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This article will show you how to build your very own electronic analog neuron, named the “Perceptron.” Creating a single neuron may not seem earth shattering in the grand scheme of things, considering that your brain has over 10¹⁰ of them!

However, a neuron is the fundamental building block of intelligence.

The Perceptron circuit provides a hands-on way to demonstrate the principles of neuron operation, and also allows you to explore basic Boolean logic functions. Last but not least, it is a cool gadget to have sitting on your desk to impress your hacker and/or nerdy friends.

Whether biological, electronic, or software, the basic operation of a neuron is fairly simple at a high level. Of course, a biological neuron is quite a bit more complicated than the others. But relatively speaking, whether we are talking about a brain or neural network program, the basic concept is that the processor is simple by itself, but when combined with many others they collectively perform a complex computation. Compare this to the computer on your desk — how many processors does it have? Essentially one, not counting some special processors in the video card and other peripherals.

The signals entering the circuit through the “PIEZOIN” connector is handled by the first op-amp (IC1A) of IC-TL072P. Choosing the right value for resistor R1 largely depends on your specific application. For example, if you want to catch sub-audio signals, the 10M value is okay.

Ready, Aim ... Fire!

Precision 16 MHz CBFET Operational Amplifier. AD-845S/AD: Precision 16 MHz CBFET Operational Amplifier. AD-845S/AD: Precision 16 MHz CBFET Operational Amplifier. AD101A: General-Purpose Low Cost IC Operational Amplifier (AA Enabled) AD201A: General-Purpose Low Cost IC Operational Amplifier (AA Enabled) AD301A. In this position, IC1a maintains the test voltage across the CUT. Since no current flows into the op amp input, any leakage current flowing through the CUT must also flow through the selected feedback resistor (R). IC1a will therefore raise its output voltage above the test voltage by I x R volts, and this difference will be shown on the DMM.

A neuron receives one or more input signals and produces an output signal (Figure 1).

FIGURE 1. Neuron Model.

What happens in between comprises the fundamental properties of a neuron — Neuron Property #1 and Neuron Property #2.

Neuron Property #1: A neuron computes a weighted sum of its inputs.

Neuron Property #2: A neuron performs an activation function on the weighted sum.

The activation function, which is also called a “squashing” function in neural network jargon, is usually nonlinear and also serves as a limiting operation to keep the output signal in a bounded range.

Mathematically, the neuron’s operation is represented as:

a = squash(∑(i_iw_i))

where:

• i_i is input i to the Perceptron
• w_i is the weight for input i
• a is the activation (output)
• and:

squash(x) = 1; if x > threshold
= 0; otherwise

In English, the weighted sum is computed by multiplying each input by a weighting factor (simply called a weight), which dictates the importance of that input to the neuron’s computation. A small weight means that input is not as important as an input that has a larger weight. The weight may also be negative, which means that the input tends to inhibit the output of the neuron. By summing all of these individual multiplications, we get an output that depends on how big the inputs are and how important each input is (as dictated by its weight).

The second operation is to perform a squashing function on the sum that was calculated in the first step. Neural network researchers experiment with different types of squashing functions, but they generally do two things.

First, they limit the range of the output value, and second, jump suddenly between states. A simple but useful type of squashing function is a simple threshold. If the sum is above a certain value — the threshold — the function jumps to the maximum value, otherwise it jumps to the minimum value.

Decades ago it was realized that in order to perform interesting computations in a neural network, the neuron needed to exhibit this nonlinear behavior.

Our electronic Perceptron is designed to perform this two-step computation of weighted sum plus threshold operation. We will see how this can implement simple, but interesting, functions including simple Boolean logic.

From the outside, the Perceptron consists of two inputs each of which is controlled by a switch and a potentiometer knob. The output is indicated by a single dual-color LED. Inside, an analog amplifier chip and a handful of other components perform the computation. A single 9V battery is used to supply power.

Circuit Operation

The schematic for the Perceptron is shown in Figure 2.

FIGURE 2. Perceptron Schematic.

The circuit is based on an LM324 quad op-amp (IC1), which was chosen for its tolerance to supply voltage used. A basic design goal was to have the Perceptron run off a single 9V battery. However, the circuit needs both positive and negative voltages to represent positive and negative weights. Therefore, a single-supply divider consisting of R13 and R14 is used to create a virtual ground and simulate positive and negative voltages.

