Simple flow-air detector

by Ciro Carbone


This simple circuit is a singular circuit: is a simple flow air detector that uses the filament of the incandescent lamp exposes to air without its protective bulb glass. Here following there is a brief description of functionality and relative images that I've made during my work.



The functionality is based over incandescent lamp ( I choose once 220V-40W) inserted among others electric components.
Electric diagram is visible in figure below:

Principle is based on decreasing of electrical resistive value of the lamp's filament when some air flow crosses and cools it.
In normal condition, the filament's voltage (pin 3) must be higher then reference voltage of the trimmer (pin2). Because pin 2 is inverting input and voltage is lower then not-inverting input, the output of the operational amplifier (pin 1) will be of high level (approximately 12Volt) and therefore led and buzzer will be off.
When an air flow crosses the filament, voltage at pin 3 will be lowered and it become inferior then pin 2, therefore the inverting input prevails and the output (pin 1) switches to the low level (about 0Volt) concurring the lighting of the led and buzzer.
I have chosen one CA358 dual op-amp chip. CA358 is used like voltage comparator. In dual chip, the second op-amp module is not necessary and configured with negative feedback like voltage-follower. Output (pin 1 and pin?) drives the led lighting and beeper buzzing through BD138 – pnp transistor.
Moreover a 12V voltage stabilizer/regolator IC (LM7812) is needed to stabilize the 12V voltage value to avoid current fluctuation in the lamp filament.


To make device, I have used universal electric education board and a voltmeter tester.
Only one calibration is required: 10Kohm variable trimmer-resistor connected to pin 2. To adjust it, you can rotate the trimmer at the end point to light the led and then rotate slowly in the other side ( clockwise) untill the led turns off.
However, it is advisable to calibrate the device with a voltmeter/tester, comparing two voltage values at pin the 3 and pin2: i.e. in normally condition whether the value at pin 3 is approximately 6.5V , you will stop the adjustment of the trimmer to just value between 6.3-6.4Volt at pin2. Through trimmer calibration you can obtain greater or smaller sensibility.



More boring could be the break down of the glass bulb at the same time being attention at not damage filament inside. However, by a vise, transparent tape and a cutter and whatching short video above, the operation is easy in two minutes.

Then the filament can be inserted "ad hoc" in one of those tubes coils used in kitchen absorbent paper/napkin .

Parts list:

- n.1 integrated circuit LM7812

- n.1 integrated circuit CA358

- n.1 direct current buzzer

-n.1 transistor BD138

-n.1 trimmer 10Kohm

-n.1 led

-n.2 resistors 150 ohm 1/4W

-n.2 resistors 6800 ohm 1/4W (or one 3300ohm resistor 1/4W)

-n.1 resistor 220 ohm 1/4W

-n.1 resistor 680 ohm 1/4W

-n.2 resistors 1000 ohm 1/4W

n.1 incandescent lamp 40W