Frequency to voltage...

[Tony Stewart]

Ian, If I understand your schematic which gives me the idea , inspite of many polarity errors,
the trigger, as I read is Vcc/3 on pin2 , yet C5/C6 are equal so the pulse starts at Vcc and gets divided in half and never reaches Vcc/3

So C5 must be much smaller than C6. Make C6 bigger or C5 smaller.

 

[moty22]

You can try to program pic12f and test it on external supply of 2.5V. I'm quite sure Microchip make only one type and then take out those that work down to 2V and sell them for more.
You can verify my claim with Tony Stewart, he probably knows about all the tricks in the industry.

 

[Tony Stewart]

From the specs I am reading, Both PIC12LF and PIC12F work down to 1.8V @ 4MHz, but the L version is guaranteed for lower current. Both have the same threshold voltages.

The reason 12F is spec'd to 5V and not the 12LF, is if you wanted the guaranteed lower power version, presumably binned, is that you would design it for 3.3 or less. Otherwise if you run at 5V , you can't be too concerned about uA.

 

[moty22]

From the specs I am reading, Both PIC12LF and PIC12F work down to 1.8V @ 4MHz, but the L version is guaranteed for lower current. Both have the same threshold voltages.

The reason 12F is spec'd to 5V and not the 12LF, is if you wanted the guaranteed lower power version, presumably binned, is that you would design it for 3.3 or less. Otherwise if you run at 5V , you can't be too concerned about uA.

I checked it too, you are right. I hope Ian wont be broken hearted for wasting a day waiting for the bits to arrive.
I also hope that Nigel wont use this opportunity to tell him "I hate it when this happens".

 

[Ian Rogers]

Tony!! I really don't know the real values... The circuit is "Approximately" how I see it... Folk can get the jist...

I make Anemometers.. I use a small pic on my system to generate 4~20mA I use feedback so I can maintain a good output signal. I read a chopper wheel directly into the pic and produce a decent voltage that can be converted to mA.. These are from other manufactures.... I think their circuit is Neanderthal... Who uses this type of input conditioning...

Anyhwo! It works so repairs are required.... I have already ordered the LF type... I need it to work for a bit so I will opt for the "Guaranteed" version.... Customer pay's anyway!!

 

[Ian Rogers]

I checked it too, you are right. I hope Ian wont be broken hearted for wasting a day waiting for the bits to arrive.
I also hope that Nigel wont use this opportunity to tell him "I hate it when this happens".

It's weekend... Can't do Jack until Monday !!! Nigel will take any opportunity.... I won't argue... I think he's black belt, so anything goes.....

 

[moty22]

It's weekend... Can't do Jack until Monday !!! Nigel will take any opportunity.... I won't argue... I think he's black belt, so anything goes.....

It's good you are ready for the revenge of the black belt.
The analogue output of the pic in your Anemometers is it PWM or a none disclosable method?
Car manufacturers say that one of the criteria for reliability is using well proven circuits and this is why the design is old.

 

[Tony Stewart]

Anemometers with 1shots using a POT are usually tachometer type for analog average DC out vs RPM. No PIC needed. Designs using a 555 are rarely high quality.

 

[Ian Rogers]

Right!!! It wasn't the LT555C.... but now I have a pretty good trigger waveform....

It would appear that something else is causing the 555 to have no output... Still!! If I manually trigger it, it has an output!!!....

ericgibbs ..... What should I be seeing on the Threshold / Discharge pins whilst acting as a monostable... I only get 0v... permanently.. Reset is high and cv is tied to a decoupling cap.... When simulating the output mimics pins 6 and 7...

This maybe where the fault lies..... I have 35k resistance to 2.5v but I cannot determine which cap is to ground.... If I don't get a pulse on pin 6 and 7, I'm not going to get a pulse on pin 3... BUT!! shorting pin 2 to ground fires pin 3.... I dont get it... The pulse looks good enough to trigger the 555????

 

[Ian Rogers]

Here is the simulation.....


Here is the actual device


The second trace is on pin 3...

 

[Tony Stewart]

If you had a variety of fixed resistors to drop the trigger DC voltage, what is the largest value that forces a trigger? Then you can see if the device current is faulty.

