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I thought I was smart enough

Started by amdx February 24, 2020
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
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
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
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
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. >>> &nbsp;&nbsp;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. >>> >>> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 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.&nbsp; 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? > > &nbsp;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. > > > &nbsp;> How about a CMOS part and a 1 nF capacitor instead? > > &nbsp;I'll get the exact part #, not sure what he has. I have had him > &nbsp;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 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. >> &nbsp;&nbsp;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. >> >> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Thanks, Mikek > > Your R2(470K) C2(1uf and higher) is suspect.&nbsp; 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.&nbsp; You want R2*C2 << R1*C1 > > I watched your newsgroup conversation with him.&nbsp; 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?&nbsp; 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.
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. > > &nbsp;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. > > > > &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 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