Hello,

In your earlier post you stated that pin 1 changed between +5 and + 2 Volts.
Measure the voltages now in the same way as in this post.

Greetings,
Bertus

 

Hi Bertus,

By adjusting the potentiometer I can do get a 3.25 to 4.5v volt different in dark and light.
I do have a LED that I wanted to use to test the circuit... right now the LED is plugged at the pin 4 of the UA123 and ground... Is that sounds correct?

Thanks a bunch

pierre

 

Hi Bertus,

The measure at pin 1 are very variable (very sensitive to light) I did a careful test and the value are changing from 4.8v with intense light to -4v in ~ total darkness.
Sorry for the confusion... this is my first electronic experience.

pierre

 

Hello,

When you have +4.8 at light and -4 in darkness, the pin 12 should alaso react.
See attachment for the working of a comparator.

Greetings,
Bertus

 

Hi Bertus,

Yes you are right the pin 12 is now reacting to the light as well.
What we should we do next?

Do you know where we should put our control LED
to control the output of UA123....

We really do appreciate your help and patience.

pierre

 

Hello,

Take te 1 K resistor from pin 10 and put it to pin 12 to trigger the 123.
Did you put a current limiting resistor in series with the led on pin 4 of the 123.
I should take a 470 Ohm resistor.

Greetings,
Bertus

 

thanks a lot bertus, I'm taking an hour break and test your last post.
as before thanks a lot

pierre

 

Hi Bertus,

I has to quit the discussion for a few hours..

I did connect the pin 12
with the 1k...
my LED connected to pin 4 of UA123 is still not showing any change light / darkness.

I'll attack the problem again tonight .. so if you have any idea please
post them I'll try to do my best to follow your steps.

thanks again

pierre

 

Hi Bertus,

Is there any chance I can plug the LED to the UA123 to test the circuit?
Or test the tension at some specific pins?


thanks for your patience.

pierre

 

Hello,

Take pin 2 from the 123 and connect it to + 5Volt.
Connect pin 3 of the 123 also to + 5 Volt.
See also attached datasheet.

Now Pin 1 will be triggered when the signal goes down.

Greetings,
Bertus

 

I can't see the schematic because it was made too small and was saved as a very fuzzy JPG file type instead of as a very clear GIF or PNG file type.

If the ICs are old fashioned TTL then you need to know their spec's:
1) Input low voltage is 0.8V or less. 0.4V allows for some noise immunity.
2) Input low current is a max of 1.6mA. Therefore for an input to be pulled down to 0.4V with a resistor then the resistor must be 0.4V/1.6mA= 250 ohms or less. A few ICs have less input current so might work with a higher value.

So the photo-transistor (or is it a photo-diode?) would be working very hard to create a logic high because the minimum logic high voltage is 2.0V, or 2.5V with some noise immunity. The 250 ohm pull-down resistor needs 2.5V/250= 10mA. The photo-transistor must conduct 8.4mA which is a lot of current for a photo-transistor.

Instead, if the photo-transistor is connected to ground to pull the input to a logic low then its current is only 1.6ma plus a little for a pull-up resistor.

 

Hi Audioguru,

I'm sorry I'm a very first electronician...
Not sure I've got what you mean.
Attached another picture of the schema.

pierre

 

ok

Thanks for datasheet.

* Take pin 2 from the 123 and connect it to + 5Volt.
* Connect pin 3 of the 123 also to + 5 Volt.

So does that mean that pin 16 should be disconnected?
as well as pin 15 and 14? what about pin 8 ?

I'm totally lost where is my input signal ....

* See also attached datasheet.

*Now Pin 1 will be triggered when the signal goes down.


I thought pin 1 was the input signal...

Again sorry for all these question but I do not follow.. I need some more baby steps.

pierre

 

..... I totally lost... for the last part. PLEASE I NEED HELP.

 

Hello,

The rest on the 123 is connected correctly.
Pin 14 and 15 are for the timing circuit.
Pin 16 is the + 5 Volt.
Pin 8 is for ground.
The resistor to +5 volt on the 123 is to high, maximum usable is 180 K, see datasheet.


Greetings,
Bertus

 

I've got your message. Where should I plug the LED in pin 4?
I'll spend some time in family and
contact you tomorrow.

I really really want to thank you.

good night and thank again.

ps: Can I put two 75k in series or should it be very close to 180k?



pierre

 

Hello,

The value of the resistor may be between 5 K and 180 K, depending on the time of the output pulse.
In the datasheet there are formulas to calculate the pulse time.

Greetings,
Bertus

PS at the moment it is 9 o'clock in the evening over here. (Amsterdam, Holland).

 

Good night bertus,

ok Thanks I'll use a 150k first.
One last thing for tonight... where should I connect the LED to test the circuit..
such that the LED will flash for each pulse..

thanks again

pierre

 

Hello,

The led can be connected to pin 4 or pin 13 for opposite reaction with an 470 Ohm resistor in series to limit the current.

Greetings,
Bertus

 

Bertus.... you are a GENIUS..... The circuit looks great.
I'll have to test it later with an oscilo //to test the duration of the pulse
and see whether or not I can detect the TTL.... any suggestion for that purpose ?//
I would need a brief TTL of 5v.

Then, I'll make a copy of the final diagram and picture of the circuit.. to put it on the forum...

We really want to thank you for your patience and kindness,

Best wishes from Paris.

pierre