Frequency to voltage...

[Ian Rogers]

I sure one or two of you will know the answer, but as I don't do SMT very often ( mainly because I can't see it ) I'm hoping it'll be something simple..

I have three Anemometers I need to repair... One was easy.. Crack in PCB (sorted )... The second had a mechanical issue on the chopper wheel ( glued and sorted )... The third has a component fault... I'm pretty sure that a 555 has done a bunk, but as I said.. Tight circuit and SMT.... The chopper ( IR led / receiver combo ) is working... The buffer opamp is also working.. This op amp is supposed to fire the 555 monostable via a small 1nf cap.. But!! On the scope at this point there is the most feeble pulse that cannot drive the trigger... But the trigger has a flyback diode to 2.5v( sorry that's the positive..Its actually Vcc - 2,5v.... ie.. 21.5 ~ 24 v ) Anywho.. There is also a 47k pullup and another decoupling cap that I'm sure is to the 2.5v... The problem is the 555 isn't triggering... If I trigger it manually ie.. short pin1 and pin 2... It works as expected... So I don't fancy taking the tiny 555 off and replacing unless absolutely necessary..

My gut feeling is there is something up with the impedance on the trigger pin, which is keeping it high... I cant see anything else causing it.. I have removed the diode and the caps ( tough going ), tested them out of circuit...

Anyone else think its the 555 IC???

 

[moty22]

I do, but that's only because I agree with you on nearly everything.

 

[ericgibbs]

hi Ian,
The Vsup max for a 555 TTL is only 18V, did you say you have 24v.??

E

 

[Ian Rogers]

No! Its only bootstrapped to the Vcc rail.. this can be 18~36v after its converted there is another dual op amp that generates the 4~20mA....

The 555 and the two op amps only see a Vsup of 2.5v ~ 3.0v... I have decided to rip the working one apart and compare the voltages to and from the 555.... If I have to replace it I'll meander down to my local PCB shop and see if they will replace it...

I have tried to recreate the circuit, but it might not be entirely accurate...




Remember the components may not be correct

 

[KeepItSimpleStupid]

Can you not be creative and substitute sections from the working boards?

Or breadboard the 555 externally?

 

[jpanhalt]

No! Its only bootstrapped to the Vcc rail.. this can be 18~36v after its converted there is another dual op amp that generates the 4~20mA....

The 555 and the two op amps only see a Vsup of 2.5v ~ 3.0v... I have decided to rip the working one apart and compare the voltages to and from the 555.... If I have to replace it I'll meander down to my local PCB shop and see if they will replace it...

I have tried to recreate the circuit, but it might not be entirely accurate...

View attachment 90927


Remember the components may not be correct

The lower VCC spec for a 555 is 4.5V; although, some may work to 3.0 volts (Source: AN170).

John

 

[KeepItSimpleStupid]

In that case, the CMOS version will operate from 2 to 18 V. Any chance someone replaced it with the wrong one?

 

[Ian Rogers]

In that case, the CMOS version will operate from 2 to 18 V. Any chance someone replaced it with the wrong one?

I don't think so.. The TLC555 is a CMOS 555 so it fits the bill.... The second op amp should have a cap between the two resistors to ground , low pass filter... I'm just going to try a new 555 as I can't for the life of me see anything untoward on the discreet side... As I said .. diode is ok, cap seems ok...

I hate analogue electronics...... Give me a programming algorithm any day!!!

 

[moty22]

I hate analogue electronics...... Give me a programming algorithm any day!!!

This is a PIC replacement for 555
https://www.microchip.com/forums/m672148.aspx

 

[jpanhalt]

moty22 I believe the power pins are reversed on that chip vs. a 555. You might be interested in checking out High Tech Chips (HTC), which years ago duplicated many 555 functions with 12Fxxx chips.

John

 

[moty22]

moty22 I believe the power pins are reversed on that chip vs. a 555. You might be interested in checking out High Tech Chips (HTC), which years ago duplicated many 555 functions with 12Fxxx chips.

John

I'm quite sure that tomorrow morning Ian will try our suggestions even before having his breakfast.

 

[ericgibbs]

hi Ian.
I would agree with John, IMO at 2.5Vsup the bog standard 555 TTL is being powered below the bottom limit spec' of 4.5V.
Its past correct operation was probably only just marginal at 2.5V, with ageing etc, its now failing.

Is that 1n0 really across the clamp diode.?

E
You could try adding another 1n0 across the existing 1n0 from the OPA drive, give the 555 a bigger pulse

 

[Ian Rogers]

This is a PIC replacement for 555
https://www.microchip.com/forums/m672148.aspx

Ah! yes... Unfortunately, as it has such a small footprint i wouldn't be able to re-route the power rails... a small pic like the 12f675 would fit, but there is no room to "hang" it anywhere.... Space is very tight.. there is a chopper wheel sandwiched between the two PCB's ... both less than 1" diameter and about 5~7mm between them.... My 52 year old eyes are having a problem.... Even the tiny diode was hard to solder back on!!!

 

[Nigel Goodwin]

My 52 year old eyes are having a problem.... Even the tiny diode was hard to solder back on!!!

Tell me about it!

 

[moty22]

I can't see them too due to age.
This must be another classic case of 'blind leading the blind'.

 

[Ian Rogers]

Looking at the 555 emulator.... If I tweak the pins and place it upside down the Vcc and Vee pins will be okay

This means RA4 and RA5 are potentially pins 6&7 and as they are tied in a monostable I have an ADC on RA4
I have a ADC on (now) pin 2 for the trigger....

Seems like the pic12f1822 is a drop in replacement after all... I have a couple of these so I'll give it a go.

One thing is for sure.... The next guy will get a shock when he opens it up...

I can even ditch the coupling cap as I can generate a pulse on (now pin 3) whenever I want...

Cheers moty22

 

[moty22]

The next guy will get a shock when he opens it up...

If the next guy is a member of ETO he will immediately guess that Ian did it. We all know about you burning love to microcontrollers.

 

[Ian Rogers]

I have just realized that the pic12f1822 are not low voltage.. I'll order a couple of pic12LF1822...

 

[jpanhalt]

Flipping will put Pin4 (!MCLR, RA3) in the position of Pin5(RA2). RA3 does not have an ADC, so will this part of the code then be a problem:

I can't read C, but it is hard to see how you could truly emulate an NE555/LM555 without having Pin5 (Control) read as an analog input. I have joked many time about using a 12Fxxx (or 12F1xxx) to emulate the NE555 for people wanting FYP's, but that is mostly as a parlor trick. If you simply flip the PIC on its back, I suspect you may still have to physically swap some pins.

In the real situation you describe, I think the low voltage you are seeing could be a result of a problem further upstream. Or, if it was designed that way and simply worked on the fringes of the NE555's capability for awhile, then substitution with a CMOS version might be a more practical solution.

I would rule out an upstream problem first.

John

 

[Ian Rogers]

I'm not going to... I'm going to watch the other one that works on a scope and mimic the output..

Pretty sure a pic can read a pulse on the trigger and output a pulse for specific time... I'm not even going to pay attention to pin 4 and 5...