A Dozen Small Battery-Powered Alarm Projects

  
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This is a selection of small self-contained alarm circuits. The main features of each alarm are described on the circuit diagram itself. They all have a very low standby current. So they are ideal for battery operation. Each pair of circuits will print out on an A4 sheet. Although they are described as alarm circuits - they will have other applica
A Dozen Small Battery-Powered Alarm Projects - schematic

tions. Sw1 is drawn as either a micro-switch or a magnetic-reed contact but - so long as it does the job - you can use whatever type of switch you like. Use more than one switch if it suits your application. The output device is a "piezo" buzzer - requiring a current of about 10mA. But - you can replace it with a relay - and use the relay contacts to switch whatever device you like. Just make sure that the relay coil doesn`t draw more than about 50mA - otherwise the transistor might be overloaded. The Cmos 4093 is the Schmidt-Trigger version of the Cmos 4011. For present purposes the two are interchangeable. However, the 4093 has an improved switching performance. It`s most noticeable when the time periods are extended substantially. The precise length of any time period will depend on the characteristics of the actual components you`ve used - especially the tolerance of the capacitors - and the exact switching voltages of the Cmos Gates. In the case of circuits 11 & 12 - treat the values of R6 & R7 as a rough guide. The switching point of Gate 3 and the characteristics of the thermistor will determine the actual temperature range available. Changing the value of R6 will allow you to access different areas of the temperature scale - while changing the value of R7 will allow you to alter the width of adjustment available.



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