Replies to Minipulse Plus

[green]

A 0.47 gram nugget (7.25 grains) can be emulated with a 9mm x 9mm aluminium square from the side wall of a Cola can. Your 10mm x 10mm square of aluminium foil is much thinner, so I doubt you'll be able to detect it.

Thanks for the information. It's the first time I remember seeing a target size referenced to a nugget size. I recorded some data with the two targets. Did it with two different coils to see if the time constant stayed the same. Got .6usec time constant for foil and 2usec time constant for the can.

[green]

Pulltabs are still very loud. So I am thinking about making the reject pot much larger, maybe 250k?

Ran the time constant for an aluminum pull tab. 10.7usec.

[Qiaozhi]

George, could you please post a sort of specification on timings in form of spans; delay to first sample (min, max), first sample duration (min, max), delay to EF (min, max)
Thanks.

The main sample delay is set by (Reject pot [50k] + R6 preset [100k]) * C8 [220pF]. If the Reject pot is turned to zero, and you adjust R6 to lower its resistance, it should be possible to get the minimum main sample delay down to 6-7us, although the datasheet for the MC14538BCP specifies 10us as the lowest value. The maximum sample delay will depend on where you set the minimum delay. So, if R6 is adjusted to 45.45k to give a 10us delay, then (45.45k + 50k) * 220pF = 21us (max). If you want to have a bigger maximum delay (which would probably be a good idea), then increase the value of the Reject pot and/or C8.

The secondary sample delay is set by (R10 [10k] * C10 [10nF]), which equals 10k * 10n = 100us. This value is added onto the main sample delay to give a range from 110us to 121us (if the minimum main sample delay is set to 10us). Again, if you want the secondary sample delay to be at 200us (for example) then increase R10 to 20k.

Both the main and secondary pulse widths are set by (R11 [22k] * C11 [2n2]), which equals 22k * 2n2 = 48.4us. However, due to the diodes D4 and D7, the actual pulse width is 58us.

As you can see, the range of values depends on how you adjust the minimum main sample delay.

[joop]

Ran the time constant for an aluminum pull tab. 10.7usec.

Thanks, will keep that in mind.

[johnandles]

Just a quick note on Rev C build document.
At Step 3 Q2 (2n3904) is not mentioned to be installed, but Rev B it is...so go with "B"...

[Qiaozhi]

Just a quick note on Rev C build document.
At Step 3 Q2 (2n3904) is not mentioned to be installed, but Rev B it is...so go with "B"...

I have updated the errata list for REV-C to include this information.

Although U2 was in the list of components in Step 3 of the REV-B document, it was removed for REV-C because of the sync problem (which has now been resolved).

[Davor]

Davor have you built your time base ? how does the 4017 cycle through with the clock connected to the reset ?

Maybe I dont understand how a 4017 works,
the counters need to be reset at some point in the sequence,but if the 555 pulse is connected to the reset pin (15), (not the clock pin 14), the 4017 will reset the counters back to zero with each 555 pulse, how do the counters cycle if the 555 is continuously resetting the 4017 with each pulse.

Hmmm is U4 somehow running as a independent clock
how does this thing operate Davor ?

6666, I'm not ignoring your posts, it is only that your repeated questions kinda confuse me. I answered them already, but well...
For clarity, yes, there are actually 3 time bases. The main one is set by the 555 and it controls the rest of shebang. The Schmitt triggers are responsible for a second time base. I need a time base for samples, and as the samples are mere multiples, the timebase is constant for sample, twice the GB, and EF pulse. The third time base is a bit hidden, as it is the first pulse of a time base which is variable to account for different GB exponents. I designed it to operate as a ratio against the sample duration, and thus the GB setting remains valid for some range of sample durations.

We had a lot of bad weather here, and as my "lab" is a table under the stars, I can make something only if the weather is nice. Trust me, I'm already twitching in expectation to see this thing working.

[joop]

Just some info after a little test... with delay set to a minimum, which is 6 uS in my case, I need a 250k pot to reject pull tabs.

[multieagle]

Am looking forward to this as my next project. I have a question regarding coil connections. Perhaps it is mentioned elsewhere in this thread, but have not stumbled across it whilst speed reading through the material. It relates to the three PCB connections. First, am I correct that "Screen" refers to the coil and cable shielding? My coils are constructed with a copper shield. The shield has a wire soldered to the coil ground wire, which is in turn connected to the cable shield. Thus at the PCB there are only two connections: shield to negative and central conductor to positive. Can this type connection be used with the Minipulse? Or must I used a two conductor cable from the coil, with the coil shield connected to the cable shield, then connected to the third (Screen) PCB connection?

