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Simple Shop Alarm: Build a 3-5 Second Beep Timer
Description
This represents a straightforward - uncomplicated - alarm circuit. For power - a small 9-volt battery was utilized. However - the circuit will operate effectively from 5 to 15-volts – simply select a buzzer appropriate for the voltage being employed. The circuit’s standby current is negligible – therefore – the battery lifespan is quite extended.
When SW1 is connected as a magnetic-reed switch - a brief tone produced by the Buzzer is triggered each time the door opens. In unattended retail environments or reception areas - the sound of this tone provides immediate notification of a customer’s presence. The duration of the output is determined by the values of R2 and C2. Utilizing the values presented – the output will last for approximately 3 to 5 seconds.
Nevertheless – adjusting these values can extend the output time to up to thirty minutes or even longer. Consequently - by substituting the Buzzer with a relay – and utilizing the relay to activate a Siren – a Basic Intruder Alarm can be installed in a wide array of locations.
A magnetic-reed switch was depicted as SW1 – however – any switch type suitable for your application can be employed. For protecting multiple doors or windows – multiple switches can be utilized – simply connecting all switches in series.
Changing the Output Time
Generally - the length of the output duration is proportional to the values of R2 and C2. Specifically – if the value of either R2 or C2 is doubled - the output time will also be doubled. Conversely – if either R2 or C2 is halved – the output time will be halved.
For example - replacing R2 with a 4.7M7 resistor will increase the output time by approximately a factor of five. Substituting C2 with a 470uF capacitor will increase the output time by approximately a factor of one hundred. Combining a 4.7M7 resistor and a 470uF capacitor will increase the output time by a combined factor of approximately 5 multiplied by 100 – resulting in an output duration of roughly half-an-hour or more.
To achieve an accurate output time – utilizing a variable-resistor (or preset) in place of R2 is recommended. Then – simply adjust the resistor to obtain the desired output duration.
Veroboard Layout
Parts List
