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Door Entry Alert

Discussion in 'General Electronics Discussion' started by Kolero, Dec 8, 2010.

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  1. Kolero


    Dec 8, 2010
    Hi all. I'm new here, so I hope this is in the right forum.

    Here's the situation: I have friends who just opened a sandwich shop. They have two doors: One to the outside, on on a perpendicular wall leading to the lobby of an office building. Normally, only the two of them are working and if they're both in the kitchen, they'd like to be alerted to a customer's presence. They both hate the sound of door chimes, or bells on the door, or a motion sensor that goes off when something passes in front. They need a sound to go off in the kitchen where customers won't hear it. They also have limited funds since they just opened the store.

    As it gets hot in their store when they're cooking, they also like to leave one door open to let the heat out (and I suppose to tease the office workers with the smell of fresh cooked food). A magnetic reed switch won't work if a customer walks in the already-open door.

    My solution: an infrared proximity sensor, mounted to the door frame above the door (will be shaded from direct sunlight). I bought a Sharp GP2Y0A21, which has a detection range of 4" to 32". For a 7' high door, this should mean that anyone 4.5' tall should trigger it. It has an input voltage of 4.5-5.5 volts, and the output voltage, depending on how close to the sensor an object is, is 0.4-3.1 volts, approximately. I expect that people who are five to six feet tall would cause the sensor to output 0.5 to 0.9 volts.

    I have a separate circuit, with two 555 timers, as an alarm. It just goes beep-beep-beep-beep with trim pots to vary the tone and duration. This circuit runs on a 9v battery.

    My Problem: How can I link the two together? So the proximity sensor causes the alarm to go off when the voltage exceeds 0.5 volts. The proximity sensor doesn't output enough voltage to drive the 555 timers.
  2. eptheta


    Dec 20, 2009
    Operational Amplifier ? It's actually very straight forward and it can sense voltages in the mV range without the need to draw current.

    You just need to set your reference voltage to something slightly lower than your expected 0.5V and it will give you a nice output on the other end. Basically using an Op-Amp as a voltage comparator.

    LM741 should do the trick (and they're cheap!)
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