This is a common op-amp technique, but it is normally advocated for AC signals only. However, by carefully referencing all appropriate signals to the virtual ground, the technique works fine for DC signals.

The first part of the circuit handles the inputs and weights. Since our input signals — as represented by the two toggle switches — are binary (either on or off), we can use a simplifying trick to simulate the weighting operation.

To represent each input weight, potentiometers R2 and R3 are connected across the full supply range. If the pot is turned toward the positive supply rail it represents a positive weight, and a negative weight when turned toward the negative supply. When the pot is in the center of its range, it represents a zero weight. Each toggle switch is used to simply indicate whether an input is active (binary 1) or not (binary 0) by connecting/disconnecting its corresponding pot.

The first op-amp IC1A is configured as a non-inverting summing circuit, to sum the voltages from the two inputs. Resistor R12 is used to tie the summing point to ground in case of no inputs. That takes care of Neuron Property #1. In order to implement Neuron Property #2, we need the squashing function. To realize this, the output of summer IC1A is fed into op-amp IC1B, which is configured as a comparator. The third pot R4 on the negative input sets the threshold of the neuron. Since the amp operates in open loop mode, the high gain will drive the output of the amp to the positive supply if the positive input is above the threshold voltage, and toward the negative supply voltage if it is below. In this mode, it acts mostly as a hard step function, although there is some “play” around the zero voltage point. The LED — with its limiting resistor at the output — indicates whether the output is high or low. The bidirectional LED given in the Parts List results in a green or red signal, which is more interesting than a single-color LED.

This is all that is needed to implement the basic neuron operation. To make things a little more interesting, a third op-amp IC1D from the LM324 is configured as an analog inverter and can be used to feed back the inverted output as an input to the neuron. This can be used to realize a simple oscillator, which will be covered later in the article.

Construction

A printed circuit board pattern is provided in the downloads below if you want to create a PCB along with the parts layout. But you don’t have to take this approach — the circuit can be constructed using simple perfboard and soldered wires. If you are not very good at soldering, you should consider using a 14-pin socket and place the chip in after you are done soldering. If you use a socket, it is easier to solder this in first — otherwise you can save soldering the IC for later in order to minimize the risk of heat damage.

Next, solder the resistors and capacitor to the board, followed by the 9V battery clip. You don’t have to be paranoid about too much when soldering but as always, you should try to make good solder connections. The rest of the components — switches, pots, and LED — will be soldered using insulated wire. I like to use hookup wire scavenged from a ribbon cable. The wires are thin and flexible and I can peel off as many conductors as I want for the particular component I am connecting. However, feel free to use single insulated wires or multiconductor hookup wire.

It is always good to double-check the supply and ground connections on the chip and battery connections, especially if you are using perfboard.

The Parts List includes a case with a battery compartment that the circuit can be built into.

Parts List

Figure 3 shows the completed circuit board inside its enclosure.

FIGURE 3. Circuit Board in Enclosure.

For the case listed in the Parts List, it was necessary to remove some material from two of the corners of the PCB. Figure 4 shows the completed Perceptron.

FIGURE 4. Finished Unit.

Testing and Using the Perceptron

The inputs of the Perceptron are controlled by a switch and a potentiometer (knob). Each knob controls the weight on its corresponding input. Clockwise is intended to be positive and counterclockwise is negative, with the middle of the range being approximately zero or “no weight.” The switches are used to indicate if the input is “on” or not. You may have to experiment with the switches to get them in the right polarity.

There is also an important note on the switch for input 1. You probably noticed that it is a different type of switch. This switch actually has three positions: the center is “off” and when you move the toggle either way, it selects one of two inputs. The first input is the normal input and the second is a feedback signal from the output, which we will talk about later. The third knob is the threshold knob, which — amazingly — sets the threshold of the neuron.

You will want to make sure your potentiometers are connected in the right polarity. You want full counterclockwise to connect the center lead to the negative supply voltage and clockwise to connect to the positive supply. If you have a multimeter, you can easily verify this by varying the knob and checking the voltage on the center lead. Without using a meter, you will have to experiment a bit.

