The 741 Chip

The ‘741’ Chip
An important and very useful group of Integrated Circuits is the “Operational Amplifier” or “op-amp” group.   These devices have a very high gain, an ‘inverting’ input and a ‘non-inverting’ input.   There are many op-amps but we will look at just one popular type called the “741” which has an ‘open-loop’ gain of 100,000 times.   All operational amplifiers work in the same way in theory.   The way they operate in a circuit is controlled by the external components attached to them.   They can operate as inverting amplifier, a non-inverting amplifier (i.e. a ‘buffer’), a comparator, an astable multivibrator, and various other things.   The symbol and connections for a 741 op-amp are:

We can connect the 741 chip to act as an amplifier with any set gain level that we choose:

Here, the gain is set by the ratio of the 220K resistor to the 22K resistor.   This circuit has a gain of 10 times, so the input signal at point ‘B’ will generate an output signal at point ‘C’ which is ten times larger, provided that the output signal does not approach the battery voltage.   If it does, then clipping will occur with the top and the bottom of the output waveform chopped off at about a volt away from the battery voltage levels, approximately 1 Volt and +11 Volts in this example.

Operational amplifiers are generally designed to operate from a dual power supply.   In the above example, the supply would be created by using two 6 Volts batteries instead of one 12 Volt battery.   To avoid the inconvenience of this, a mid-point voltage is generated at point ‘A’ by using two equal resistors in series across the battery.   This gives a central voltage of +6 Volts which is fed to the IC.

This circuit can be used in many applications. Here is a circuit for a meter to measure sound intensity:

This circuit is two copies of the previous circuit.   Each 741 chip has a reference voltage of half the supply voltage created by a voltage-divider pair of 1K resistors.   This voltage is fed to pin 3 of the chip, which is the non-inverting input.

At point ‘A’, a microphone or small loudspeaker is used to generate a signal voltage when sound reaches it.   This voltage is fed to the 741 op-amp via a 1 microfarad blocking capacitor.   This passes the audio signal through while blocking the +4.5 Volts DC on pin 3.   The first 741 has a gain of 22, set by the 10K and 220K resistors (220/10 = 22).

Point ‘B’ then receives an audio signal 22 times larger than the signal produced by the microphone.   This signal is still quite small, so the second 741 boosts it further.   The gain of the second 741 is variable and depends on the resistance set on the 1M variable resistor.   If the variable resistor is set to zero ohms, then the gain of the second 741 will be controlled by the 4K7 resistor at point ‘C’ alone and so will be 1 (4.7/4.7 = 1).   If the variable resistor is set to its maximum value, then the gain of the second 741 will be some 214 (1,004,700/4,700 = 213.8).

The two op-amps together have a combined gain which ranges from 22 to 4702.   The amplified audio signal arrives at point ‘D’ and it can be adjusted to a respectable value.   This alternating voltage is now rectified via the diodes at point ‘E’ and it builds up a DC voltage across the 47 microfarad capacitor there.   This voltage is displayed on a voltmeter.   The result is that the voltmeter shows a reading directly proportional to the sound level reaching the microphone.

The 741 can be wired as a buffer.   This is the equivalent of an emitter-follower circuit when using transistors.   The set up for the 741 is:

Difficult circuit – huh! Are you sure you can afford all the extra components?   This circuit utilises the full gain of the 741 chip.   The output follows the input waveform exactly.   The input requires almost no current, so the circuit is described as having a ‘high input impedance’.   The output can drive a serious load such as a relay, so the circuit is described as having a ‘low output impedance’.

The 741 chip can be wired to act as a comparator.   This is the circuit:

Are you sure you are up to such a difficult circuit?   Bit complicated – huh!   This is the basic operational form for an operational amplifier.

If the voltage at point ‘A’ is higher than the voltage at point ‘B’ then the output goes as low as it can go, say 1 or 2 volts.

If the voltage at point ‘A’ is lower than the voltage at point ‘B’ then the output goes as high as it can go, say 10 volts or so.

Having seen how transistor circuits work, you should be able to understand why the 741 chip circuitry (which is a transistor circuit inside the 741 package) needs some voltage inside the supply rails to provide an efficient high-current output drive.

Here is a 741 version of the light-operated switch:

This circuit is set up as evening falls.   We want the relay to have minimum voltage across it in daylight, so the voltage at point ‘A’ needs to be higher than the voltage at point ‘B’.   As the 1K variable resistor is across the supply voltage, its slider can be set to any voltage between 0 Volts and +12 Volts.   To make this easy to do, we choose a ‘linear’ variable resistor as the logarithmic variety would be hard to adjust in this application.   With the ‘linear’ version, each 1 degree of rotation of the resistor shaft causes the same change in resistance, anywhere along the range.   This is not the case for the logarithmic variety.

Anyhow, we adjust the variable resistor downwards until the relay voltage drops to a minimum.   When the light level has fallen to the level at which we wish the circuit to trigger, we adjust the variable resistor to make the relay click on.   The 741 chip has a very rapid output voltage swing when the input voltages swap over, so the relay switching will be decisive.   The switching can be made even more positive by adding a resistor between the output and point ‘B’.   This acts like a Schmitt trigger when switching occurs by providing some additional positive feedback, lifting the voltage at point ‘B’.

If you wish the circuit to trigger on a rising light level, just swap the positions of the 10K resistor and the ORP12 light-dependent resistor.   The same circuit will operate as a temperature sensing circuit by substituting a ‘thermistor’ (which is a temperature-dependent resistor) for the ORP12.

