Product Application Guide- Transducers- External Drive- Piezo
Part Numbering System
Transducer Series (PT / AST)
Part Number Structure PT – 1224 P E-05 Q
Series
PT = Piezoelectric Transducer
Unique Identifier
Drive Type & Termination
FP = Feedback Design & PC Pins P = External Drive & PC Pins
FW = Feedback Design & Wires W = External Drive & Wires
Special Designator
RoHS: Q = RoHS Compliant
Part Number Structure AST 605 TR Q
Series: AST = Surface Mount
Unique Identifier
Termination: TR = Tape-N-Reel
RoHS: Q = RoHS Compliant
Page 2
Piezoelectric Electronic Alarm Construction
The above cross section picture shows the basic elements used in a piezoelectric audible
alarm. The area in front of the transducer element including the front hole opening forms
an acoustic cavity that lets the sound radiate out with the most efficiency (i.e. loudest
sound level). If the alarm is an indicator that contains a circuit board, the circuit board is
attached to the piezoelectric sounder element via soldered wires.
The above picture can be interpreted to represent a board mount package with pc pin
terminations, but the same concept is used when building audible alarms in other
mounting configurations such as SMT, Flange Mount, and Panel Mount alarms.
If the back of the alarm is sealed with epoxy or other material, the “guts” of the alarm (including
the circuit board and components) are protected against fluid intrusion. However, fluid sitting
inside the front cavity can obstruct the operation of the device causing the sound level to
decrease significantly. If you need to wash the alarms after a soldering operation, it is strongly
recommended to use an alarm that comes with a wash label that keeps the washing fluid from
getting inside of the front cavity.
Page 3
Operation of Piezoelectric Audible Alarms
Piezoelectric electronic audible alarms work by converting the user input voltage to an
appropriate oscillating signal that is applied to a sounder element that is mounted in a housing.
The piezoelectric sounder element consists of a metal disc that has a special ceramic material
bonded to it that physically bends when voltage is applied to it.
The above picture shows a bare piezoelectric sounder element. By applying a sinusoidal wave-
form at an appropriate frequency, the transducer will physically deflect in one direction and then
in the opposite direction following the shape of the input wave-form. If this oscillation occurs in
the audible frequency range (1 Hz to 20 kHz), then air pressure waves are produced that the
human ear interprets as an audible sound.
The larger the voltage of the applied wave-form, the larger the amplitude of the air pressure
waves resulting in a louder sound level. However, the ceramic portion of the transducer can only
bend so far before there is a risk of a catastrophic failure. This maximum voltage is somewhere
around 40 to 50 volts. However, it is rare to apply this much voltage to a transducer as you reach
a point of diminishing returns for voltages much greater than 32 volts.
By itself, the sound level produced by a transducer element is insignificant. To increase the size
of the air pressure waves (and thus the sound level), the transducer element must be mounted
inside an acoustic chamber that is optimized for the transducer size and resonant frequency.
Every transducer has one frequency where it flexes more efficiently producing the louder sound
levels. This frequency where the transducer performs the best is called the resonant frequency.
Self-Drive type devices provide a 3rd terminal that connects to an isolated portion of the
piezoelectric transducer. This third terminal provides a feed-back signal that is 180 degrees out
of phase with the drive signal. This signal can be fed back into the circuit to allow the sounder
element to self-tune itself to the transducer’s resonant frequency.
Page 4
Decibel Sound Level Scale
The decibel sound level scale is an arbitrary scale that ranges from 0 dB (threshold of
hearing) to 130 dB (threshold of pain). The chart below shows where some common
sounds fall on this dB scale. Audible alarms are available that have sound levels as soft
as 55 dB at 2 feet and as loud as 110 dB at 2 feet.
Page 5
Fundamental Frequency & Harmonics
Below is a frequency scan of a piezoelectric audible alarm that has a resonant frequency
of 2,800 Hz. As you can see, there is a strong frequency peak at 2.8 kHz and several
smaller frequency peaks that follow called harmonic frequencies. The table below the
chart shows that the size of the harmonic frequencies are significantly smaller than the
fundamental frequency for this particular alarm unit. Because this alarm has a large
fundamental frequency and much smaller harmonic frequencies, the sound quality of this
part will be very good. When this alarm is activated, the listener will hear one clear
frequency (also called sound pitch) from the alarm. Other electronic alarm technologies
such as electro-magnetic or electro-mechanical type alarms often have much larger
harmonic frequency components resulting in less clear tone.
Page 6
Circuit to Increase Turn-On Voltage
Below is a circuit that can be used to prevent the alarm from sounding until a certain
voltage is reached. This particular circuit has a turn-on voltage around 10 Vdc due to the
10 volt Zener Diode, but you can just substitute other values of Zener Diodes to get the
needed turn-on voltage for your circuit.