First (since we are using a dual color LED), if the power switch is on and all battery and power connections are good, the LED should show some color. Next, as a simple test, put all knobs in the middle of the range. Now vary the threshold knob back and forth — it should cause the LED to change between green and red.

Now, set the threshold knob in the middle of its range. Set input switch SW1 to its middle position. Now vary the input 2 knob. If nothing changes, then the input switch SW1 is off. Switch it the other way and vary the knob. If you do not see a change now, then you should go back and check your wiring. Next, tTurn the input 2 switch back off and try the same thing with input 1. Another thing to check is the polarity of the LED — a positive output is intended to show as green.

The Perceptron can be used to emulate simple Boolean logic gates. First we will try an OR gate, which has a truth table as shown in Figure 5.

FIGURE 5. Boolean Logic Tables.

In Perceptron terms, the output will be on (green) if either input is on. To configure the Perceptron for this operation, set the threshold knob to a little over halfway and each weight knob well over halfway. Now try the switches in different combinations. With either switch on, you should see a strong green light. Now try a NAND gate configuration. For this function, set the threshold knob most of the way counterclockwise and set the weight knobs a little more clockwise than the threshold.

The idea is that the threshold is strongly negative and each weight is somewhat negative. In this way, there will be a positive output unless both inputs are on and combine to drive the sum below the threshold.

A Neural Oscillator

Now, let’s talk about the third setting of switch SW1. Our Perceptron is a highly idealized model of a real biological neuron, and it would take some complex electronics to model all of its functions. However, the Perceptron does include one additional behavior, which brings us to an interesting fact — Neuron Property #3.

Neuron Property #3: A neuron has a built-in time delay.

A neuron’s operation is a complex function of time, but for our purposes we will consider a simple time delay. As with all real world devices, a neuron cannot instantaneously produce an output and, in fact, operates quite slowly (on the order of milliseconds) compared to electronic circuits (nanoseconds). But as we will see shortly, time delays are actually useful in the same way that clocking functions are critical to digital circuit operation.

One of the extra amps on the LM324 chip has been used to create a second neuron. This neuron only has a single input which is the output of the first neuron. But it also has a time delay, which is implemented by the combination of C1 and R6. When switch SW1 is moved to its third position, it connects an inverted version of the first neuron’s output back as an input to itself. However, because of the delay circuit, this doesn’t happen until after about a second. Since the signal is inverted, it causes the first neuron to change its state, and then keep switching back and forth about once a second. The result is a simple oscillator that has been created using only neurons.

Wrapping Up

If you are new to the concept of neurons and neural networks, the Perceptron offers a practical hands-on introduction to a neuron’s operation. If you are a seasoned pro, I think you will find it to be an entertaining diversion that is simple to build, and makes a fun accessory for your desk. NV

About the Author

Christopher McCarley is currently a software architect at Viziqor Solutions. He has done postgraduate research in VLSI analog neural network design and has 20 years experience developing a variety of technologies including middleware, robotics, and laser systems.

Downloads

The Perceptron Circuit.zip

What’s in the zip?
Parts Layout and PCB pattern files

1. IC 741

The most commonly used op-amp is IC741. The 741 op-amp is a voltage amplifier, it inverts the input voltage at the output, can be found almost everywhere in electronic circuits.

Pin Configuration:

Let’s see the pin configuration and testing of 741 op-amps. Usually, this is a numbered counter clockwise around the chip. It is an 8 pin IC. They provide superior performance in integrator, summing amplifier and general feedback applications. These are high gain op-amp; the voltage on the inverting input can be maintained almost equal to Vin.

It is a 8-pin dual-in-line package with a pinout shown above.

Pin 1: Offset null.

Pin 2: Inverting input terminal.

Pin 3: Non-inverting input terminal.

Pin 4: –VCC (negative voltage supply).

Pin 5: Offset null.

Pin 6: Output voltage.

Pin 7: +VCC (positive voltage supply).

Pin 8: No Connection.

