But what i main encountered is the when the ir beam faced toward the base of phototransistor. The phototransistor's emiiter doesnt have enough power to operate the npn transistor (base of npn). So i guess this why the trigger is "high".

 

hi T,
OK.
Keep the resistor and LED connected to pin #3.
With a short piece of wire touch across pin #2 [ trig] to 0v, this will simulate the the action of the photo-transistor and the 555 should switch the LED.
Lets know.
E
Dont disconnect the photo transistor parts

Do you have a voltmeter.??

 

hi T,
OK.
Keep the resistor and LED connected to pin #3.
With a short piece of wire touch across pin #2 [ trig] to 0v, this will simulate the the action of the photo-transistor and the 555 should switch the LED.
Lets know.
E
Dont disconnect the photo transistor parts

Wont light up too. I guess the ic is burned . But by theory this circuit should work like a charm right?

 

Hello,

I see that the phototransistor is in the open.
External light might already have such an influence that the circuit does not react.
Please place the phototransistor is a piece of tubing, so that external light is shielded.

Bertus

 

hi,
The circuit will work OK.
Have you checked that your 470R and LED test circuit is OK.?
To check it, connect the 470R to the +9v and the free end of the LED to 0v, it should light.
E

Do you have a test voltmeter.?

 

Hello,

I see that the phototransistor is in the open.
External light might already have such an influence that the circuit does not react.
Please place the phototransistor is a piece of tubing, so that external light is shielded.

Bertus

I placed it very near. So maybe external light is not a big deal i dont know. But the problem is i afraid the phototransistor's emitter will release over-voltage to the base of the npn transistor. Or too less voltage to the base of npn.

 

Do you have a test voltmeter.?

 

hi,
The circuit will work OK.
Have you checked that your 470R and LED test circuit is OK.?
To check it, connect the 470R to the +9v and the free end of the LED to 0v, it should light.
E

Do you have a test voltmeter.?

They worked very well.

 

Do you have a test voltmeter.?

Yes

 

OK,
Measure between pin #2 and 0V, is it at 0V or +9v??

 

OK,
Measure between pin #2 and 0V, is it at 0V or +9v??

U means pin 2 to gnd?

 

Yes, measure pin 2 to 0V voltage

 

Yes, measure pin 2 to 0V voltage

50mV something..

 

OK,
Connect your test resistor and LED to pin #3
Unplug the photo transistor, pin 2 should then go to ~9v, remeasure pin#2 to 0v

 

OK,
Connect your test resistor and LED to pin #3
Unplug the photo transistor, pin 2 should then go to ~9v, remeasure pin#2 to 0v

Yes

 

Good, keep the phototransistor unplugged, repeat this test.
With a short piece of wire touch across pin #2 [ trig] to 0v, this will simulate the the action of the photo-transistor and the 555 should switch the LED.

 

Good, keep the phototransistor unplugged, repeat this test.
With a short piece of wire touch across pin #2 [ trig] to 0v, this will simulate the the action of the photo-transistor and the 555 should switch the LED.

Even i switched the pin 2 wire to gnd led still no light up.

 

ok,
That suggests that the 555 is faulty, do you have another one handy.??

 

ok,
That suggests that the 555 is faulty, do you have another one handy.??

Nope. may i know will the phototransistor's emitter possible destroy the npn transistor?

 

OK,
The photo-transistor should not destroy the transistor, if it had done, when you unplugged the photo transistor pin #2 it would have not switched to 9v.
The 1k in series with the PT [photo transistor] would have limited the current into the transistors Base to a safe value.

What is the value of the capacitor on pins 6/7 of the 555.? and R4

As post #24 by Bertus explains, in the final circuit when you have it working OK, you should shield the PT from the room lighting.