Hi again hex reader , i have got the parts for the circuit in real time , i have used a 100K pot , but the led is always turned on without sending any signal , i wonder what you've done to make it work ?

 

I wired the circuit exactly as in your posts and adjusted the pot until the light went out.

Put your oscilloscope on "test point" (with scope ground to circuit ground) and make sure that you have a good voltage swing.

You should see about 0.3 Volts or less with no signals, then nice square pulses of about +3.5 Volts or more peak when IR is received.

If the transistor output is good, then let me know. We can explore the logic circuitry.

If the transistor output is bad, then put the oscilloscope probe to the IR receiver chip output. Expect a high with no signal, then pulses of low when IR is received.

 

Hi hexreader again , i have checked the test point , and i have about a 5v pulse on the oscilloscope but with DMM it does not appear ( maybe because its at high frequency ?) .

 

with DMM it does not appear ( maybe because its at high frequency ?) .

See post #15. Put your DMM in a cupboard and leave it there.

The circuit is working well up to "test point" which is good.

Set your pot to the end of travel that gives 100k ohms, wait 20 seconds, then check the voltage on pins 4,5,6,7 and 14 with your oscilloscope. (scope ground connected to circuit ground)

Make sure that no IR signal is being sent at any point.

Let me know the results.

 

I get 5 volts on pins :14, 5,6 ; 0v at 7 and 4 .

I have just checked the pot and it not working as supposed , changed it with a 50K one and the result is still the same

 

Those voltages look reasonable, except that I would hope for about 3V at pins 5,6

I must admit... I cannot make the circuit work correctly any more. I think there must have been something wrong when I tried it first time.

I think the logic circuit is flawed - and works either intermittently or not at all.

50k Ohms is good for the pot. Try this:

Connect the wiper of the pot to U1 pins 5 and 6
Connect one end of the pot to ground
Connect the other end of the pot to +5 Volts

Adjust the pot until the LED is just on
Send an IR signal and watch the LED go out.

I know this is not the task that you were set, but it is the closest that I can get to something that works.

Are you absolutely certain that the diagram that you posted matches what you were given?

 

Ok thanks a lot , i guess i will get a brand new IC and i hope it from that .

 

I see nothing to suggest that the IC is broken (but by all means try a new one)

Either the design is flawed, or I my understanding is flawed.

Somehow, the input to the IC needs to get to well below 2 Volts to give a high out (and turn off the LED) I do not see how a resistor to +5V can achieve a good low input to the gate.

..... there just seems something very wrong with that circuit - or maybe I just don't understand


I don't suppose this could be a break-beam detector could it? Continuous 38kHz reception (modulated) normally, then LED lights when an obstacle breaks the beam? ... just a thought.

 

I see nothing to suggest that the IC is broken (but by all means try a new one)

Either the design is flawed, or I my understanding is flawed.

Somehow, the input to the IC needs to get to well below 2 Volts to give a high out (and turn off the LED) I do not see how a resistor to +5V can achieve a good low input to the gate.

..... there just seems something very wrong with that circuit - or maybe I just don't understand


I don't suppose this could be a break-beam detector could it? Continuous 38kHz reception (modulated) normally, then LED lights when an obstacle breaks the beam? ... just a thought.

Maybe , i will find a way to make it work .

I appreciate your time .