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 Simple presence Alarm devices circuit
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Posted - 01/12/2018 :  02:00:23 AM  Show Profile  Send kynixliu an AOL message  Reply with Quote
You leave this unit on in some way and when someone approaching it will emit a series of beeps and turn on some LEDs that will flash for a few seconds. It is an ideal assembly to be placed in the entrance of shops, offices announcing the presence of customers or even in places to which the attention of the passers should be called. Professional applications include alert for entry into restricted or dangerous locations next to machines and other devices. The circuit is simple and powered by common batteries.

Devices that are automatically triggered by the presence of people can be used in many important practical applications.

In a store, for example, it can be placed next to a gift counter, attracting the attention of customers passing by.

In the children's room, undoubtedly, it will mean an extra fun and even a kind of alarm, as it will make noise at night if someone does get up or come in and turn on the light.

And of course, there are the professional applications indicated in the introduction.

The unit is powered by standard batteries or a 9 V battery and its current consumption in standby condition is very low. Assembly is simple because it uses common components and the few adjustments needed do not require special instruments.

The circuit has two modes of operation that can be programmed by the way the sensor is turned on: by cutting / decreasing the light intensity or by increasing / increasing the light intensity.


* Power supply voltage: 6 to 9 V

* Consumption at rest: 0.5 mA (tip)

* Consumption in action: 15 mA

* Timing: 2 to 20 seconds


The sensor is an LDR (photo-resistor) that can be connected in series with a potentiometer of two forms. In the original form, shown in the diagram, it triggers a monostable 555 when the ambient light intensity increases. However, if we connect this sensor as shown in the figure we will have the drive by the cut of light or the decrease of its intensity.

Modification for operation with shadow or dark.

The P 1 potentiometer will adjust the sensitivity of the appliance according to ambient light.

The integrated circuit 555 as monostable has a timing given by capacitor C 2 and set in P 2 . This circuit will determine how long we will have beeps and LEDs on when the sensor circuit detects something.

The capacitor C 2 can be changed in the range of 10 #956;F to 220 #956;F in order to obtain other drive times, according to the wishes of each assembler.

When the 555 monostable trip occurs, its output corresponding to pin 3 goes high. This means that the LEDs corresponding to the indicator system light up continuously for the programmed time.

For an intermittent ignition (blinking) we can excite the LEDs with transistors connected to pin 4 of the integrated circuit 4093, as shown in figure 2. Transistor BC548

Flashing drive.

At the same time that the LEDs light up, two ports of the integrated circuit 4093 (CI- 1 a and CI- 1 b are activated in order to oscillate. The first oscillator generates an audio tone (beep) determined by R 2 and C 3 , while the second one generates the blinking given by R 3 and C 4 . These components can be changed over a wide range of values #8203;#8203;in order to change the type of sound output.

The two signals are combined in the other two ports of the 4093 circuit that act as digital buffers and amplifiers that deliver the signal to a piezoelectric transducer a pulsed signal for playback.

To operate with a higher audio level (but also higher power consumption) we can drive a loudspeaker with the circuit shown in figure 3.

Speaker excitation.

In this circuit, the power transistor must be mounted on a heat radiator. Power FETs that must also be mounted on heat radiators may also be used.

Types like the IRF640, IRF620 or any N-channel equivalent can be used in this amplifier stage.

For this case, if the use is in advertising, it will be interesting to feed the circuit with a source because the consumption is greater. A source for this purpose is shown in figure 4.

Power supply for alarm.

In this case, the amount of LEDs fed by the circuit suggested in figure 2 may be appreciably increased.


In figure 5 we have the complete diagram of the apparatus.

Diagram of the apparatus.

The arrangement of the components on a printed circuit board is shown in figure 6.

Plate suggestion for the alarm

For integrated circuits ( will be interesting to use sockets if the assembler is not very experienced. The LDR can be of ordinary round type, small or large.

The LEDs are ordinary red or other color and the resistors are all 1/8 W or larger.

The electrolytes must have minimum working voltages indicated in the material relation. The transducer is of the ceramic piezoelectric type or even a microphone or ceramic headset.

Note that the LDR must be inside a small opaque tube to capture light changes from a person's passageway just in front of you.

The use of a converging lens in front of this tube increases the sensitivity and directivity of the device. The lens should be positioned so that the sensor (LDR) is in focus, thus achieving the condition of greater directivity and sensitivity.


To adjust the appliance turn on the power supply and set P 2 to the minimum timing (least resistance) position.

In the version that operates by increasing the brightness (reflection) that is the basic point the tube with the LED to a wall or dark object (a black card, for example) and P 1 setting until the trigger threshold is reached.

The passage of a lighter object in front of the sensor, a person in light clothing for example, should cause the trip of the circuit to blink and beeps.

In the version that fires in the dark, aim the tube at a light wall or white paper and adjust P 1 to obtain the trigger threshold.

After getting the trigger in both versions, set P 2 to the desired timing, ie the duration of the beeping time.

If you want to change the beeps, change C 3 and C 4 or R 2 and R 3 . Proof of operation is to install the unit pointed at the light or dark place, redo the adjustment to the local conditions according to the version and let it work.


CI- 1 - 555 - integrated circuit, timer

CI- 2 - 4093 - CMOS integrated circuit

LED 1 , LED 2 - Common red LEDs or any other color

Resistors: (1/8 W, 5%)

R 1 - 10 k#937;

R 2 , R 5 - 47 k#937;

R 3 - 470 k#937;

R 4 - 680 k#937;

P 1 , P 2 - 1 M #937; - potentiometer


C 1 - 10 #956;F / 12 V - electrolytic

C 2 - 47 #956;F / 12 V - electrolytic

C 3 - 47 nF - ceramic or polyester

C 4 - 1 #956;F / 12 V - electrolytic

C 5 - 10 #956;F / 12 V - electrolytic


BZ - piezoelectric transducer

B1 - 6 or 9 V - 4 small batteries or battery

S 1 - Single switch

Printed circuit board, battery holder or battery connector, sockets for integrated circuits , mounting box, buttons for potentiometers, LDR opaque tube, convergent lens (see text), wires, solder, etc.

vivi liu

Edited by - kynixliu on 01/30/2018 01:06:42 AM
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