INTRODUCTION
Need help in making up your mind? Just ask the SM-200K Decision Maker. Press the ask button and the right
answer will be given to your question. The Decision Maker has six light emitting diodes (LEDs) that are driven
ON and OFF one at a time in sequence. When the ASK button is pushed the LEDs will flash and the buzzer
will sound. After a brief period of time only one LED will remain lit. Above the lit LED is the answer to the
question asked. In a minute or two the LED will get dim and gradually go out.
The SM-200K uses very small surface mounted components. By building this kit you will obtain an interesting
electronic device and also gain valuable experience in surface mount technology.
THEORY OF OPERATION
Figure 1 shows the block diagram of
the Decision Maker circuit. It consists
of 6 LEDs driven by a decimal counter
and two oscillators. One oscillator
drives the decimal counter and the
other drives a buzzer. The counter and
the oscillators are controlled by two
timers. We shall proceed to study the
circuit in detail.
DECADE COUNTER
The 4017 IC is a 5 stage divide by 10 counter. Figure 2 shows a
diagram of the IC. The IC has 10 outputs and a clear input. Only
one of the 10 outputs will be high at any given time. The other 9 will
be low. Let us assume that output 1 is high. If a pulse is fed to the
clock input, output 1 will go low and output 2 will go high. Each
clock pulse will move the output one position. If we connect an LED
to the output, it will light only when the output goes high. It is
obvious that when the clock is running, the LEDs will flash ON and
OFF with the speed of the clock. When the clock stops only one
LED will be lit.
In this design 6 LEDs are used per IC. But the counter has 10
outputs. If the clock stops at an output without an LED, nothing will
light. To prevent this the 4017 IC is preset after hitting the 6th
output. This is done by tying the 7th output to the clear pin (pin 5
and pin 15 shorted together).
LIGHT EMITTING DIODES (LED)
The operation of the LED is very simple. When current flows through the LED it will emit light. Note that the
LED is connected between an IC output and ground through a resistor. When the IC output goes high the LED
will light. The resistor limits the current so that the LED will not be damaged.
-2-
Figure 1
Figure 2
Timer 1
Osc 1
Decade
Counter
Osc 2
Timer 2
Button
Ask
B+
Buzzer
LEDs
Block Diagram
Clock
Clock
Enable
Reset
Q0
Q1
Q2
Q3
Q4
Q5
Q6
Q7
Q8
Q9
C
OUT
3
2
4
7
10
1
5
8
9
11
12
14
13
15
V
DD
= Pin 16
V
SS
= Pin 8