Page 7
Controlling Sound Level- Electronic Method
For piezoelectric type audible alarms, the larger the voltage signal applied to the piezoelectric
transducer, the louder the sound level. This property can be used to electronically control the
sound level of these devices.
For audible alarm model MSR320R,
the sound level will vary from 65 dB
at 2 ft. at 3 Vdc up to 80 dB at 2 ft. at
20 Vdc. By varying the voltage from
3 to 20 Vdc, the sound level can be
varied by 15 dB. A 10 dB drop in
sound level will make the alarm sound
half as loud.
There are several ways of electronically controlling the voltage including:
1. Using a manual or digital potentiometer.
2. Using a selector circuit and different values of resistors.
3. Using PWM voltage signals from microcontrollers.
In all cases, care must be taken to make sure that the circuitry and voltage signals used do not
interfere with the internal circuitry of the audible alarm. This method of controlling the sound
level will probably not work well in the following cases:
1. The sound level of electro-magnetic type alarms do not vary much over the voltage range of
these devices, so most electro-magnetic type alarms are not suitable for this method.
2. When you only have a narrow operating voltage range to work with. For example, if you are
using the MSR320R at 5 Vdc, the sound level change from 5 Vdc to 3 Vdc is not very much.
3. When the voltage range of the audible alarm is narrow. For example, panel mount model
SC307NR only has a voltage range of 3 to 7 Vdc. Over this range, the sound level will vary
by only 6 dB. This sound level change is significant, but may not be enough to make a
difference in the application.
Page 8
External Drive Transducer Circuits
External Drive Transducers require a sine or square wave type signal at the appropriate
frequency for the transducer. A sine-wave type signal will produce more sound level
than a square-wave type signal. The higher the voltage level (up to the maximum
allowed) the higher the sound level will be. Below are two circuits that can be used with
external drive transducer devices:
It should be noted that the component values given are starting points only. For example,
the inductor shown is optional and the value of the inductor will definitely have to be
adjusted depending on the transducer used. Also, the oscillator portion of the transistor
circuit will have to be adjusted to match the resonant frequency of the transducer.
Page 9
Tube and Tape-n-Reel Counts- All P/N’s
Part Number
Tube
Count
Reel
Count
Part Number
Tube
Count
Reel
Count
ASI301Q
35
AST612Q
35
ASI301TRQ
250
AST612TRQ
450
ASI401Q
35
AST7525MATRQ
1000
ASI401TRQ
250
PB-1220PQ
40
AST100Q
25
PB-1221PQ
40
AST100TRQ
500
PB-1224PE-05Q
40
AST1109MLTRQ
1000
PB-12N23P-01Q
40
AST1240MLTRQ
1000
PB-12N23P-03Q
40
AST1440MATRQ
600
PB-12N23P-05Q
40
AST1575BMATRQ
300
PB-12N23P-12Q
40
AST1628MATRQ
800
AST1750MATRQ
400
AST200Q
48
AST200TRQ
1000
AST501Q
35
AST501TRQ
450
AST605Q
35
AST605TRQ
300
Page 10
Typical Failure Modes of Piezoelectric Audible Alarms
Component/Subsystem
Failure Mode
End Result
Occurrence
Circuit Components
(Resistors, Capacitors,
Diodes, IC’s, etc.)
Over-voltage by customer’s
application
Unit ceases working.
Vast
Majority of
Returns
Transducer/Wire Solder
Operation
Not enough wire strands in
solder joint
Wire breaks after
period of time & unit
ceases sounding
Rare
Physical Assembly
Transducer wire pinched,
adhesive/epoxy run down
onto transducer, or RTV
adhesive seal failure
Intermittent operation
Rare
Soldering Operation
Incorrect Solder Temperature
or Time Causing Cold Solder
Joint
Intermittent operation
or unit ceases working
after period of time
Very Rare
Circuit Components
Random Component Failure;
Wrong Component Used;
Missing Component
Unit ceases working
under normal
operating conditions
Very Rare
Transducer Wire
Defect in Wire;
Wire Strands Damaged in
Production
Wire breaks after
period of time & unit
ceases sounding
Very Rare
Piezo Transducer
Incorrect Polarization by
Manufacturer;
Glue Bonding Failure
Sound volume level
decreases over time.
Exceedingly
Rare
Notes:
1. Customer returns of Mallory audible alarms for failure to operate are very rare. Of the
few parts returned each year, the vast majority of the root cause of failure is an over-
voltage or voltage spike condition caused by the customer’s application.
2. All Mallory alarms are, at a minimum, function tested 100% during production, and a
final audit is performed. Mallory SC/SBM/SBT/SBS/SNP/LSC/VSB/MSR/MSO/ZA
series of alarms are audited 100% at final test by checking that sound level, frequency,
and current are within specification limits from 2 to 4 different voltage levels.