The main pins in the 741 op-amp are pin2, pin3 and pin6. In inverting amplifier, a positive voltage is applied to pin2 of the op-amp; we get output as negative voltage through pin 6. The polarity has been inverted. In a non-inverting amplifier, a positive voltage is applied to pin3 of the op-amp; we get output as positive voltage through pin 6. Polarity remains the same in non-inverting amplifier. Vcc is usually in the range from 12 to 15 volts. When two supplies (+Vcc/-Vcc) are used, they are the same voltage and of opposite sign in almost all cases. Remember that the operational amplifier is a high gain, differential voltage amplifier. For a 741 operational amplifier, the gain is at least 100,000 and can be more than a million (1,000,000). That’s an important fact you’ll need to remember as you put the 741 into a circuit.

There are many common application circuits using IC741 op-amp, they are adder, comparator, subtractor, integrator, differentiator and voltage follower.

Below is some example of 741 IC based circuits. However, the 741 is used as a comparator and not an amplifier. The difference between the two is small but significant. Even if used as a comparator the 741 still detects weak signals so that they can be recognized more easily. A comparator is a circuit that compares two input voltages. One voltage is called the reference voltage and the other is called the input voltage. It is a circuit which compares a signal voltage applied at one input of an op-amp with a known reference voltage at the other input. The 741 op-amp has ideal transfer characteristics (output ±Vsat); and the output is changed by increment in the input voltage of 2mV.

2. LM324

LM324 is a quad op amp integrated circuit with high stability, bandwidth which was designed to operate from a single power supply over a wide range of voltages. They have some dissimilar advantages over standard operational amplifier types in single supply applications. It is a 14-pin dual in-line package, contains four internally compensated and two stage operational amplifiers, shown in figure.

• Pin 1, 7, 8 and 14 are the outputs of comparator
• Pin 2, 6, 9 and 13 are the inverting inputs of compactor
• Pin 3, 5, 10 and 12 are non-inverting inputs of comparator
• Pin 11 is ground (0V)
• Pin 4 is supply voltage; 5V

Features:

• Internally frequency compensated for unity gain
• Large DC voltage gain 100 dB
• Wide bandwidth 1 MHz
• Wide power supply range: Single supply 3V to 32V
• Essentially independent of supply voltage
• Differential input voltage range equal to the power supply voltage
• Large output voltage swing 0V to V+ − 1.5V

Potential dividers of LM323 are connected to the inverting and non inverting inputs of the op-amp to give some voltage at these terminals. Supply voltage is given to +V and –V is connected to ground. The output of this comparator will be logic high if the non-inverting terminal input is greater than the inverting terminal input of the comparator. When the inverting input is more than the non-inverting then logic low (0) will be the output.

Working of LM324:

• When the power is applied to non-inverting terminal which is less than the inverting voltage of op-amp then the output becomes zero which means there is no current flow. Because we already know that when “+ > – = 1”. Here the ‘+ ‘sign indicates non-inverting terminal and ‘-‘sign indicates the inverting terminal.
• If the non-inverting voltage is greater than the inverting voltage then the output will be high.
• In this output of LM324 is internally connected to some resistance and it has some arrangement inside the IC, which makes a lot of difference to other comparators.
• It is internally pulled-up, so no need of any resistor connection from the supply.

3. LM339

The LM339 is a most commonly used comparator, designed for use in level detection, low−level sensing and memory applications in automotive and industrial electronic applications. It has four inbuilt comparators; it compares two input voltage levels and gives digital output to show the bigger one.

These comparators additionally have a unique characteristic in that the input common-mode voltage range includes ground, in spite of the fact that they are operated from a single power supply voltage.

• Pin 1, 2, 13 and 14 are the outputs of comparator
• Pin 3 is supply voltage; 5V
• Pin 4, 6, 8 and 10 are inverting inputs of comparator
• Pin 5, 7, 9 and 11 are non-inverting inputs of comparator
• Pin 12 is ground; (0V)

Features:

• Signal or dual supply operation
• Wide operating supply range (VCC=2V~36V)
  • Max Rating: 2 V to 36 V
  • Tested to 30 V: Non-V Devices
• Input common-mode voltage includes ground
• Low supply current drain (IF=0.8mA)
• Open collector outputs for wired and connection
• Low input bias current 25nA
• Low output saturation voltage
• Output compatible with TTL, DTL, and CMOS logic system
• Differential input voltage range equal to the power supply voltage

Potential dividers of LM339 are connected to the inverting and non-inverting inputs of the op-amp to give some voltage at these terminals. Supply voltage is given to +V and –V is connected to ground. The output of this comparator will be logic high if the non-inverting terminal input is greater than the inverting terminal input of the comparator.

