Hello minipulsers

[qute1]

Built minpulse beach rev e followed the instructions exactly and all was well until i connected the coil (mono style 3 and 1 5 bridge) fine till i put the bridge in but now r10 gets very hot really fast and drains the battery if i take out the bridge its fine again but no detection. Circuit makes all the right noises but cant seem to get a step 3 waveform now either, does anyone have a clue what might be going on ? I have built many electronics kits over the years but this has me beat?

[Qiaozhi]

Built minpulse beach rev e followed the instructions exactly and all was well until i connected the coil (mono style 3 and 1 5 bridge) fine till i put the bridge in but now r10 gets very hot really fast and drains the battery if i take out the bridge its fine again but no detection. Circuit makes all the right noises but cant seem to get a step 3 waveform now either, does anyone have a clue what might be going on ? I have built many electronics kits over the years but this has me beat?

OK ... first question:
How did you manage to complete the project through to step 9 without having previously connected the coil in step 4?

Second question:
I assume you must have an oscilloscope in order to have correctly adjusted the TX oscillator?

[qute1]

Yes but found the connection diagram confusing as could not see how the bridge bit didn't short out until i found an answer on here (might be better show wires to board connection as some don't use commercial coils so no jack).
i have several pocket style oscilloscopes and didn't seem to be a problem all fine up until day after completed then reconnected and fine till i put in the bridge then burned pinky on resistor (sniff).

[qute1]

Only changes i made were c8 in document says 1uf but only had a 10nf in the kit and getting same result with 150uh 250uh and 600uh coils also found step 6 difficult as only have one channel so needed to use two scopes.... fortunate that i had scopes at all as there is no mention on the silverdog website that a scope is prerequisite.
I have built two Surfpi's without problems i just want to know what is the likely cause if its the tx or tx oscillator then what should i replace 555 is fine so think osc is ok but not sure about d7 q2?

[Qiaozhi]

Only changes i made were c8 in document says 1uf but only had a 10nf in the kit and getting same result with 150uh 250uh and 600uh coils also found step 6 difficult as only have one channel so needed to use two scopes.... fortunate that i had scopes at all as there is no mention on the silverdog website that a scope is prerequisite.

In the Build Document (on page 3) it says:
"Please note (for this revision) it is imperative that you have access to an oscilloscope during the
setup procedure. Given the adjustable nature of this version, it would be extremely difficult to set up
the detector without one. It is also mandatory that you possess a multimeter."

You will not be able to use two single-channel scopes instead of a dual-channel scope. The idea in this case is to trigger on one channel while comparing the delay on the second channel. Using two separate scopes does not serve the same purpose.

I have built two Surfpi's without problems i just want to know what is the likely cause if its the tx or tx oscillator then what should i replace 555 is fine so think osc is ok but not sure about d7 q2?

Assuming your coil is ok, I would first check that the TX oscillator pulse rate and pulse width are both set correctly. The coil inductance should be around 400uH. If you want to use the 150uH or 250uH coils, then you'll probably need to decrease the TX pulse width and/or decrease the pulse rate.

Then I would measure the average coil current with a multimeter in series with the battery. If the average current measures much higher than 200mA, I would [again] try adjusting the TX settings.

If you're still having problems, then set the TX pulse rate to 1000pps, and the pulse width to 100us. These are failsafe settings that will allow you to check the detector is working as expected.

[qute1]

Thanks i will give it a go still a bit miffed cos nowhere on silverdog does it mention needing a dual channel oscilloscope before you have bought the kit and an incorrect component set is very confusing also perhaps they could offer alternative basic configuration for those without dual channel oscilloscopes?

[Qiaozhi]

Thanks i will give it a go still a bit miffed cos nowhere on silverdog does it mention needing a dual channel oscilloscope before you have bought the kit and an incorrect component set is very confusing also perhaps they could offer alternative basic configuration for those without dual channel oscilloscopes?

Despite some people managing to build a metal detector without access to an oscilloscope (and I'm referring to other kits, such as Baracuda, Surf-PI, or previous incarnations of MPP) trying to do so is like having one hand tied behind your back. A metal detector project is not an easy starting point for a begineer, and not being able to "see" what's actually going on inside the circuit puts you at a major disadvantage. If you're extremely careful (and lucky) with some designs that have fewer adjustments, you can end up with a working unit. The majority of problems occur when the luck runs out and the detector fails to function. Trying to figure out what's wrong with only a multimeter is extremely difficult. I have fixed a few detector kits for some members who were at their wit's end, and in nearly all cases there were components in the wrong place, faulty ICs (damaged during the build), solder blobs that were shorting tracks together, etc. These would have been almost impossible to track down without a scope.

By the way, you might be able to get away with a single-channel scope if it has a Z-axis trigger that can be used to trigger off one signal while viewing another.

I've just checked the Silverdog website, and the Minipulse Rev-E Document is there in the left-hand pane. On page 3 it does say "it is imperative that you have access to an oscilloscope". Granted, it doesn't state a dual-channel is required, but you can get away with a single channel.

[qute1]

Thanks for your time and input i will just have try a few things and wing it i will try my other scope just waiting for bnc probe to arrive today, i think it has z-axis trigger so fingers crossed. just that that r10 is instantly very hot gives me pause as not much time to adjust any thing before it lets the magic smoke out (been lucky thus far).
Oh and i wish i had read the document before purchasing the kit as i might have chosen a different one.

[qute1]

Meh still drawing 1.054 AMPS no matter what i twiddle as soon as connect the bridge. Bottoms........

[waltr]

R10 (3R3 - 2W) is in series with the Coil. If this is ok without the jumper on the connector (bridge) then you have a SHORT on the RX side.
Re-check the soldering and connections and part values. Ensure you have the bridge on the correct pins and connector soldered to the correct pads on the PCB WITHOUT any solder shorts.
The RX side is a 1k Ohm series resistor and will limit current so something here is definitely wrong. As Qiaozhi suggests and commonly it is something very simple so get out your magnifier and Ohmmeter and re-check.

[Qiaozhi]

Meh still drawing 1.054 AMPS no matter what i twiddle as soon as connect the bridge. Bottoms........

Can you post a drawing of the way you've connected the coil, just in case there's a mistake there?

[qute1]

[qute1]

Thanks walter just going to do that as you say its prolly summat simples and i have been concentrating on the tx side.

[qute1]

Still no joy i have been over with a fine tooth comb all resistor correct no obv shorts i agree it must be giving a direct path to earth to get that hot but off for another look just wierd cos all fine till that bridge is in..... Goggles on again.....

[Qiaozhi]

[ATTACH]42856[/ATTACH]

Looking at your drawing, I note that the coil is actually connected to the RX input. In that case, without the bridge, the TX will be running without the coil connected.
Try connecting the coil to the TX input without the bridge. I would bet that R10 will still get very hot, even with the bridge removed. This will point the finger at the TX oscillator.

Are you sure the oscillator is actually running?