I have built an IR proximity sensor using a normal 940nm IR LED, Tsop 4856 kHz IR photodiode. I have used 555 timer to generate pulses to the IR led. the circuit is working correctly as far as I know.

Using an oscilloscope, I can see 5V straight line without obstacle, a 0V straight line when there is a obstacle and a square wave between 0 and 5 V when the obstacle distance varies. But the range is only 8 - 10 cms.

I have tried the circuit with a transistor in the output of 555 timer. But the range remains same.

I have to detect an obstacle at a distance of 1.5 meters. How could I achieve this range.

Major problem: I have projected normal IR source on the Tsop sensor. the output LED blinks even for that source which shouldn't be going on. As per my knowledge the sensor should not give output when is subjected to other IR frequencies except for 56 kHz.

What do you think is wrong and what should I do to eliminate other frequencies?

I have attached my circuit diagram.

Schematic 1

Schematic 2

This is my first project in electronics. I want to learn by doing. It would be very helpful if you could explain in detail.


2 Answers 2



The IR sensors I use (Sharp) with different distance attributes use the same PCB but differing lens assemblies.


1 - I assume your schematic showing pin 1 if the 555 floating is a mistake. It must be tied to ground.

2 - I assume you are using 5 volts to power the 555. If so, unless you have a really hefty LED, you need to increase the 47 ohm resistor to about 330 ohms.

3 - The Tsop48 needs at least 4.7 uF between pins 3 and 1. See the data sheet, page 2. I suspect that, for low light levels, the chip is oscillating.

4 - The output of the Tsop is already a logic level. Running it through a second op amp will not gain you anything.

5 - Again, read the manual. The Tsop is not intended for sensing continuous (non-burst) data. From page 3:

After each burst which is between 10 cycles and 70 cycles a gap time of at least 14 cycles is necessary.

I don't know if your symptoms are related to this, but I don't know if they are not.

  • \$\begingroup\$ thanks for the reply. pin 1 is tied to ground. I forgot to draw it. for 56 khz, the on time is 10usec and off time is 8 u sec. is it sufficient. Is that what you meant by gap time between bursts(8usec gaptime between 10usec bursts in my case) could you also tell me how could i increase the range of detection to 1.5 meters \$\endgroup\$
    – tony
    May 22, 2014 at 8:45
  • \$\begingroup\$ Your LED waveform should be OK. Please read the data sheet: farnell.com/datasheets/1675558.pdf to learn about gap time. You are running your LED continuously. The Tsop expects to see bursts of pulses, with periods between the bursts when the LED is not pulsed. As I say, I don't know if this is a problem. What I suspect is a problem is your LED. You don't say what it is. The data sheet (Read it!) specifies a high power LED driven with 350 mA. It may be that you just aren't putting out enough optical power. \$\endgroup\$ May 22, 2014 at 13:04

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