I thought I was smart enough

[amdx]

I'm try to help the guy with the LDR triggering a 555.
I'm slowly walking him through it and now just triggering it with a wire
momentarily to grd, to pull Pin 2 low, through a capacitor.
But it won't trigger.
Here's the circuit, what is needed to make the circuit trigger properly.

https://www.dropbox.com/s/fwxu5vqazzqist4/one%20shot%20wire.jpg?dl=0

Ignore the LDR for now I realize a slow change is a problem.

Thanks, Mikek

 

[Phil Hobbs]

On 2020-02-24 11:05, amdx wrote:

I'm try to help the guy with the LDR triggering a 555.
I'm slowly walking him through it and now just triggering it with a wire
momentarily to grd, to pull Pin 2 low, through a capacitor.
 But it won't trigger.
Here's the circuit, what is needed to make the circuit trigger properly.
https://www.dropbox.com/s/fwxu5vqazzqist4/one%20shot%20wire.jpg?dl=0


Ignore the LDR for now I realize a slow change is a problem.

                            Thanks, Mikek

When you release the short you're discharging that monster cap through
the protection diodes on the chip, which probably dumps a huge amount of
charge into the substrate. All sorts of weird things can happen. Does
it work properly if you short out the cap on pin 2?

Also 470k is a pretty large resistor to use with a bipolar 555. How
about a CMOS part and a 1 nF capacitor instead?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[amdx]

On 2/24/2020 1:48 PM, Phil Hobbs wrote:

On 2020-02-24 11:05, amdx wrote:
I'm try to help the guy with the LDR triggering a 555.
I'm slowly walking him through it and now just triggering it with a
wire momentarily to grd, to pull Pin 2 low, through a capacitor.
  But it won't trigger.
Here's the circuit, what is needed to make the circuit trigger properly.
https://www.dropbox.com/s/fwxu5vqazzqist4/one%20shot%20wire.jpg?dl=0


Ignore the LDR for now I realize a slow change is a problem.

                             Thanks, Mikek

When you release the short you're discharging that monster cap through
the protection diodes on the chip, which probably dumps a huge amount of
charge into the substrate.  All sorts of weird things can happen.

Maybe, but often a differentiated signal is used to trigger a 555,
because the if the trigger is held low, it doesn't allow the output to
time out. You just want a pulse on the trigger.


> Does it work properly if you short out the cap on pin 2?

Yes, it does

Also 470k is a pretty large resistor to use with a bipolar 555.

I started smaller, but when it didn't work I increased the value hoping
to be able to pull the trigger low easier.


 > How about a CMOS part and a 1 nF capacitor instead?

I'll get the exact part #, not sure what he has. I have had him
try from .001uf, 0.01uf, 0.1uf, 1uf, 10 uf.

Thank, Mikek

 

[ehsjr]

On 2/24/2020 11:05 AM, amdx wrote:

I'm try to help the guy with the LDR triggering a 555.
I'm slowly walking him through it and now just triggering it with a wire
momentarily to grd, to pull Pin 2 low, through a capacitor.
 But it won't trigger.
Here's the circuit, what is needed to make the circuit trigger properly.
https://www.dropbox.com/s/fwxu5vqazzqist4/one%20shot%20wire.jpg?dl=0


Ignore the LDR for now I realize a slow change is a problem.

                            Thanks, Mikek

Your R2(470K) C2(1uf and higher) is suspect. You want a trigger pulse
that is a lot shorter than the desired output pulse. There are no
values given for R1 & C1, which set the output pulse duration. Try
a much smaller cap for C2. You want R2*C2 << R1*C1

I watched your newsgroup conversation with him. You were
absolutely right to take it to private (email) because he
was getting too many different answers from different
responders, which makes it real confusing for him.

Can you breadboard the circuit and send him a photo
once you have it working the way he wants? That might
help him.

Ed

 

[John Robertson]

On 2020/02/24 2:08 p.m., amdx wrote:

On 2/24/2020 1:48 PM, Phil Hobbs wrote:
On 2020-02-24 11:05, amdx wrote:
I'm try to help the guy with the LDR triggering a 555.
I'm slowly walking him through it and now just triggering it with a
wire momentarily to grd, to pull Pin 2 low, through a capacitor.
  But it won't trigger.
Here's the circuit, what is needed to make the circuit trigger properly.
https://www.dropbox.com/s/fwxu5vqazzqist4/one%20shot%20wire.jpg?dl=0


Ignore the LDR for now I realize a slow change is a problem.