Working of LM339:

• When the power is applied to non-inverting terminal which is less than the inverting voltage of op-amp then the output becomes zero which means there is no current flow. Because we already know that when “+ > – = 1”. Here the ‘+ ‘sign indicates non-inverting terminal and ‘-‘sign indicates the inverting terminal.
• If the non-inverting voltage is greater than the inverting voltage then the current flow will be in the device.
• The LM339 is act as an open-collector that’s why we connected the resister from the supply, if we remove the resister then there is no current flow in the circuit.

4. LM258

The LM358 op-amps are used in transducer amplifiers, dc gain blocks and all the conventional op-amp circuits which now can be more easily implemented in single power supply systems. For example, the LM358 op-amp can be directly operated off of the standard +5V power supply voltage which is used as a part of digital systems and will easily provide the required interface electronics without needing the extra ±15V power supplies.

It comes in an 8-pin DIP package is shown in below.

Pin Description:

• Pin 1 and 7 are outputs of comparator
• Pin 2 and 6 are inverting inputs
• Pin 3 and 5 are non-inverting inputs
• Pin 4 is ground (GND)
• Pin 8 is VCC+

Features:

• Internally frequency compensated for unity gain
• Large dc voltage gain: 100 DB
• Wide bandwidth
• Wide power supply range: single supply: 3V to 32V
• Very low supply current drain essentially independent of supply voltage
• Low input offset voltage: 2 mV
• Input common-mode voltage range includes ground
• Differential input voltage range equal to the power supply voltage
• Power drain suitable for battery operation

Advantages:

• Two internally compensated op amps
• Eliminates need for dual supplies
• Allows direct sensing near GND and VOUT also goes to GND
• Compatible with all forms of logic
• Power drain suitable for battery operation

Working of LM358:

The inverting input of the comparator LM358 i.e., pin 2 is given to the fixed voltage i.e., in the ratio 47k:10k and the non inverting input of the comparator is pulled down and is given to sensing terminal. When the resistance between the positive supply and the non inverting input is high then resulting is the non-inverting input less than the inverting input making comparator output as logic low at pin1. And when the resistance falls making available a voltage to the non-inverting input higher than inverting input, so that the output of comparator is logic high.

5. CA 3130 Op Amp

It is excellent Op Amp that requires very low input current requirements. Its output will be in the zero state in the off mode. CA3130 is the 15MHz BiMOS IC with MOSFET inputs and a bipolar output. MOSFET transistors are present in the inputs that provide very high input impedance. The input current can be as low as 10pA. The IC shows very high speed of performance and combines the advantage of both CMOS and bipolar transistors. The presence of PMOS transistors at the inputs results in common mode input voltage capacity down to 0.5 volts below the negative rail. So it is ideal in single supply applications.

The output has CMOS transistor pair that swings the output voltage within 10mV of either supply voltage terminal. IC CA3130 works off 5 to 16 volts and can be phase compensated with a single external capacitor. It also has terminals to adjust the offset voltage and strobing.

6. CA 3140 Op Amp

It is the 4.5MHz BiMOS Op Amp with MOSFET inputs and bipolar output. It has both PMOS transistors and high voltage bipolar transistors inside. Is inputs have gate protected MOSFETs (PMOS) that provides very high input impedance typically around 1.5T Ohms. The input current requirement is very low around 10pA. It exhibits very fast response and high speed of performance. The output has protection against damage from load terminal shorting. The input stage has PMOS FET which helps in common mode input voltage capability as low as 0.5 volts. The IC is internally phase compensated for stable operation. It also has terminals for additional frequency roll off and offset nulling.

7. TL071 Op Amp

It is a low noise Op Amp with JFET inputs. It operates in wide common mode and consumes very little current. It requires very low input bias and offset currents. It’s output is short circuit protected and has very high slew rate of 13 V/us and exhibits latch free working.TL0 71 is ideal for high fidelity and audio preamplifier circuits. TL071 and TL0 72 contain only one Op Amp inside while TL074 is a Quad OpAmp with 4 operational amplifiers inside.

