Can anyone plez suggest me how to build a range finding sensors with the help of a 555 timer and a TSOP1738.
Quite a tall order. You could make an in range/out of range sensor that way, but not a real range finding sensor to reliable report distance.
A thing to note about some IR-receiver modules, among them the TSOP17xx, is, that you cannot use a continuous wave, as this will be filtered out as an error signal (makes sense when you think of what they're designed for).
From the data sheet:
After each burst which is between 10 cycles and 70 cycles a gap time of at least 14 cycles is necessary.
For each burst which is longer than 1.8ms a corresponding gap time is necessary at some time in the data stream. This gap time should have at least same length as the burst.
So, to use a TSOP1738 for your purpose, the best strategy is to use eg. 1ms bursts with 1ms pauses.
Some IR-receivers takes this error suppression a bit farther and won't even allow such a repeated signal with no variance, but I don't recall if that's the case with the TSOP17-series.
For best result, you need a 950nm IR LED, if using eg. an 850nm wavelength, the sensitivity will be down to approx. a third.
All that said, you'd only get a go/no go reading, but if that's all you need, it's fairly easy - although a 555 alone won't be able to make the burst/pause signal.