555 not timing,what am I doing wrong?

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markelectro

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Hello to all

I have a 555 timer circuit that I can't seem to get timing.

The circuit is using a trigger from an LED. and a photo transistor.The use of this circuit is for one of my other threads-

https://www.electro-tech-online.com/threads/1-sec-pulse-from-30-sec-clock.31429/#post235809

Basically when a pendulum swings in a clock it will break the beam of light to the light sensor (photo transistor) when this happens the the 555 will run for a given amount of time sending power through to an external device.For the purpose of the experiment I used and LED. for the external device.This does happen but the 555 does not time out.I want the 555 to time out after about 3 sec's.

When the transistor is receiving the light then the circuit runs ok (no power going through to the external device)

Due to my lack of electronics knowledge I am now at a loss.I have been at this for hours trying to get it to work.

Can you please Help?

Regards Mark
 

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The timining resistor and capacitor should be chosen according to this formula: T = 1.1 R C; for 3 s you can use 68kΩ and 47 uF, for example. As you know, tolerances affect the ideal behaviour.
You didn't placed a current limiting reistor at pin 3.
Have you checked the output with a multimeter, without the LED?
 


Thanks eng1

I did as you sugested and put a multimeter inplace if the LED. and when no light going through to the transistor the meter shows the 9v that is flowing through the circuit.

regards mark
 
IRL3103 said:
There are 2 free programs to calculate 555 timer components. SS55511.exe and ic_555.exe.
Click to expand...


Hey thanks for that.!!!!

Sorted!!! I just put into the 555 calc what I wanted and it gave me a workable capaciter and resistor.

Cheers.

Mark
 

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heres a simple edge triggered 555 schematic i built a while back-it should give you a few pointers how to go about using the 555 another way.........
 

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Markelecto, I agree with eng1 you still need a current limiting resistor in series with the LED on pin 3. I have also found that most electrolytic capacitors greater than about 10uF cause timing problems because of the leakage current. A better solution is use a higher value of resister in the timing calculations.

Sheldorstv, your diagram for your circuit should have about a 1K resistor in series with the 1M pot if the pot is adjusted near 0 ohms the current to pin 6 and 7 could exceed maximum value.
 
Hi all

Been palying around with it and if I put a resistor of 560 or 680 then the led does not light but if I go down to 1k then it does light up.

Now I am confused.

Regards Mark
 
I got rid of the awful black background, added a supply bypass capacitor and added a current-limiting resistor for a red 2V LED.
 

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markelectro said:
Hi all

Been palying around with it and if I put a resistor of 560 or 680 then the led does not light but if I go down to 1k then it does light up.

Now I am confused.

Regards Mark
Click to expand...
Lower the resistor, brigther the LED. Did you a use 560 ohm resistor? I expect the LED current to be about 10 mA with a 560 ohm resistor. That's reasonably enough?
 
All

By jo I think I got it!!!

Cheers eng1 for that.I got my resistor values mixed up.

You will now see the led. glowing like a light house.thanks to hm:

Cheers audioguru for the new schema!!! I added mine a logic level just so you all could see the flow.I am now very happy to see it up and running corectly.I THINK!!!

Regards Mark
 

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Hi Mark!

Glad to see you've got the sim working! Sorry I didn't have time to answer your earlier post in the original thread earlier today.


Good luck with the project!

Torben
 
Torben said:
Hi Mark!

Glad to see you've got the sim working! Sorry I didn't have time to answer your earlier post in the original thread earlier today.


Good luck with the project!

Torben
Click to expand...


Hey Torben


Cheers for that.It took a while to get the little circuit going but I did learne a lot doing so.

Cheers Mark
 
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