Hi guys i want to know how we can replace the infrared receiver with a 3 pins tsop1738 .
This is the schematic :

This is a simple car parking sensor circuit

 

If the 3-terminal device is like most 3 terminal devices, one pin is supply positive, the next is the signal output line, and the third pin is signal common and supply negative. Those descriptions fit exactly the three dots on the column of D5 and R5. Since I have no clue about how a 3 pins tsop1738 operates that is the best that I can offer. I hope that this input is useful.

 

The tsop1738 is more than just a IR diode. It is a device with a 38KHz filter so it will not respond to ambient IR, but to a 38KHz pulse stream.
the circuit above is looking at the analog value produced by the IR brightness on the diode, so the tsop1738 will not work for you.

 

Can't be done.
The TSOP devices are intended to receive the modulation of an IR signal modulated at about 38kHz (different models have filters for different frequencies) and the output is a digital version of the modulation envelope. Your circuit expects and analog representation of the IR intensity.
[edit] Snap!

 

What range do you need ?
About a 3:1 improvement in Rx by adding a conical reflector, maybe 15 mm dia. to 3 mm exit, 30 mm long.
For better range might try a modulated IR system. Tx., 930 nm @ 10 kHz, Rx. maybe OPTEK photologic OPL561-OC ( open collector ). Does not require double modulation , choose frequency. Output low with IR, available inverted.

 

I don't recall seeing any statements or even comments about the intended function of the circuit, and knowing that is sort of important in knowing how to improve it. So an explanation of the intended operation could allow folks to provide useful information.