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ULTRASONIC BURGLAR ALARM
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| Circuit designed by Phil Townshend 1995 This alarm system works using ultrasonic sound. It transmits a sound wave at around 40KHz which is way above human hearing range. This sound is received and monitored for any change in its signal. For example if someone walks in the path of the sound wave this will cause a change in the reflected wave and cause the circuit to respond. The unit has a range of between 3 and 4 metres and is ideal for small rooms and enclosures. The ultrasonic transducers can be mounted on the PCB directly or mounted on the case with short connecting wires. If a long lengths of wire are required then thought should be given to using screened cable, particularly for the receiver transducer. The ultrasonics trigger a 555 monostable when a signal is detected. This acts as the timing for the alarm duration which will reset after a set time, dependent on VR2/C4 using the formula t=1.1 x CR. The values shown set the alarm time between 2 and 200 seconds. A high output siren can be connected since the monostable has a Darlington driver on its output giving a current capability of around 500mA. Any more than that and you should either add a heatsink or replace the BFY51 transistor for something with a higher collector current (Ic). Operating voltage is from 9-12 volts and the unit consumes around 10mA when not triggered. This will not really be suitable for a PP3 but a pack of 6 AA batteries would suffice. Ideally rechargeable AA's should be used, but due to them being only 1.2 volts instead of 1.5, you would need to use 8 of them to give you 9.6 volts. |
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Block diagram: (click for pdf download)
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| Setting up this circuit will ideally need an oscilloscope. Set the transducers facing away from you and pointing into the room in the same direction. Attach a probe to the bottom of the 2K2 (R9) resistor connected to T4 and monitor the amplitude of the signal on the oscilloscope. If there is no or very little signal then aim the transducers at a closer wall to get a better reflected signal.
The photo shows BC109 transistors for T3 and T4. These are metal case equivalents for the BC549. If you use BC109 then you can touch the probe to the case since it is the collector connection Adjust the 47K preset carefully looking for the maximum signal on the scope. This is the resonant frequency of the transducers and is around 40KHz. If you do not have access to an oscilloscope the same procedure can be taken using a multimeter set to AC volts and an average reading taken. Adjust to get the highest reading |
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All components are fairly standard. Take care when soldering in the 0A91 germanium diodes as theyare quite sensitive to heat. Use a CMOS version of the 555 timer as this will improve on current consumption.
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