8. TL082 Op Amp

Op Amp Circuits

It is a dual OpAmp with separate inputs and outputs. It has JFET inputs and bipolar outputs. The IC shows very high slew rate, low input bias. It also has low offset current and low offset voltage. Its inputs can be biased with very low input currents. Output of the IC is short circuit protected. TL082 exhibits latch free operation and it has the internal frequency compensation.

9. LM 311 Op Amp

It is a single OPAMP capable of driving DTL, RTL, TTL or MOS circuits. Its output can switch up to 50 volts and 50mA current. It works on wide range of supply voltages from 5 to 30 volts and requires only single supply. It can directly drive relays, solenoids etc if the current requirement is less than 50mA.The pin connection of LM311 is different from other OpAmps. Here the pin3 is inverting input and pin2 Non inverting input. Output also is different. It has two outputs. Pin7 is the Positive output that sinks current while Pin 1 is the negative output.

Pin 7 is connected to the collector of the NPN output transistor. Pin1 forms the emitter of output transistor. Normally the output transistor is in the off state and its collector will be pulled to Vcc. If its base gets more than 0.7 volts, it saturates and turns on. This sinks current and the load turns on. So unlike other OpAmps, LM311 sinks current and output turns low when triggered.

10. IC 747

The 747 is a general purpose dual operational amplifier containing two 741 op-amps. The two operational amplifiers have a common bias network and power supply leads. Otherwise, their operation is completely independent. The characteristics of the op-amp are no latch-up when input common mode range is exceeded, freedom from oscillations. It is a 14-pin dual in line package (DIP), shown in figure below:

Pin Description of 747 Op-amp:

Pin1 – Inverting input terminal of op-amp1

Pin2 – Non-inverting input terminal of op-amp1

Pin3 – Offset null terminal op-amp1

Pin4 – Negative supply voltage (-V)

Pin5 – Offset null terminal of op-amp2

Pin6 – Non-inverting input terminal of op-amp2

Pin7 – Inverting input terminal of op-amp2

Pin8 – Offset null terminal of op-amp2

Pin9 – Positive supply voltage (+V) of op-amp2

Pin10 – Output of op-amp2

Pin11 – No connection (NC)

Pin13 – Positive supply voltage of op-amp1

Pin14 – Offset null terminal of op-amp1

Features of 747 op-amp:

• Dual supply voltage ±1.5V to ±15V
• No frequency compensation required
• Short-circuit protection
• Wide common-mode and differential voltage ranges
• Low power consumption
• Unity gain stable
• No latch-up
• Balanced offset null
• Supply current is less than 300 μA per amplifier at 5 V

How to test an Op Amp IC?

Operational Amplifiers are widely used in electronic circuits as amplifiers, comparators, voltage follower, summing amplifier etc. Most of the commonly used Op Amps like 741, TL071, CA3130, CA3140 etc have same pin configurations. Hence this tester is useful to check the working of the Op Amp during trouble shooting or servicing. It is an easy to make tool which is essential in the work bench of a hobbyist or technician.

The tester is wired around an 8 pin IC base into which the IC to be tested can be inserted. Pin 2 (inverting input of IC) is connected to a potential divider R2, R3 that gives half supply voltage to pin 2. Pin 3 (None inverting input) of IC base is connected to the VCC through R1 and a Push to on switch. Output pin 6 is used to connect the visual indicator LED via the current limiting resistor R4.

The design is a voltage comparator. Insert the IC in to the socket with correct orientation. The notch at the left side of the IC should match with the notch in the IC base. In this comparator mode, the output of IC1 goes high when its pin 3 gets a higher voltage than pin 2. Here pin 2 gets 4.5 volts (if battery is 9V) and pin 3, 0 volts.

So the output remains low and LED will be dark. When S1 is pressed, pin 3 gets higher voltage than pin 2 and the output of IC turns high to light the LED. This indicates that the circuitry inside the IC is working.

Testing topologies:

There are three testing topologies in op amp

• The two operational amplifier test loop
• Self test loop
• Three op amp loop

Ic1a Op Amplifier

Now you have got an idea about the pin configuration and opam IC’s if any queries on this topic or on the electrical and electronic projects leave the comments below.

Video Showing comparision of the first 4 ICs

Ic1a Op Amp Diagram