Or, have I completely misinterpreted it?

[Qiaozhi]

Am looking forward to this as my next project. I have a question regarding coil connections. Perhaps it is mentioned elsewhere in this thread, but have not stumbled across it whilst speed reading through the material. It relates to the three PCB connections. First, am I correct that "Screen" refers to the coil and cable shielding?

Yes.

My coils are constructed with a copper shield. The shield has a wire soldered to the coil ground wire, which is in turn connected to the cable shield. Thus at the PCB there are only two connections: shield to negative and central conductor to positive. Can this type connection be used with the Minipulse? Or must I used a two conductor cable from the coil, with the coil shield connected to the cable shield, then connected to the third (Screen) PCB connection?

That will work fine.
Have a look at Fig.7 (page 13) of the Build Document, and note the wiring for a mono coil. Essentially you connect your shield wire to 0V.
The most flexible way to wire your coil connector is to take one wire from the middle pin of PL2 and connect it to pin 1 on the coil connector, and a separate wire from the middle pin of PL4 to pin 5 of the coil connector. Pin 3 of the coil connector goes to 0V (which is any of the outer pins of either PL2 or PL4). Then, when you wire the coil plug, you can connect 0V (coil shield) to pin 3, and the "hot" wire to both pins 1 and 5 (this is for a mono coil). For a coil with separate TX and RX windings you can connect 0V (coil shield to pin 3), the TX wire to pin 1, and the RX wire to pin 5 (obvously without the short).

[multieagle]

Thanks. I knew it must be mentioned somewhere.

[6666]

6666, I'm not ignoring your posts, it is only that your repeated questions kinda confuse me. I answered them already, but well...
For clarity, yes, there are actually 3 time bases. The main one is set by the 555 and it controls the rest of shebang. The Schmitt triggers are responsible for a second time base. I need a time base for samples, and as the samples are mere multiples, the timebase is constant for sample, twice the GB, and EF pulse. The third time base is a bit hidden, as it is the first pulse of a time base which is variable to account for different GB exponents. I designed it to operate as a ratio against the sample duration, and thus the GB setting remains valid for some range of sample durations.

We had a lot of bad weather here, and as my "lab" is a table under the stars, I can make something only if the weather is nice. Trust me, I'm already twitching in expectation to see this thing working.

Hi Davor, no drama, its all good,
Sorry if you answered me somewhere else but I missed that reply
my questions where more a response to silver dollar's reply #325, sorry for confusion .

I built the time base but could not get it to work properly so pulled it apart, but on retesting the 4017
it tests faulty, and does not cycle correctly, so will build the time base again , thanks for the reply cheers.

[CAS]

In the schematic of Rev C there is a 100pf cap from pin 4 on U2 going to collector of Q2. Is this another mod as it's not on pcb layout.
Also is the addition of the sync cap on pin 7 the only difference in circuit between rev c and rev b?

[josewashere]

Well guys after a few hours of trying different opamps not much result to report.

Lm4562 in place of the ne5532 gives a tiny bit more sensitivity with a corresponding increase in instability

TL072 in place of the NE5532 shifts the threshold further up the adjustment pot and really smooths out the threshold. The threshold is far more stable and sensitivity is same to maybe a touch less. The chattiness as you advanced the threshold pot past the slow ticking sound is gone!

LF353 in place of the NE5532 is very similar to NE5532

Of course all this was tested in a room full of electronics so in the field the far from interfering sources the LM4562 could be better with its apparent higher gain. Has anyone tried using a matched set of 470nf for C13 C14? I had 6 of these on hand but couldn't match two.. Amazing the spread of values 447 to 483nf from same batch.
I really like this detector and hope to add a GB to it so if anyone has a diagram for one I would be keen to try it

[johnandles]

In the schematic of Rev C there is a 100pf cap from pin 4 on U2 going to collector of Q2. Is this another mod as it's not on pcb layout.
Also is the addition of the sync cap on pin 7 the only difference in circuit between rev c and rev b?

What version board do you have?.
You mean pin 7 on U2? to Q2 collector
that is the sync cap.
The first board has also a link to be added from pin 3 to pin 5 on U2