Data Sheet

-3-
SENSITIVITY
The potentiometer, P1, adjusts the sensitivity of the
Sound Activated Switch. Varying the resistance of
P1 will vary the DC voltage at the input of the first
one-shot. Rotating P1 counter-clockwise causes
the voltage at the input of the first one-shot to
increase. This means that a louder clap is required
to activate the first one-shot, making the Sound
Activated Switch less sensitive to sound. Likewise,
rotating P1 clockwise causes the voltage at the
input of the first one-shot to decrease, making the
Sound Activated Switch more sensitive to sound.
THE ONE-SHOT
A one-shot, or monostable multivibrator, is a circuit
that, once triggered, will switch its output logic level.
The output will remain at this new logic level for a
predetermined period of time, after which the output
will switch back to its previous logic state.
The 4011 IC is a quad 2 input CMOS NAND gate.
There are two separate one shots, each using 2
NAND gates. Both one-shots are configured in
such a way that the normal steady state output is
equal to a high voltage, or logic 1.
When the first one-shot is idle, waiting to be
triggered, the capacitor C4 is completely
discharged. When the input at pin 1 goes low, due
to the waveform from C3, the capacitor C4 begins to
charge. The output of the one-shot changes from a
high voltage, logic 1, to a low voltage, logic 0. The
output will remain at a logic 0 until the capacitor C4
charges through resistor R8. The time the first one-
shot remains at a logic 0 is approximately .25
seconds. After the capacitor C4 charges, the output
of the one-shot switches back to a logic 1.
When the first one-shot’s output switches back to a
logic 1, the second one-shot is triggered, causing
capacitor C7 to charge. The second one-shot will
remain at a logic 0 for approximately .7 seconds.
Now, if the first one-shot is triggered again by
another clap within .7 seconds, both one-shot
outputs will be at logic 0. The outputs of both one-
shots are connected to the input pins of a NOR
gate. When both inputs of NOR gate U2A are at a
logic 0, the output will be at a logic 1. This output
pulse is then coupled to the input of the flip-flop.
THE FLIP-FLOP
A flip-flop, or a bistable multivibrator, is a circuit
whose output logic level changes when a pulse is
applied to the input. The output will remain at its
logic state until the next pulse is applied. The only
two possible output states for a flip-flop are a logic 1
and a logic 0.
The 4001 IC is a quad 2 input CMOS NOR gate.
The first NOR gate is used as its primary purpose,
a NOR gate. The next two NOR gates are
configured as a flip-flop.
When pin 4 is at logic 1, or 9 volts, pin 10 will be at
a logic 0. The voltage divider of R15 and R14 sets
pin 6 at approximately 2 volts. The voltage needed
at the input of the NOR gates to switch the outputs
from one state to the other is between 4.5 and 5.5
volts. It can be seen that the voltage at pin 6, when
pin 4 is at logic 1, is biased at less than the trigger
voltage. Capacitor C9 couples the output pulse
from the first NOR gate to the input of pin 6. The
pulse is now “riding” on the DC level at pin 6. The
peak of the pulse is now high enough in amplitude
to reach the trigger level of the flip-flop. The flip-flop
will now trigger causing its output logic state to
change.
When pin 4 is at a logic 0, pin 10 will be at a logic 1.
The trigger pulse is then coupled into pin 8 through
capacitor C8 to change the output state of the flip-
flop.
The last NOR gate is used as an inverting buffer to
separate the input of the transistor switch from the
output of the flip-flop.
THE TRANSISTOR SWITCH
When a transistor is biased on by a high base
current, its collector to emitter saturation voltage is
very low making the transistor look like a closed
switch. When a transistor is off, no base current, the
collector to emitter current is very low making the
transistor look like an open switch.
Resistor R16 sets the current through the base of Q2
at about 1 milliamp when pin 11 of the 4001 is at a
logic 1. This base current turns on Q2 causing
current to flow through the LED and R17. When
current flows through an LED, it will emit light. R17 is
used to limit the current flow through the LED so that
the LED is not damaged.
Although not included in the kit, a relay can be
connected to the two points indicated on the PC
board. When the LED is turned on, the relay will
pick. The relay could then be used to power up
other electronic circuits. If using a relay, we
recommend using a 7 to 9 volt with a coil resistance
of 500 ohms or greater.