Sketchy – Integration

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Mar 022024
 

Sense Wire Loop

The first step to prepping for installation is to clip the sense wire used for testing and convert it into a loop – to act as a terminal on the board. The plan is to clip the existing sense wire in the controller and insert “sketchy” into that line. In order to make that work we need to have an easy way to connect both ends of the clipped sense wire. A loop on each end of the board will do nicely.

Find Power

The next step will be to identify the cleanest way to get power to our board. As it turns out the controller has regulated 5V rails in the middle of it’s board AND (lucky for us) there are two adjacent holes on those rails exactly in the right place for our ribbon wire. Here you can see the red and black ribbon wire matched up with the rails… carefully inspecting the circuit to make sure the polarity is correct and precisely where these wires will come through the controller board.

Not visible in this shot, on the top side of the board near the top of this picture I identified the 5V regulator and it’s pins to make sure which of the two rails in the middle are positive and negative.

Power Leads Inserted Top Side

Here I’ve inserted the power leads from the amplifier into the controller. They fit correctly flush up against the board and it’s clear that the colors for + (red) and – (black) match up with other connections on the board. This is another good sign that I have the polarity right.

Power Leads Inserted Bottom Side

Viewed from the bottom side we can see the wires coming through the holes in the power rails on the controller. It all looks correct (again checking carefully) so we can solder it in.

Power Leads Soldered

Here is a close up inspection of the new connections soldered into the rails. No unwanted solder bridges. Nice smooth, shiny solder joints. Some of the other nearby joints leave holes uncovered but otherwise they look fine… so I resist the urge to reheat them.

No Melted Insulation Top Side

One of the reasons I resisted the urge to re-heat those other joints is because I don’t want to begin melting insulation or causing other trouble. The connections themselves are electrically just fine – -it doesn’t matter at all (except maybe aesthetically) if the holes aren’t completely filled in. But, it DOES matter if the insulation melts potentially causing a shorting hazard.

Here we can see that the newly installed wires to power “sketchy” are not melted at all. That means there was just enough heat for just long enough to make a good solder joint.

Clip Existing Sense Wire

Now that power is available to the amplifier board we need to put it in line with the sense wire. The existing sense wire is the white wire in the maw of the clippers. I have dry-fit the board into that location in the box and so I know I can clip the sense lead right in the middle of that location. One side of it will bend back to the input of the amplifier and the other side will bend back to the output. This will leave just enough extra lead to safely solder the connections and move the new board around to the right position in the box.

Carefully Calibrate Stripper

A tricky part of the installation will be trimming the insulation off of the freshly clipped sense wire. It’s important to get this right the first time… because if this step goes wrong then there will be a lot more work to do replacing that wire and it’s other connections.

The wire strippers have a small bolt and nut that can be slid into the correct position to calibrate the strippers and then tightened down. So, with the calibration bolt loosened, I take a similar wire that I can use as a gauge and carefully close the strippers on it until they just barely cut all of the insulation without nicking the conductor. Then I carefully tighten the bolt and re-check that the calibration didn’t move during that process. It seems like a lot of work – but sometimes there is no substitute and I “really” don’t want to have to replace those wires if I can avoid it so it’s worth the effort.

To make sure I have it right, I strip a few more bits of insulation off of the test wire and inspect the conductor to ensure it’s not nicked; all the while making note of the pressure required to squeeze and pull the insulation so I know what will feel right when I do the real thing.

Sense Lead Cut, Stripped, and Tinned

Once I’ve stripped off about half a centimeter of insulation from both halves of the sense wire I tin the ends so that it will be easy to get these soldered into place. With tinned wires, one can push them through the connector loops and give them a quick hit with the soldering iron to get them connected. Then when both are done it’s easy to go back and re-heat them and fill them in a bit without having to worry much about the board or connections moving around during the process.

Sense Leads Inserted Into Loops

Here is a shot of the board in place, top-side up, with both ends of the sense wires “tacked” in place.

Sense Leads Soldered

With that done, we can go back and re-heat the connections and fill them in a bit to make things good and solid. Here is a shot of that from the top at about the angle I saw soldering them.

Side View

Here is a shot more from the side.

Sense Lead Output Close Insepection

Just to make sure the connections are solid I used the camera to take close-up inspection shots — here is the output lead close up.

Sense Lead Input Close Inspection

Here is the input lead close up. This shot is a bit motion-blurred, but it’s clear that the connection is solid and shiny.

