Distance using modulated led

[samarsingla]

hello,
I want to measure distance uto one meter using modulated ir and implementing a bandpass filter at the receiver. Pulse generation att 20 khz is ok using a 555. I got this link
**broken link removed**
for a filter at 20 khz. Can someone please tell me how is this circuit working?? and what is the output voltage? is it analog voltage?? please help.

 

[samarsingla]

And, one more question... does duty cycle of the input pulse to led play any role? if yes what?

 

[evandude]

If you're bandpass filtering and then peak detecting, your output is going to be essentially a DC voltage corresponding to the strength of the received signal. Well, it may look more like a sawtooth wave with a DC offset, depending on the time constant of your peak detector.

duty cycle determines the brightness of the LED. Obviously, a 100% duty cycle would make the LED be constantly on (bright as possible), and 0% would make it be completely off. However, here you need your signal to be modulated at 20KHz so you can't run it at either extreme. 50% duty should put the most energy into the 20Khz frequency component, which is what you want... any energy you waste in the harmonics won't contribute any performance to your sensor system.

 

[samarsingla]

thanx for the reply. i couldnt understand everything though. how should i decide wether i should be driving 555 in monostable or astable mode. both produce square pulses only.
what is the ideal frequency to be used for this? on what factors does it depend?
can you suggest a link on distance ranging using modulated ir. i need analog data corresponding to distance. i googled a lot but could not find anything to suit my needs.

 

[Sceadwian]

The frequency isn't overly important, just keep it a decent distance away from 35-40khz as most consumer remote controls modulate at that frequency. Keep in mind though IR distance sensing isn't absolute, it will depend on the objects ability to reflect IR light in the first place. A highly IR reflective object is going to be able to be sensed from a much further distance and appear 'closer' than an object that is made from an IR absorbing material. What are you trying to sense?

Since the 555 is just being used as an oscillator, you want astable. Monostable will only emit a single pulse.

If you need absolute distance you're going to have to use some method other than IR, such as acoustic.