                             Thanks, Mikek

When you release the short you're discharging that monster cap through
the protection diodes on the chip, which probably dumps a huge amount
of charge into the substrate.  All sorts of weird things can happen.

Maybe, but often a differentiated signal is used to trigger a 555,
because the if the trigger is held low, it doesn't allow the output to
time out. You just want a pulse on the trigger.


Does it work properly if you short out the cap on pin 2?

 Yes, it does


Also 470k is a pretty large resistor to use with a bipolar 555.

I started smaller, but when it didn't work I increased the value hoping
to be able to pull the trigger low easier.


 > How about a CMOS part and a 1 nF capacitor instead?

 I'll get the exact part #, not sure what he has. I have had him
 try from .001uf, 0.01uf, 0.1uf, 1uf, 10 uf.

Thank, Mikek

As Phil pointed out a large cap may have discharged too much current
into the 555, so it may be damaged.

Have the fellow start with a fresh 555 and replace C2 with a 1N400X
diode and get rid of R3, and change R2 to perhaps 10k. Pin 2 needs to
get down to 1/3 of Vcc to trip. the diode will protect pin 2.

If it MUST be a capacitor isolated input (why?) then put the cap first
in series with the diode and make it perhaps 100ufd. You will need to
put a pair of resistors joined at the junction of the cap and the diode
- perhaps 20K to Vcc and 10K to ground. I think R2 will still work at
10K, but it might need to go up to 47K - I'd have to breadboard it to be
sure.

The diode solution is simpler...

John :-#)#

 

On Monday, February 24, 2020 at 2:48:21 PM UTC-5, Phil Hobbs wrote:

On 2020-02-24 11:05, amdx wrote:
I'm try to help the guy with the LDR triggering a 555.
I'm slowly walking him through it and now just triggering it with a wire
momentarily to grd, to pull Pin 2 low, through a capacitor.
 But it won't trigger.
Here's the circuit, what is needed to make the circuit trigger properly..
https://www.dropbox.com/s/fwxu5vqazzqist4/one%20shot%20wire.jpg?dl=0


Ignore the LDR for now I realize a slow change is a problem.

                            Thanks, Mikek

When you release the short you're discharging that monster cap through
the protection diodes on the chip, which probably dumps a huge amount of
charge into the substrate. All sorts of weird things can happen. Does
it work properly if you short out the cap on pin 2?

Did you miss R3 at 5.6K in series with the cap??? That's hardly a HUGE amount of charge. It won't damage anything.

Also 470k is a pretty large resistor to use with a bipolar 555. How
about a CMOS part and a 1 nF capacitor instead?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[amdx]

On 2/27/2020 12:45 AM, ehsjr wrote:

On 2/24/2020 11:05 AM, amdx wrote:
I'm try to help the guy with the LDR triggering a 555.
I'm slowly walking him through it and now just triggering it with a
wire momentarily to grd, to pull Pin 2 low, through a capacitor.
  But it won't trigger.
Here's the circuit, what is needed to make the circuit trigger properly.
https://www.dropbox.com/s/fwxu5vqazzqist4/one%20shot%20wire.jpg?dl=0


Ignore the LDR for now I realize a slow change is a problem.

                             Thanks, Mikek

Your R2(470K) C2(1uf and higher) is suspect.  You want a trigger pulse
that is a lot shorter than the desired output pulse. There are no
values given for R1 & C1, which set the output pulse duration. Try
a much smaller cap for C2.  You want R2*C2 << R1*C1

I watched your newsgroup conversation with him.  You were
absolutely right to take it to private (email) because he
was getting too many different answers from different
responders, which makes it real confusing for him.

Can you breadboard the circuit and send him a photo
once you have it working the way he wants?  That might
help him.

Ed

We finally got it triggering properly. The output timing was 1.1
seconds. The last final addition was adding a resistor from pin 2
to ground, setting the dc voltage on Pin 2 just above the 1/3 Vcc
threshold. He used a 4.7uf cap. Once it worked I didn't ask him to
alter anything. It was a long slog with more than 40 email exchanges.
The last thing I mentioned was replacing the LDR with an infrared
sensor and emitter.
Mikek

To the one that ask whether the cap was needed, yes, if Pin 2 is held
low, the output follows that instead of it's timing sequence. So we make
it a pulse.
There is an additional problem, If the input sensor has a slow change,
it may not make it through the cap to Pin 2. That's a future fix.