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Is there any sensor which can detect any object placed anywhere within that sensor range?(minimum 3 inch to 4 inch with low cost)
please send me the link of website where i can get the knowledge regarding that sensor.
Optical sensors work fine for that kind of thing. The Range is pretty much as far as you want it to be. The sensor typically sends out a modulated IR signal, bounces it off a reflector and triggers it's alarm line when an object interrupts the beam. Alternatly it can detect the IR beam being reflected by a nearby object, but the reflectance in IR and the angle is important then. Do a Google search for 'optical sensor' and you will find more than a few companies that sell them.
There's electronics of some kind in that unit colin. From a 2us input pulse it generates a 200us 40khz burst and detects incoming 40khz and toggles the I/O line. It uses only a single digital I/O line and could be run from a loop easily in basic. Your suggestion while mentioning cheap doesn't mention where you can buy those sensors cheap from and requires significantly more resources on the pic to implement by FAR. The PING module requires no tone generating and requires no tuned buffer to detect the 40khz tone. So apparently nothing you said has any validity to it whatsoever. 30 bucks seems a bit much, I'd be happier with 20, but unless you can get the ultrasonic sender and receiver module dirt cheap elsewhere and you want to bother with all the extra code it's worth it.
So figure 12 bucks for sensors, 5 bucks for the strip/perf board and an hour for assembly and testing, not to mention the time spent writing the code if that code you linked to doesn't run on your micro controller. Cause the ping module albiet expensive can be done from a simple counting loop in basic. I would seriously give the ping module thought. Personally I'd go the discrete sensor route but thats' okay because ultrasonics can be used for communicating as well if you have direct access to them.
I've made a 40kHz sensor, similar to the PING))) board, and I've used the PING))) board. The PING board is MUCH more advanced that what I came up with. For example, when my 40KHz emitter would send a signal (chirp) the receiver would resonate which would look like a reflection. My MCU would have to learn this initial resonation and filter it out. The problem is that when the object to sense was very close to my board, the reflected signal would interfere with the initial oscillation. Depending on the exact distance, the interference would either be constructive or destructive. It made short range sensing VERY difficult (about 1").
The PING))) board is much more complex. It has multiple selectable op-amp gains and it adds a varying amplitude PWM signal to the received signal to eliminate the signal seen by the initial resonation. My board was basically a fixed gain -> rectifier -> peak hold circuit while the PING))) was ... well, all around better.
If you only need one board, its worth it to spend $30 on the PING))) board than to come up with your own solution.