Help reading a schematic please

[bigal_scorpio]

Hi to all,

I am in the process of building an ESR meter from here **broken link removed** and just getting a bit puzzled about a couple of things.

Firstly as I have not had a lot to do with MOSfets I am worried that I can't tell which is the Source and Drain on the BS170 at the top left.

Also I am puzzled what the Backlight and Credits part does, its below the right hand side of the LCD in the schematic, does the circuit imply that a switch would short RC0 and RC1 and connect them to RB2 when the switch is made? If it does can anyone explain why? And would it need a latching switch?

Also I am assuming that the on and off switches are momentary push to make types, and as there is no other means of disconnecting the battery I wondered if it would be continuously pulling a small amount of current?

Maybe it would be better to add a further switch directly after the battery, maybe a toggle or slider?

Any thoughts?

Thanks in advance for any help.................Al

 

[kchriste]

Firstly as I have not had a lot to do with MOSfets I am worried that I can't tell which is the Source and Drain on the BS170 at the top left.

The circuit is badly drawn. Looking at the circuit, the drain connects to the 1N4007 below it. Also, you could sub a 1n4001, 1n4002, etc for the 1n4007. They don't show Vss of the PIC being connected to Vss of the LCD, but on the PCB layout it is correct.

Also I am puzzled what the Backlight and Credits part does, its below the right hand side of the LCD in the schematic, does the circuit imply that a switch would short RC0 and RC1 and connect them to RB2 when the switch is made? If it does can anyone explain why? And would it need a latching switch?

Yes, it looks like the switches connect RC0 and/or RC1 to RB2. Why the author did it this way I don't know. It might have to do with the fact that he planned to have a keypad and then abandoned the idea. Looking at the source code comments it seems like the switches are momentary though they are drawn as latching on the schematic.

Also I am assuming that the on and off switches are momentary push to make types, and as there is no other means of disconnecting the battery I wondered if it would be continuously pulling a small amount of current?

Yes, those are momentary for sure. The only current it would draw when OFF would be leakage current through the BS170. No need to add another switch.
But you could simplify the circuit by putting a toggle switch in the B+ line and eliminating the BS170, the 1n4148's, the 100nf caps, and some resistors if you wanted to.

 

[mvs sarma]

I doubt the connection of LCD pin 3 to pin1. It would be dark squares across the display. The contrast pin needs to be treated properly. I don't think that this could be the final version of the project.

 

[Roff]

Hi to all,

I am in the process of building an ESR meter from here **broken link removed** and just getting a bit puzzled about a couple of things.

Firstly as I have not had a lot to do with MOSfets I am worried that I can't tell which is the Source and Drain on the BS170 at the top left.

The source is the top pin.

 

[bigal_scorpio]

struggling a bit still

Hi guys,

Thanks for all the info, I have some more of it sorted now due to the advice.

Also I have substituted a 2n7000 for the BS170 as it seemed close in spec! Is it OK?

Thanks for the orientation Ron.

Thanks kchriste for the tips, would the double pole switch be momentary too?

Since I already etched the board I will try to stick with the original plan, but I have another small problem, I can't see where the OFF switch should go on the silkscreen! Can any of you show me, I have sussed the ON switch but can't find the OFF pins!

Also if there is an obvious ground pin I must have missed it, the pos has been brought out to the bottom right, but where is the ground?

Also I have taken on board the contrast suggestion and will fit a pot into the design, well spotted, thanks Sarma!

Also did anyone notice that the orientation of the two 1N4148 diodes is different in the silk to the photo, though it doesn't seem to matter!

And again thanks for the replies guys

 

[kchriste]

Thanks kchriste for the tips, would the double pole switch be momentary too?

I don't remember a double pole switch, but if you meant the "toggle switch" I mentioned, then no, make it latching.

but I have another small problem, I can't see where the OFF switch should go on the silkscreen!

