Op-Amps

Op-Amp 741 is an 8 pin I.C. Integrated Circuit. We only use the 741 Op-Amp as a Voltage Comparator. (but it can be used as an amplifier).

The symbol shows a - and + sign. They do not mean minus or plus voltages.

The 741 has two inputs at Pins 2 and 3 and an output Pin 6.
Pin 3 is the non-inverting input with the sign + and is supplied with a fixed voltage, normally half the supply voltage by a Potential Divider.
in 2 is the inverting input with the sign --

The inverting input voltage at Pin 2 is made to go higher or lower than the fixed non-inverting input at Pin 3.

When the voltage at Pin 2 (the inverting input - ) goes higher than the fixed voltage at Pin 3 (the non-inverting input + ) then the output voltage at pin 6 goes low. (about 1.8 Volts for 9 volt supply).

When the voltage at Pin 2 goes lower than the fixed voltage at Pin 3 then the output voltage at Pin 6 goes high (about 8 Volts for 9 Volt supply).

The word "Inverting" means when one value goes high, the other value goes low. The opposite is true. When one goes low the other goes high.

The point at which the voltage at Pin 2 and 3 are equal is called the THRESHOLD voltage. It is the point at which the output at Pin 6 switches from high to low. (or low to high)

INPUT STAGE

"Fixed" Non-inverting input

The + non-inverting input at Pin 3 is supplied with a fixed voltage set at half the supply voltage. This is achieved by using a simple Potential Divider network with two equal 10K resistors.

Using two 10K resistors produces a small current at the I.C and a low battery drain. Current through Potential Divider is:-

I = V/R = 9/(10K + 10K) = 9/20000 = 0.00045 = 0.45mA

"Varying" Inverting input

The "Sensor" is placed in another Potential Divider network with a variable resistor to allow the THRESHOLD level to be set.
From a previous experiment, it was found that the resistance of a Light Dependent Resistor LDR was 12K at a dark setting. If the Variable Resistor is set at 12K then the Potential Divider network will give half the supply voltage to the inverting input Pin 2.
As the two inputs are equal this is the Threshold voltage.
If voltage from the sensor Potential Divider network increases, then the output at Pin 6 goes low. If it decreases, the output goes high.

Effects of varying the light level.

More Light.
If more light shines on the LDR, then resistance falls to 2K6
The voltage now on Pin 2 is found using the following formula.
Voutput = Vsupply x R2/R1+R2 = 9 x 2.6/14.6 = 1.6 V
As the voltage on Pin 2 (1.6V) is below the voltage at Pin 3 (4.5V), the output at Pin 6 goes high (8.0V).

Less LIght.
If less light shines on the LDR, then resistance rises to 50K
Voutput = 9 x 50K/62K = 7.3V
As the voltage on Pin 2 (7.3V) is above the voltage at Pin 3 (4.5V), the output at Pin 6 goes low (1.8V).

Effects of battery life.

The voltage of a PP3 battery falls from its original value of 9 volts until it becomes unusable at about 5.5 volts.
The fall in battery voltage has no effect on the circuit because both the inputs to the 741 Op-Amp are feed from the same battery through Potential Divider networks.
(source of graph:- Every Ready Battery data book 1989)

OUTPUT STAGE

The output can be used to indicate the On/Off state of a sensor or control further circuits. These can be placed between the output at Pin 6 and either the Top Rail (9 Volt) or the Bottom Rail (0 Volt)

Output to Top Rail.

An LED and resistor are put between Pin 6 and the 9 Volt Top Rail.

When the output goes low (1.8 V), the Potential Difference across the LED and resistor goes to 9 - 1.8 = 7.2 Volts.
The LED turns ON.
When the output goes high (8 V), the P.D. drops to 9 - 8 = 1 Volt
The LED turns OFF.

Output to Bottom Rail.

An LED and resistor are put between Pin 6 and the 0 Volt Bottom Rail.
When the output goes low (1.8 V), the Potential Difference across the LED and resistor goes to 1.8 - 0 = 1.8 Volts.
This is just sufficient to turn on the LED
When the output goes high (8 V), the Potential Divider rises to 8 - 0 = 8 Volts
The LED turns ON.
In this circuit design the output Pin 6 never drops below 1.8 Volts and can cause problems so it is always better to place the Output Devices between Pin 6 and the Top Rail.

20mA is the maximum current that can be supplied from the output Pin 6. When a circuit requires a higher current, the output can be feed to the base of a transistor.

Op-Amp Circuits

Thermistor input with LED output

The 5K Thermistor R3 is placed in the top half of the Potential Divider. The LED goes ON when the Thermistor heats up.    RAPID 61-0410   5K @ 25 degC   300R @ 100 degC.
To use this circuit as a room thermostat, Change the Variable Resistor VR1 to 4K7 and its fixed Resistor R4 to 3K9.
If you want to get the LED to go ON when the Thermistor cools down, reverse the position of the Thermistor in the Potential Divider network. Connect it to the negative rail.

Light Dependent Resistor LDR input with two LED's output

Window Comparator

The following circuit allows the output to be ON for a range of input voltages between low and high reference voltages. The output will be OFF when the input voltage is below the low reference voltage and above the high reference voltage.

V.in = Input Voltage at Pin 3 of top 741 and Pin 2 of bottom 741
Yellow LED D3 is ON when V.in is below the LOW reference voltage set by VR2 at Pin 3 of IC2
Green   LED D3 is ON when V.in is below the LOW reference voltage set by Sensor/VR2 at Pins 3 (IC1) and Pin 2 (IC2)
Red      LED D3 is ON when V.in is above the HIGH reference voltage set by VR1 at Pin 2 of IC1