Infrared Alarm Barrier Circuit

This infrared alarm barrier can be used to detect persons passing through doorways, corridors and small gates. The transmitter emits a beam of infrared light which is invisible to the human eye. The buzzer at the output of the receiver is activated when the light beam is interrupted by a person passing through it.

Infrared Light Alarm Transmitter Circuit Schematic

Infrared Alarm Barrier Receiver Circuit Schematic

How to make an infrared barrier alarm

The transmitter and receiver circuits of the infrared alarm system shown here have been designed for a range of several meters, almost independent of ambient light conditions. Only in the rare case of the receiver sensor being exposed to bright, direct sunlight, some screening measures have to be added.

The transmitter does not emit a continuous infrared signal, Rather, it is modulated, that is, the 36-kHz carrier used to pulse the IRED (infrared emitting diode) on and off is itself switched on an off at a rate of about 300 Hz. The reason for doing so is that most infrared sensors, including the ones suggested in the diagram do not respond very well to continuous incidence of infrared light. Switching the IR source off, even for a small period, allows IR detectors to ‘recuperate’, and so optimise their ability to minimize the response to ambient light.

The transmitter consists of two oscillators built around the ubiquitous 555 IC. Here, the current-saving CMOS version TLC555 (or 7555) is used. Alternatively, the two 555’s may be replaced by a single TLC556 (or 7556). IC1 is the 300-Hz generator, IC2, the 36-kHz source. The IRED type LD274 is pulsed at a relatively high peak current via driver transistor T1. If in your application the distance covered by the IR beam is relatively short, the value of resistor R5 may be increased to save on current consumption. Preset P1 is adjusted for a carrier frequency of 36 kHz exactly (failing test equipment, adjust it for optimum range).

The receiver is equally simple and also based on a CMOS 555. As long as the sensor picks up infrared light from the transmitter, the reset input of the 555 IC is held low and the buzzer is silent. Components D1 and C2 act as a low-frequency rectifier to cancel the effect of the 300-Hz modulation on the transmitter signal. When the infrared light beam is interrupted, the oscillator built around the 555 is enabled and starts to produce a warning tone.

Finally, the test values indicated in the infrared barrier alarm circuit diagram are average dc levels measured with a DVM, under light/no light conditions. In fact, most test points carry rectangular or sawtooth waveforms.


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  • Sangita Mitra

    The receiver circuit is not working with NE555.Can it be solved by using TLC555.Plz reply and I will be obliged if u give any other solution.

    • Ernie

      In the Jaycar Catalogue page 56 under linear ICs, is an IC , called an LM7555. It works. There catalogue part number is ZL-3455

  • Jim Keith

    Reduce the resistance of P1 until the receiver fails to respond–this provides minimum output pulse width and minimum LED power dissipation–observe short voltage pulses at collector of T1.

    The oscilloscope can be used to focus the beam accurately by observing the voltage across C2 while rotating the the transmitter in either axis–tune for minimum voltage of the sawtooth waveform–if it hits zero V, further reduce the value of P1. Then rotate receiver while observing the same point to get minimum voltage again–then fix orientations of both transmitter and receiver.

    • Ernie

      Thanks Keith for your comments, I best I could manage was to do a bit of both of your suggestions, adjust P1 to zero ohm, & while RX is buzzing, start adjusting P1up. I did this outside (but with RX not in direct sunlight) & progressively adjusted P1 & separated units farther apart until I got a good 3/4m apart & RX stopped buzzing. I did try the CRO on T1 assumed the oscillating signal would vary in amplified from 1-5V but it didn’t. I’m out of my depth with that I think. The unit works well now so I’m happy with it. Thanks again.

  • Ernie

    Hi all, Can anyone please help me tune circuit regards component P1. I do have an oscilloscope but don’t know which pin to connect it to.

  • Ernie

    Thanks for posting the article “Infrared alarm barrier circuit”, (using 555 Ic’s) Works great. I see u also posted a ” IR High Power Transmitter ” version using a 4047 IC. Would this work with the other Receiver circuit?

    • Jim Keith

      This should also work with the receiver on this project, but the range will probably be less–while it perhaps offer more average power, the peak power is lower than the transmitter indicated in this project. The peak power of the short pulses is what gives it good range. The object is to set the transmitter for sufficient power, but not excessive power as any additional power serves only to increase the LED temperature rise–the LED may already be operating close to its maximum temperature rating.

  • patrick

    Hi still wainting on reply

  • patrick

    Looks Great do you have pcb design or veroboard design you could send to me.
    Many Thanks

  • jhunronics

    hi,do you know the distance between the receiver and the transmitter cant operated normally.thanks

  • stephanie

    how long does the alarm sounds when i just interrupted the signal for 1 sec?\
    pls repz ASAp

    • Leila

      Out of the box, these worked percfetly in our Nissan Pathfinder. I was so pleased & relieved I would not have to hassle with returning them through the mail. The only (minor) thing I don’t like about them is that they don’t adjust small enough for very little heads. They should be fine for my 7 year old, though. MUCH better than wasting time trying to search for the same kind that came with our vehicle no need!!

  • Pistike

    for tan :1N5819

  • eng

    any one have this circuit by proteus program