It's not on there and neither is the 3.3K resistor feeding the gate of the BS170! You could replace the jumper (Red line in upper right hand corner of silk screen) with a 3.3K resistor and tack a wire onto the BS170s (Q4) gate. So it looks like the layout is a different version than the schematic. It also looks like it'll be a pain to stuff because there is no parts list and the silkscreen has parts designations but the schematic doesn't and you'll be modifying the PCB anyway. You could just eliminate the BS170 and put a toggle switch across the where the source and drain would be.

 

[bigal_scorpio]

I don't remember a double pole switch, but if you meant the "toggle switch" I mentioned, then no, make it latching.


Hi kchriste,

No I didn't mean the toggle switch. I mean the switch under the right of the LCD panel with Credits and Backlight on it.

If its a momentary then I am struggling to find one that has the two poles necessary to not permanantly short RC0 and RC1!

On the other hand I have plenty of latching double pole switches.

Any ideas?

Thanks................Al

 

[ericgibbs]

I don't remember a double pole switch, but if you meant the "toggle switch" I mentioned, then no, make it latching.


Hi kchriste,

No I didn't mean the toggle switch. I mean the switch under the right of the LCD panel with Credits and Backlight on it.

If its a momentary then I am struggling to find one that has the two poles necessary to not permanantly short RC0 and RC1!

On the other hand I have plenty of latching double pole switches.

Any ideas?

Thanks................Al

Hi Al,
All the 'credits' switch does is display the author/designer names in a sequence of 4 labels. Not required IMHO.

The LCD backlight is a latching switch, it seems a waste of two PIC pins to enable the backlight via the PIC.

 

[bigal_scorpio]

Thanks Eric,

On your advice a I will not bother with the credits and put the backlight on a seperate switch!

BTW did you notice any other flaws in the schematic to copper transition, kchriste and others seem to be a little concerned about some parts, do you think it will work?

Thanks again Eric...........Al

 

[ericgibbs]

Thanks Eric,

On your advice a I will not bother with the credits and put the backlight on a seperate switch!

BTW did you notice any other flaws in the schematic to copper transition, kchriste and others seem to be a little concerned about some parts, do you think it will work?

Thanks again Eric...........Al

hi,
I'll have a quick look.

Ran the program in Oshonsoft Sim, see the two LCD shots.
The 'credits' is what you get with that switched on, the 'esr' is what you see at power ON.

 

[ericgibbs]

hi Al,
My first thoughts on the schematic is that it was drawn on the back of a 'fag packet'.
Its laid out on the sch in a confusing hard to follow way.

The program ran first time in the OS Sim, which is promising.
He has also gone to the trouble of defining all the PIC registers in the program, why.??

The designer seems to gone to a lot of trouble to ensure all the PIC pins are used.?
eg:
Used all 8 LCD data lines. Driving the backlight via the PIC and a transistor.
Sw to select the credits display. etc.

As pointed out by Sarma, the LCD Vo contrast control.

Why he has got a 'fussy' power ON/OFF control is puzzling.?

I would recommend that you configure this circuit on a project board before considering an artwork.
Its not possible to say whether it will work or not.

IMHO the circuit to the left of the PIC [dwg] could be simplified.

 

[bigal_scorpio]

Hi Eric,

Well thanks to you I know the circuit should work!

But I am getting deeper and deeper all the time with the board itself!

Each time I trace where a component goes I find something strange eg. when I was going to place the BS170, and found that the Gate seems to come from the pos supply through the 1N4007 instead of the drain. So if he has cocked up on that maybe other parts pins are wrong on the board?

Also I seem to have struck a wall in the number of transistors, 5 on the schematic and mysteriously 6 on the layout, so I may be at the end of this project now! I wish I hadn't etched the board already, and it was my last biggish piece of copper too!

Any help appreciated mate, and thanks so far.

Regards...............Al

 

[ericgibbs]

Hi Eric,

Well thanks to you I know the circuit should work!

But I am getting deeper and deeper all the time with the board itself!

Each time I trace where a component goes I find something strange eg. when I was going to place the BS170, and found that the Gate seems to come from the pos supply through the 1N4007 instead of the drain. So if he has cocked up on that maybe other parts pins are wrong on the board?