Installed

With all of the wires connected it’s time to flip the board over into it’s permanent location. Part side down, directly over the input connections. This protects the parts by keeping them (as much as possible) in free space away from the top, and makes the circuit easily accessible for any probing I might want to do later.

Smoke Test – Green Light!

A quick look around to make sure everything still seems correct and then we turn on the power. Hurray! No magic smoke has escaped and as expected the green light is on! The tuner is virtually always in a tuned state so this is what we expect. It looks like the project is a complete success.

Hrmmm, That’s Not Right

… except, after letting WSPR tune up on a few bands I notice that the green light never seems to go out. Occasionally, as I recall, the amber light should come on while the tuner is working it’s magic… and that doesn’t seem to be happening.

So, I hook up the meter to the input side of the amplifier via some clips and I see it’s barely a diode drop below the 5V rail… or should I say a BE (base, emitter) drop.?

It seems that my sketchy open collector signal amplifier works too well. It’s clear that the signal line from the tuner is not open, and it’s also not shorted, nor is it being pulled to ground as expected. In previous tests I noted that the voltage did vary quite a bit as if some signal were making it through that line and into the controller… so I expected that I could amplify that signal and get the desired result.

As it turns out, that was not a good assumption… and I will have to go back to head-scratching, and perhaps go and investigate connections out near the antenna to see what’s actually happening there.

In any case, this was a fun little project and it does do what it’s designed to do… it’s just that this sketchy idea has turned out not to be the right solution. I guess that makes it a successful failure.

Sketchy – Prototype

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Nov 112023
 

I thought “Ok, so let’s throw some parts at this thing and see if it works like the simulation…” and so I did:

As it turns out, I had a few different parts laying around the bench and using those would be easier than hunting down the specific values I had in my original simulation… those values aren’t critical really, so I just used what was already lying on the bench and updated the schematic in EveryCircuit to match:

Built up on the breadboard it looked like this:

I find that with a little effort and creative thought it’s not difficult to get breadboard layouts to closely resemble schematic diagrams… and from there it’s not too much of a leap to get to what the prototype(s) and final build(s) will resemble.

A clockwise tour of the circuit on the breadboard…

Here you can see from top to bottom on the breadboard matching roughly from left to right on the schematic: There is a 47K input resistor that I’ve connected to a white wire that will act as the switch.

I describe that as a “wire” so that I can reserve the word “jumper” for the short color coded jumpers I use to make patches between segments of the breadboard. These are fantastic, by the way, because they are color coded for the number of points they span. This saves a lot of time when building up more complex circuitry or trying to trace where one or more of these jumpers go…

See the red jumper connecting 2 points away, and the orange 3, and so forth… very nice. I bought a bunch of boxes of these and I use them all the time. They’re so cheerful and perky. Full of potential – like a fresh box of colored pens and a new graph pad. I love that… but I digress.

You can see the input resistor go directly to the base of the PNP transistor, and on the back side of that transistor you can see the filter capacitor between base and emitter; an orange jumper connecting that to the positive rail 3 points away.

Moving down from there we come to the MOSFET acting as a switch. A red jumper connecting the collector of the input transistor to the gate of the switching MOSTFET and then from there you can see another 47K resistor pulling that down to the negative rail on the right.

In parallel with that a yellow jumper connects the source of the MOSFET to the negative rail (which will be common ground)… so that the other side of the switch is a typical “open drain” version of the “open collector” type of switch. The key idea being that it’s either open, or connected to ground.

Speaking of the “open drain” you can find an orange jumper crossing the centerline gap to the left and acting as the “output” of the circuit. Over on the left side you see couple of header pins stuck in the breadboard as a test point connector of sorts and then a panel mount LED connected on it’s other side to the positive rail through a 470 ohm resistor. (It was laying right there, and I didn’t care about the LED being a little bit dimmer, so I just went with it!)

You may have a little trouble spotting the header pins because there is a white alligator clip chomping on them… and if you look over to the right you’ll see a black alligator clip chomping on another set of pins plugged into the negative rail. These two clip leads make it easy to keep the DVM in circuit to measure the voltage across the output of the circuit… that way we can see just a bit more detail than whether the LED is on or off.

Testing Sketchy with a floating input

The guess is that the tuner controls the status line as an open-collector type of output. This means that the long wire running from the shack out to the tuner would see a very high impedance and would be essentially “floating” whenever the open collector is “off” at the tuner.

