Step Alarm Circuit

This Circuit can monitor the door steps or Staircase. When a person crosses the steps, the alarm sounds indicating the entry. The circuit is too sensitive and operates in day light.

The circuit uses an NPN Darlington phototransistor L14F1. It senses the intensity of light through its exposed base and passes current in the collector- emitter path. When there is light, the phototransistor conducts and C1 charges up to its full voltage level. The alarm circuit uses a Monostable timer built around IC NE555. The triggering threshold of IC1 is adjusted using VR1. So normally the trigger pin 2 of IC1 remains high as set by VR1. When the shadow of the moving person masks the phototransistor, it turns off allowing C1 to discharge through R1. This momentarily makes the trigger pin 2 of IC1 low and the timing cycle starts. With the given values of R2 and C2, Buzzer sounds for two minutes.

Step Alarm Circuit


The circuit can be constructed on a small piece of Perf board. The circuit is designed to operate in day light. Switch off the circuit in the evening to avoid false alarm. If a lamp is provided opposite to the phototransistor on the opposite wall of staircase, the circuit can be used as day and night alarm. Align the lamp in the line – of – sight of Phototransistor. Adjust VR1 till the circuit becomes silent.


Join the conversation!

Error! Please fill all fields.
  • jignesh soni

    I have make transmiter and receiver when supply given to ir transmiter it work ok it emite ir waves bt same way photo transiator receiver. Pickup single time my requirement when any object pass between trans.& receiver then reciever active every operation also i require to hold output 1 or 2 mnt pls help me….

  • KD

    can u give some cautions

  • amit

    Sir i constructed step door alarm with the help of ur circuit diagram but it will creat a false sound in room pls sir any solution for this problem


    what is vr1 1m?

  • rayed

    sir culd a ldr be used instead of the L14F1

  • D.Mohankumar

    Dear Vincent
    L14F1 is the darlington NPN transistor with high gain. It has a built in lens in front of it to focus light into its pn junction .It is too sensitive and highly reliable.Other photo transistors like 2N 5777 can be used. But sensitivity will be low compared to L14F1.Keep the base pin of the photo transistor unconnected.

  • Vincent

    Dear sir, is that have another component that can replace the L14F1 phototransistor?

  • Ali

    Isaw your intersting site ,but Ididnot find a clear word about schmatics for security systms which ar working with magntic tags. thse systems are used in stores as an antithef system.
    At your best regard

  • godsent

    mahn you are off to a great kind in thinking. . . . . .
    could you just help me out in these kinda things. . . . . . .i need your help in my upcoming projects. . . .myself Godsent from Chennai. . .please reply for my comment. . . . . i need to talk a lot more about this. . . .

Looking for the latest from TI?