How quickly the cycle will run depends on the capacitor value. In the first cycle the LED is not glowing as the current is flowing through the resistors 6.8 K Ohm and 15 KOhm. On the second cycle the LED glows as the current is flowing through the resistor.
Error/Conclusion
As our percent error is not zero we know that there is some error involved in the measurement. Maximum error may have come from the construction of the resistance. Second largest sources of error may have been introduced from the wire resistances of the circuit and third largest contributor to the measured vs. actual resistance value came from measuring error in the Ohmmeter.
