8/18/2019 Páginas DesdeRadio Electronics July 1985-3

1/2

ULTR SONI

PEST·REPELLERS

FIG.   SCHEMATIC DIAGRAM of a

popular

ultrasonic pest repeller. Despite its

simplicity

, the device

was remarkable effec tive.

c

C

livering 16-20 watts in the ultrasonic re

gion and special high-power tweeters.

Certainly that little plastic box didn't con

tain a 20-watt power amplifier or high

power tweeters . Also, a 16-20-watt power

amplifier drawing only 4 watts from a

supply would be about as close to per-

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Now that the pest repeller had done its

work, I began to speculate on its circuit.

An early article on the use of ultrasonics

in insect and rodent control ( Electronic

Pest Control by LymanGreenlee, Pop-

ular lectron ics July 1972) indicated that

the repeller needed a power amplifier de-

The claims made for those ultrasonic pest repellers seem

fantastic at first

gl nce but

they really work. In this article,

we ll find out what makes those devices   tick .

RO ERT

 

S OTT

OVER

T H E

YEARS

, T

HERE

H AV E

BEEN

many art icles published that proclaimed

that ultrasonics, either in the form of

pulsesor a sweep signal, can be used as an

effective insect and rodent repellent.

I' ve

always been skeptical of such claims and

placed them in the same category with

those electronic devices claimed to pre

vent swa llows from ne stin g on th e

court house roof and prevent pigeons from

defiling the Stonewall Jackson statue on

the town square. Nevertheless , I filed

those articles away for investigation

sometime in the future.

Last summer, my hunting and fishing

club took possession of a farmhouse that

had been abruptly abandoned about a year

ago. The house was absolutely overrun

with mice and roaches that were bold

enoughto scamper about in full daylight.

We were at a loss as to how to get rid of

them.

Ultrasonic pest repellents had begun to

appear in mail-order advert ising and our

club president suggested that we try one.

Those devices, accordingto the literature,

generate a signal that sweeps over a fre

quency range of approximately 22 kHz to

65 kl-lz, develop sound pressures ranging

from   5 to 152 db , and repel pests in

areas of 2500 to 3500 square feel, Pow

er

consumption is typically 2 to 4 watts.

Prices range from 30 .00 to 70.00, plus

shipping. .

At first, I scoffed at the suggestion that

we purchase an ultransonic pest repeller,

but agreed to try one since theywere avail

ablefo r a 30-day trial and full refund. The

 30.00 model was available from several

sources under name s that include Pest

Control, Pest-Elim 1500, and Westronix.

Weordered one and it came within a few

days . It was shipped in a plain unmarked

carto n and weweresurprised to findthat it

did not carry a trade name or model

number. Weinstalled in the clubhouse.

Within twoweeks, mice and roaches were

nowhere to be seen--even when lights

were suddenly turned on in a dark room.

Now, we consider the clubhouse com

pletely free of pests. Not a sign of them:

even in the darkest corners and crannies.

61

8/18/2019 Páginas DesdeRadio Electronics July 1985-3

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on the line from the power supply modu

la te s t he ult rason ic freque nc y. Two

Darlington-connected NPN transistors

provide some power amplification and

drive for the speaker.

Circuit for experimenters

If you wan t to experiment with the

effects of continuous or pulsed high-fre

quency signals , ' the circuit in Fig. 3 is

idea l: it can provide either a con tinuous or

pulsed output. It was developed by Sig

netics and described in

Electronic Prod-

ucts Magazine .

Looking at the circ uit. one 555 timer,

IC2, generates the ultrasonic squarewave

at a recommended 20 kHz . That signal

can be supplied continuously or pulsed on

and off by a second 555. ICI.

Exp erimenting wi th frequency and

dutycycle is easy. Duty cycle is the  on 

time compa red to the total period, and can

be set from slightly above 50lk to almost

1009'c. In the asta ble mult ivibrator circuit ,

the duty cycle is set by the timing re

sistors . RTI and R

T

2. and is equal to RTI

  (R

T2

/R

T

I)   2R

r

. .

The on time is close to 100 lk when RTI

is chosen to be as small as practical while

limiting the current through the discharge

transistor to the maximum spec ified in the

data sheet. (The discharge transistor.

which is on-board the 555 , is an open

collector NPN device with the collector

going to pin 7 and the emitter to ground at

pin I. The maximum current through it

varies with differe nt manufacturers so you

should check the maker's data sheet' to be

sure .)

If you want a duty cycle of less than

509c 

connect a general-purpose silicon

diod e such as the IN914 across R

T

2 with

its anode at pin 7 and cathode at pin 6.

That effectively shorts R

T2

while timing

capacitor C

T

is charging, and the duty

cyc le is now (R

T2

/R

T

l)   R

T2

and it can

be varied from around 0 to nearly 1009c.

The frequency of the squarewave gener

ator can be found from 1.44/C

T

(R

T

I

 

2R

T2

), where resistance is in megohms

and capaci tance in microfarads.

If you want to vary the duty cycle of the

osci lla tor while keeping the frequency

cons tant , use the basic circuit shown in

Fig . 4 .

In that circuit , a single potentiometer is

used for the two timing resistors . In that

scheme it is possib le to set the value of

one of the two  timing resistors to zero.

As that is undesirable, two resistors , RI

and R2, have been added to set minimum

values for those timing resistors.

Use the basic circuit shown in Fig. 5

when you want to vary frequency while

keeping the duty cycle constant at approx

imately 50  .The variable element used

in that circuit, R, a and

 

b, is a two

gang linear potentiometer. Note that the

value of the two variable elements are

continued on page 82

frequency. That sweep of from 25 kHz to

65 kHz issurpr ising ly clo se to the 22-65

kHz range specified in the ads. The speak

er is a 2 inch piezoe lectric tweete r.

How the French do it

The circuit in Fig . 2 is a pest repe ller

described in the French electronics maga

zine , Le Haw Parleur. In the art icle, the

author claims that frequencies in the range

of 20 to 40 kHz cause highly uncomforta

ble cavities to form in brain fluids and

bloodvesse ls of mice and insects , causing

them to beat a hasty retreat. Rad iated

power levels can be as low as y watt .

Looki ng more closely at the circ uit, a

quad two-input NAND gate is connected as

multi vibrator operating at around40 kHz.

With the

minimum

of filtering used in the

power supply, a residual 120-Hz sawtooth

Sl

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a

FIG. 3- THISCIRCUITIS ideal for experimenting with ultrasonic signals. It can supply either a pulsed

or continuous output.

petual motion as one can come. Thus , I

couldn't wait to pry open the repe ller 's

x   x plastic case and see

wha t made it  tick .

Figure I is the circ uit of the device we

tested .' We were quite surprised to find

that the circu it was simply a 555 timer IC

connected as a squarewave generator. Its

base frequenc y is approximately 45 kHz,

as determined by the values of Rl , R2,

and C l .

The 45 -kH z  carrier

  is frequ ency

modulated by a modified trapizoidal volt

age waveform app lied to pin 5 of the 555

time r. Tha t modulating voltage is de

veloped by a network consisting of C2 ,

R3, and R4 connec ted across one leg of

the bridge rectifier. A check with an os

cilloscope showe d a sweep of approx

imately 20 kHz on eac h side of the base

FIG.

  THIS

PEST REPELLER of French design is built around a CD4 quad NANO gate. The

schematic diagram is shown in a; a block diagram of the IC is shown in b .