Also I seem to have struck a wall in the number of transistors, 5 on the schematic and mysteriously 6 on the layout, so I may be at the end of this project now! I wish I hadn't etched the board already, and it was my last biggish piece of copper too!

Any help appreciated mate, and thanks so far.

Regards...............Al

Hi Al,
Ive had a further look and there are big discrepancies twixt the schematic and pcb layout.... I think it maybe another 'doorstop' project.!

Can you sketch out a circuit diagram from the pcb artwork layout, so that we can compare the differences.. its difficult going back and forth between the pcb and sch.

 

[bigal_scorpio]

Hi Eric

Hello again Eric,

I am still messing with the circuit, having now drawn my own circuit from the schematic.

Do you have any info on the LM339 that is reliable, as I have noticed that the LM339 was getting a bit warm! Then I looked further into the schematic wondering if I should ground unused pins, and the Datasheet says yes, but what I also noticed is that the schematic says pin 12 is V+ and pin 3 is Gnd, however the datasheet says 3 is V+ and 12 is Gnd!

So my question is which is correct, after all you did a simulation that worked, so I am now confused? Would your sim software have shown the reversed pins error?

Would there be any damage to the LM339 due to reversed polarity, as I only have the one

More to the point can you confirm through use which is the correct pinout for V+ and Gnd?

Thanks for looking at my latest installment of the ESR meter of doom!

Regards.............Al

 

[Mikebits]

Do you have any info on the LM339 that is reliable, as I have noticed that the LM339 was getting a bit warm! Then I looked further into the schematic wondering if I should ground unused pins, and the Datasheet says yes, but what I also noticed is that the schematic says pin 12 is V+ and pin 3 is Gnd, however the datasheet says 3 is V+ and 12 is Gnd!

You should always follow the data sheet. The schematic is wrong.

 

[kchriste]

Well, if you used the PCB layout, then your LM339 should be OK because the power pins are opposite that of the schematic.
Here is a good site for datasheets:
Datasheets.org.uk :: Datasheets :: Datablad :: Datenblatt :: Fiche technique

 

[mvs sarma]

On further analysis of the circuit, I doubt that the circuit never works as the osc2 pin is loaded with 0.1uF cap to rectify the clock. Instead, they might have buffered the clock with a FET and then processed it's output.

For On/Off using push switches, I feel that one need not depend on the clock as a signal.

Another observation-- the 2 red LEDs used on base- emitter junctions of PNP transistors, driven by 3.3 and 33k are doubtful for lighting up at 5V supply. the one with 33K is definitely wrong.

 

[bigal_scorpio]

Thanks for the replies guys,

I have been trying to get some sense out of this circuit for weeks now! I originally built the circuit from the authors pcb layout but there were too many discrepancies to ever get it to work!

Then perhaps foolishly I built the circuit from the schematic onto veroboard, not realising that the LM339 was connected reversed polarity, so I may have killed that now!

I just wish I could find another ESR meter circuit and then I could start afresh as I am fast running out of patience and parts.

If anyone knows if a 339 can survive reverse polarity or knows of a different ESR circuit I would be very grateful.

Thanks again.................Al

 

[ericgibbs]

If anyone knows if a 339 can survive reverse polarity or knows of a different ESR circuit I would be very grateful.
Al

hi Al,
It looks as though that project was an engineering students assignment.

There is no guarantee that it did or will ever work, I cant be sure how far the project was proven.
The software displays labels, but beyond that...

The LM339 is about 40 pence to replace.

Regards

 

[Roff]

Thanks for the replies guys,

I have been trying to get some sense out of this circuit for weeks now! I originally built the circuit from the authors pcb layout but there were too many discrepancies to ever get it to work!

Then perhaps foolishly I built the circuit from the schematic onto veroboard, not realising that the LM339 was connected reversed polarity, so I may have killed that now!

I just wish I could find another ESR meter circuit and then I could start afresh as I am fast running out of patience and parts.

If anyone knows if a 339 can survive reverse polarity or knows of a different ESR circuit I would be very grateful.

Thanks again.................Al

Have you Googled "ESR meter schematic"? I've been wondering why you are beating your head against the wall on this one.