In this test you can see that the white input wire is hanging in the air on the left. The voltage at the output of our circuit is about an LED drop down from the positive rail… I guess because the DVM pulls just enough current to see that drop. 1.256v = 5 – 3.743

The LED is off… so all is well.

Testing Sketchy with a “human antenna” input

A long “floating” wire seems a lot like an antenna that might pick up all kinds of noise or even some stray RF (this is going to an antenna tuner after all). We want to make sure the circuit isn’t sensitive to that so we put in a filter capacitor across the BE junction of the input transistor. The idea is that the input resistor and the capacitor make a low pass filter that will reject almost anything that isn’t close to DC.

Connecting myself to the input wire to give it some “extra length” we can see that I do inject some noise; but not enough to cause the circuit to really switch. The LED stays off, and the voltage across the output only goes down by a couple hundred millivolts. That’s enough to see on a DVM, but not enough to matter when switching LEDs on and off. (Nor even enough for any logic circuit that might come later to care about it either.) 201mv = 3.743 – 3.542

Testing Sketchy with the input pulled high

Another design spec is that the circuit should be happy with a TTL input. So, connecting the input to the positive rail (+5V) we get the same result as the floating input: The output is off as expected!!

Testing Sketchy with the input pulled low

Testing the other side of the TTL input spec as well as the open collector input spec, we pull the input low by connecting it to the negative (ground / common) rail. As expected, the output goes to ground and the LED comes on!! Not only on, but VERY on… 11mv measured across the output.

This is a benefit of using a good switching MOSFET vs a bipolar transistor for this kind of output (open drain/collector). The “closed circuit” or “on” condition of the output with the MOSFET will have a much lower voltage than you can get from a bipolar transistor. Definitely closer to ground.

Testing Sketchy with the input pulled low weakly

Finally, since our theory about the status line problem is that it may be oxidized at the connector near the tuner, we tested again with a high resistance in place. In this case, a 220K resistor to simulate whatever that unwelcome resistance might be out there. Again success! The LED is fully on just as if the input had been pulled hard to ground.

Qapla’ !!

Sketchy Long-Run “open collector” cleanup

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Oct 102023
 

A while back we replaced the SGC-237 at the feed point of the big loop. Since then, the controller has been a bit sketchy. The indicator for a good tuning solution is intended to light either the yellow panel LED or the green panel LED; but since the upgrade the best it seems to manage is a bright or dim yellow.

I’m not sure what’s going on there, but I have a few theories. One is that the junction for the control indicator line in the box under the tree might be a bit oxidised causing extra resistance in the line. Another is that the design of the SGC-237 might have changed with the unit I have now, or that perhaps it never really was an open collector on the tuner end (that was a guess afterall). Another is that I just got lucky with the first design and that the long lead length might have something to do with my indicator not-quite getting the clear signal it needs.

I did measure the voltage across the input while the system was up doing it’s WSPR thing (where it must tune with each band change)… and I observed that the voltage from the indicator to ground seemed to get no higher than about 4V and no lower than about 3.2V… that’s weird, and suspiciously close to a silicon junction drop (about 0.8V)… almost as suspicious as frog’s breath… but of course, nothing is as suspicious as frog’s breath. Whatever is going on there has to do with how the indicator circuitry in my controller is interacting with the tuner through the controller cable and the junction box in the middle.

Anyway– at some point I will go out there and lie on the ground and “improve” the connections in the box. In the meantime, since the tuner seems to otherwise work just fine, I’m looking for a quick fix to get my indicator working properly again and generally make it more robust.

So, based on the idea that it’s probably some thing that looks reasonably like an open collector at the tuner end, or that even if it’s a TTL signal I might be able to interpret it like an open collector… I designed a circuit to clean up the indicator signal and make it FIRMLY on or off in an open collector manner.

Here is a snapshot of the schematic from Every Circuit (which was handier and a bit cleaner than my usual graph pad + phone camera… even if the N-Channel Mosfet symbol is a bit odd):

The diagram presumes I’ll be using the 5V supply from my existing controller. There is an SPST switch representing the input of the circuit which is the indicator line from the tuner. The output is the drain of the N Channel MOSFET that I’ve shown here connected to an LED via a 330 ohm resistor… but essentially that output will either be open if the input is “open” or “high”, or closed (shorted to ground) if the input is drawing current down from the 5V supply … as if it were “closed” (or trying to be) or “low” if it’s acting like a TTL signal.

