Nuts & Volts - January 2006

■ FIGURE 3. Circuit Board in Enclosure.

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 connect-ing/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

PARTS LIST

SEMICONDUCTORS

❑ IC1 LM324N quad op-amp

❑ LED1 T1 3/4 dual color red/green, two-lead LED

RESISTORS (all 1/4W, 5%) ❑ R1,R7,R8,R10-R12 100K ❑ R5 470Ω ❑ R6 1M ❑ R9 220K ❑ R13,R14 1K ❑ R2-R4 10K linear pot

CAPACITORS

❑ C1 1µF 16V ceramic or other nonpolarized

ADDITIONAL PARTS AND MATERIALS ❑ SW1 SPDT center off submini toggle switch ❑ SW2-3 SPST submini toggle switch ❑ Knobs (to fit potentiometers), case (PacTec HM- 9VB 4” x 2 1/2” x 1” or similar), LED holder, 9V battery connector, PCB or perf board, 14-pin socket, ribbon cable or other hookup wire, 9V battery

A printed circuit board pattern is provided on the Nuts&Volts website ( www.nutsvolts.com) 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