 

[moty22]

Right!!! It wasn't the LT555C.... but now I have a pretty good trigger waveform....

It would appear that something else is causing the 555 to have no output... Still!! If I manually trigger it, it has an output!!!....

ericgibbs ..... What should I be seeing on the Threshold / Discharge pins whilst acting as a monostable... I only get 0v... permanently.. Reset is high and cv is tied to a decoupling cap.... When simulating the output mimics pins 6 and 7...

This maybe where the fault lies..... I have 35k resistance to 2.5v but I cannot determine which cap is to ground.... If I don't get a pulse on pin 6 and 7, I'm not going to get a pulse on pin 3... BUT!! shorting pin 2 to ground fires pin 3.... I dont get it... The pulse looks good enough to trigger the 555????

Mark the position of the 50K trimmer connected to pin 7 and then move it to either sides, it is possible that somebody tried to repair it before you and left the trimmer on 0 ohm and that stops the cap from discharging.

 

[Ian Rogers]

Mark the position of the 50K trimmer connected to pin 7 and then move it to either sides, it is possible that somebody tried to repair it before you and left the trimmer on 0 ohm and that stops the cap from discharging.

Not possible... There is red paint on both pots ( the second pot controls the gain on the output amp ) I could move it... but I know that it's not the problem...

My take.... There must have been something wrong with the 555 as the signal on the trigger is 10 fold... Therefore there must be something wrong elsewhere... I figure a possible short on the threshold cap holding the pin low...

The pic12lf1822's will be here today so that's the route I'll be taking.... I can't spend much more time on this....

Thanks everyone for their input, I'll update when the thing's up and running...

 

[Nigel Goodwin]

I can't help thinking Ian, when you find what the actual problem is it's going to be something simple and silly

 

[ericgibbs]

Ian,
If you wish I could look at it for you.?
I don't do house calls any more, but if its not too bulky we could use the post office.
E

 

[Ian Rogers]

I can't help thinking Ian, when you find what the actual problem is it's going to be something simple and silly

I have repaired a couple of these things now... The majority have cracked PCB's ( they are prone to get wet )... I suspect this will be the source of the problem...

If you wish I could look at it for you.?

Thanks for the offer, but If I don't sort it in the next day or so, I'll be able to sell them one of mine... As they have two repaired, I'm sure they will agree.. One of the repairs already has some of my parts on it.

The trouble is.... My anemometer still has to have a separate ground whilst theirs have two wire hookup..

Once I float my circuit I can do the same.... I just need a reliable way to pin my 5v circuit to the Vcc rail....

I have made several units based on the XTR116/5 so it shouldn't be too much of an issue..

 

[alec_t]

I see the time constant of the trigger components in your post #30 sim is much greater than the time constant of the output pulse. Could that be the root of your problem in the real circuit? C3 needs enough time to recover from one trigger before getting the next trigger.

 

[Ian Rogers]

I see the time constant of the trigger components in your post #30 sim is much greater than the time constant of the output pulse. Could that be the root of your problem in the real circuit? C3 needs enough time to recover from one trigger before getting the next trigger.

Hi Alec

The chopper wheel has @ 100 PPR so at 30MPH we are looking at 1Khz.... BUT!!! It has to work from 10hz up to 3Khz... 100mS ~ 0.3mS..

I've pulled it apart already.... I am trying to attach my electronics, as I'm sure the PCB is too far gone... I've already ripped one SMD pad off....

 

[Nigel Goodwin]

The pic12lf1822's will be here today so that's the route I'll be taking.... I can't spend much more time on this....

Hi Ian,

I just happened to be looking through the 16F1827 and 16F1847 datasheets and noticed:

SR Latch:
- Multiple Set/Reset input options
- Emulates 555 Timer applications

This also applies to the 12F1822, so are you going to be using this function?.

 

[Ian Rogers]

It has changed again.... The PCB board itself wasn't giving me much faith in repairing it..

I have converted my own circuit now... As it happens I use a pic12f675 and a 12bit DAC to produce a solid 4~20mA.. I use feedback to give better control... I have attached this to the Vcc rail and now runs on a two wire hookup.... Sorted!!