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OSCILLATORS
The SM-200K uses two oscillators. The first oscillator produces
a frequency of about 2000 hertz (cycles per second) and the
other produces a frequency of about 20 hertz. Figure 3 shows
the basic oscillator circuit. The 4011 integrated circuit (IC)
contains four-two input NAND gates. Two of these NAND gates
are needed to form an oscillator. Feedback for this oscillator is
via capacitor C1 and resistors R1 and R2. These elements
determine the frequency of oscillation. Both IC1 and IC2 act as
inverters, that is when the input is low the output is high. As long
as pin 1 of IC1 is high the circuit will oscillate. If pin 1 is brought
low, the circuit will stop oscillating.
The second oscillator is similar to the one described except for a difference in its frequency controlling
components, capacitor C2 and resistors R3 and R4. C2 and R4 are smaller values causing the oscillator to
oscillate at a much higher frequency. Pin 8 of the first NAND gate is brought high at a 20 cycle rate. This causes
the second oscillation to be chopped up at the first oscillation frequency rate as shown in Figure 4. This
combination produces the unusual sound heard from the buzzer.
BUZZER
The SM-200K buzzer consists of a piezoelectric material on a metal base. When a voltage is applied to a
piezoelectric material its dimensions change. The buzzer is connected to the 2kHz oscillator. When the
oscillator runs, the changing dimensions of the piezoelectric buzzer act like the cone of a speaker to set up
sound waves.
TIMERS
Timer 1 is made up of capacitor C4 and resistor R6. Timer 2 is made up of capacitor C3 and resistor R5 (see
schematic diagram). When switch S1 is pushed both capacitors charge up to 9 volts and the oscillators run.
When the switch is released, capacitor C4 discharges through resistor R6 and capacitor C3 discharges through
resistor R5 and the base of Q1.
Note that C4 is tied to pin 1 of the 4011 IC. Thus, when capacitor C4 loses its charge, the circuit stops
oscillating. The buzzer stops and a single LED remains lit. After a minute or two, C3 loses its charge and
removes the base current from Q1. Q1 is in the ground return path of the LEDs and both ICs. Thus, when Q1
is turned off, the LED goes out. Only a very small leakage current is then drawn from the battery. This current
is so small that no ON/OFF switch is required.
Figure 3
Figure 4
