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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