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Animal and pest repeller
A family cat always clawing my
prized speaker, a dog constantly marking an area and a possum chewing
off my seedlings were problems that
I needed to solve. I designed this circuit to deter their actions.
A common fear for the animals is
the sound of a spray can, so I designed
this circuit to simulate the repetitive
quick squirt of a can along with flashing red LED ‘eyes’.
The PIR sensor module, PIR1 (Jaycar Cat XC4444) detects movement and
sends out a high pulse with a duration
based on the delay setting of the PIR
detector. When this pulse is received,
it switches on power to the rest of the
circuit as it forward-biases the baseemitter junction of NPN transistor Q1.
This, in turn, pulls current from the
base of PNP transistor Q2, supplying
9V via its collector.
If night-time only use is required,
an LDR such as Jaycar Cat RD3485
or Altronics Cat Z1621 can be wired
across the 1MW base-emitter resistor
of Q1, as shown. This will prevent
triggering during daytime by shunting Q1’s base drive current due to
siliconchip.com.au
its lower resistance when exposed
to light.
When the PIR is triggered, power
is supplied to a multivibrator based
around PNP transistors Q3 & Q6 and
NPN transistors Q4 & Q5. This alternatively drives the LEDs via Q6’s collector, and the white noise circuitry
via Q3’s collector.
When current flows from Q3’s
collector, white noise is generated
by a reverse-biased small signal silicon transistor junction (Q7). This is
then amplified by NPN transistor Q8,
operating as a common-emitter amplifier, followed by audio amplifier IC1,
which drives a miniature 8W speaker.
I built the circuit into UB3 jiffy box
with LEDs as the ‘eyes’, the PIR as the
‘nose’ and the speaker as the ‘mouth’.
For best results, set the PIR sensor to
single trigger mode and adjust its trimpots to almost minimum sensitivity
and delay.
A piezo speaker can be used instead
of a standard speaker, but it will produce less volume. It will also reduce
the overall current drawn from the
9V battery.
Australia’s electronics magazine
The majority of small-signal transistors will stop breaking down to
produce white noise at 7.7V. Highfrequency types like the BF494 will
operate down to 6.8V, so if the battery
voltage drops below 8V, white noise
won’t be generated.
One solution is to place a 3V lithium
cell in series with the 100kW resistor
to Q7’s emitter, boosting the voltage.
There is negligible current drawn from
this cell. However, I found this wasn’t
required with a good 9V battery.
Overall battery drain is less than
100µA at idle, then up to 50mA when
triggered if the amplifier is set to maximum volume via potentiometer VR1.
The power switch is an SPDT centre-off type. The lower on position
powers the PIR module only, to enable
stabilisation which can take a minute.
Then the switch is thrown over to the
upper on position, which fully powers
the circuit. As the PIR module has a
wide view angle, it may be necessary
to mask some areas off to achieve the
required viewing angle.
Warwick Talbot,
Toowoomba, Qld. ($90)
February 2021 71
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