If we would like the circuit to act as a burglar alarm, we could use the same circuit like this:

The circuit is still controlled by the voltage at point ‘A’.   Under normal circumstances, this voltage will be near +6 Volts (produced by the two 10K resistors and the 100K resistor).   The upper switch marked ‘NC’ for ‘Normally Closed’, represents a chain of, say, magnetic switches attached to doors and windows.   If any of these are opened, then the voltage at point ‘A’ will be dictated by the lower 10K resistor in series with the 100K resistor.   This will cause the voltage at ‘A’ to fall instantly to a low value, triggering the circuit.

The ‘NO’ switch (‘Normally Open’) represents one or more pressure-operated switches under carpets or rugs and/or switches which get brushed when doors are swung open, etc.   These switches are wired in parallel across each other and if any of them is closed for even a millionth of a second, the voltage at point ’A’ will be pulled down by the 1K resistor and the circuit will be triggered.

The circuit can be latched on in any one of a variety of ways.   One relay contact can be used to hold the relay on or hold the voltage at ‘A’ low.   A transistor can be wired across the relay to hold the circuit on, etc. etc.   If this is done, the circuit will remain in its triggered state until the supply voltage is interrupted.   You might prefer to use a 555 chip to limit the length of time the alarm sounds to three minutes or so.

An alternative to using a relay or semiconductor latch is to use a Silicon Controlled Rectifier usually referred to as an ‘SCR’ or ‘Thyristor’.   This device is normally “off” with a very high resistance to current flow.   If it is switched on by applying a voltage to its Gate connection, it stays continuously on until some external device stops current flowing through it.   The following circuit shows how it operates:

When the voltage is first applied to the circuit by closing switch S2, the SCR is in its OFF state so no current is supplied to the load. If the press-button switch S1 is pressed, a current is fed into the Gate of the SCR, turning it ON.   When switch S1 is allowed to open, the SCR remains in its ON state and it will stay that way until the current through it is cut off.   Opening switch S2 cuts off the current to the load and the SCR returns to its OFF state.   A very valid question would be: “Why have an SCR at all and just turn the load on and off with switch S2?”.   The answer is that switch S1 might be the under-carpet pressure pad of a burglar-alarm and it might be operated some hours after switch S2 was closed to activate the alarm system.   Stepping off the pressure pad does not stop the alarm sounding.

While this sort of DC latching action is useful, it is more common for an SCR to be used in an AC circuit.   For example, take the circuit shown here:

The 120 volt AC supply coming in from the right hand side, is converted to positive-going sine-wave pulses by the diode bridge.   This pulsing voltage is applied to the Load/SCR path.   If the voltage at pin 3 of the 555 chip is low, then the SCR will remain OFF and no current will be fed to the load device.   If the voltage on pin 3 goes high and the voltage applied to the Load/SCR chain is high, then the SCR will be switched ON, powering the load until the pulsing voltage drops to its zero level again some 1/120 of a second later.

The 555 chip is connected to form a monostable multivibrator and the timing components (the 120K resistor and the 10nF capacitor) cause it to output a 1 millisecond pulse which is long enough to trigger the SCR into its ON state, but short enough to have finished before the mains pulse reaches its zero-voltage level again.   The 555 chip is triggered by the rising mains voltage being passed to its pin 2 through the voltage-divider 100K and 120K pair of resistors, and that synchronises it with the AC waveform.   Pin 4 of the 555 chip can be used to switch the load power on and off.

In the circuit shown above, the diode bridge is needed to convert the incoming AC waveform to pulsing DC as shown in red in the diagram, as the SCR can only handle current flowing in one direction.   The AC load equipment works just as well with the pulsing DC as with a full blown AC waveform.   A better semiconductor construction is the ‘Triac’ which acts like two SCR devices back-to-back in a single package.   It is shown like this in circuit diagrams:

There are three connections to the device: Main Terminal 1, Main Terminal 2 and the Gate.   When switch ‘S’ shown in the diagram is closed, the triac conducts on both positive and negative voltages applied to its MT1 and MT2 terminals.   When the switch is open, the device does not conduct at all.

If the external circuit containing switch ‘S’ is placed inside the device as a permanently closed circuit, then the device becomes a ‘Diac’ which can be used to trigger a Triac and give a very neat circuit for controlling the power to an item of AC mains equipment as shown here:

Here, the variable resistor/capacitor pair controls the point on the AC waveform that the Triac is triggered and so controls how much of each sinewave cycle is passed to the mains equipment, and so it controls the average power passed to the equipment.   A very common use for a circuit of this type is the ‘dimmer-switch’ used with household lighting.

To return now to the 741 chip.   The 741 can also be used as an astable multivibrator.   The circuit is:

The rate of oscillation of this circuit is governed by the Resistor marked ‘R’ in the diagram and the capacitor marked ‘C’.   The larger the resistor, the lower the rate of oscillation, the larger the capacitor, the lower the rate of oscillation.

When the output goes high, capacitor ‘C’ charges up until the voltage on it exceeds the mid-rail voltage on pin 3, at which time the 741 output goes low.   The capacitor now discharges through resistor ‘R’ until the voltage on it drops below the voltage on pin 3, at which time the output goes high again.   The 10K resistor connecting the output to pin 3 provides some positive feedback which makes the 741 act quite like a Schmitt trigger, sharpening up the switching.

The same arrangement of resistor and capacitor applied to a Schmitt inverter or Schmitt NAND gate causes exactly the same oscillation:

If you would like to see additional ways of using 741 and 555 chips, I can recommend the excellent book “Elementary Electronics” by Mel Sladdin and Alan Johnson ISBN 0 340 51373 X.

Here is a very well tested and highly thought of, low-cost oscillator circuit, using a 74HC14 Schmitt inverter chip. It allows fine tuning control of the frequency and the pulse width produced. Three of the inverters are connected together to give a more powerful output current drive:

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