Either way, the circuit should clean up the input by slamming the output fully open or closed. Here is the theory:

The input is tied to the base of a PNP transistor through a 47K resistor to limit the base current. The emitter of that transistor is tied to +5V (the positive rail).

If the input represents a high impedance between the positive rail and ground then effectively no current will flow through the base of the PNP transistor and it will be off. This will happen if either the input is something close to +5V like a TTL signal, or if the input is high impedance in general like an open collector would be.

The collector of the PNP transistor drives the gate of an N Channel MOSFET which is otherwise pulled to ground through another 47K resistor. So, if there is no current on the input then there is no current through the PNP transistor and the gate of the MOSFET will be at 0V. This will turn the MOSFET off and so it’s output will be “open” to ground.

On the other hand, if any current (even a fairly small one) flows to ground on the input, then the PNP transistor will switch on and pull up the gate on the MOSFET thus turning it on. The voltage gain of the PNP transistor given a 47K load (the MOSFET gate being essentially invisible to it) will be very high so that even a tiny current through the input will be enough to pull the MOSFET gate well above it’s “ON” voltage.

The choice of a 47K resistor on the input and also on the collector/gate is the same in both cases… it’s a high impedance (but not too high) and a handy value (I have a pretty good inventory of standard values like this). On the collector side of the PNP transistor this means a lot of gain. On the base side of the PNP transistor it means that not much current can flow through the transistor no matter what the input voltage is (within reason). That said, the beta of the PNP transistor is likely on the order of 100 so whatever the input current is at the base the output current will try to be about 100 times that amount.

I measured a 0.8v change between on and off in the existing circuit. The new circuit will amplify that by about the beta of the PNP transistor since the same resistance is on the base and on the collector, so any gain as big as 6 or so will be enough to swing the collector side between the 5V rails. With a beta of 100 I’ll be more concerned about instability than missing a weak signal. (I’ll address that if it shows up; but for now, simpler is better.)

Since I have these parts around I chose a 2N2907 (the complement of the 2N2222) as the PNP transistor; and a 2N7000 (a very common switching part) for the MOSFET.

SG-237 Controller for the Big Loop

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Jan 042022
 

At the heart of the “big loop” antenna is the SG-237. Click here to see the manual for that.

The tuner can be run without a controller, but it offers a few features with a controller that are useful (and sometimes important) in practice. I took a look at the controller suggested in the manual and re-designed it a bit to better suit my purposes. First, because I wanted to use parts that I had laying around the lab, and second because I wanted a better “light-show.”

The important features are:

  • You can tell the tuner when NOT to tune. I find in practice that this can be particularly important when running digital modes as switching things around at the antenna can inject unwanted noise to your transmitted signal.
  • The tuner can tell you when it’s found a good tuning solution. Sure, it seems like this would be obvious enough once the SWR stops bouncing around… but it is awfully handy to have a nice green light tell you when the tuner has stopped looking.
  • I found this out AFTER I built my controller – The tuner can tell you when it didn’t find a solution but has given up trying! On my tuner controller there is an amber light for when the tuner doesn’t yet have a solution and a green one for when it has found a solution. One day I saw these flashing back and forth like it was shaking its’ head. I figured out that’s what it does when it gives up trying. This doesn’t seem to be documented anywhere, but it’s also a nice feature.

The first step was to draw up a schematic for the circuit and do a little math to make sure everything would work.

In the controller recommended in the manual they roll their own ~9 v regulator using a zener diode and an NPN transistor. They only use this to drive their LEDs, so I did something similar but instead used an actual 5V regulator.

The next thing I changed was the “tuned” indicator logic. Their controller pulls one side of their tuned indicator to ground when the tuner is happy. I presume this is done through something like an open collector in the tuner.

I wanted two lights instead of one so I added a 2N2222 transistor and a resistor to turn on an amber “not-tuned” LED when the green “tuned” LED is not on. Basically, the green LED and its’ current limiting resistor act like a pull-up resistor to bias the transistor on whenever the “tuned” LED is not pulled to ground by the tuner. The 47K resistor in series with the base ensures that any current that flows is tiny enough that the green LED won’t light (at least not in a way you can see it). The gain of the transistor is high enough that it will still effectively saturate in this condition thus turning on the amber “not-tuned” LED.

The rest of the circuit is essentially the same as their controller – so the tuner sees almost precisely the same signals. This consists of a DPDT switch, a momentary SPST push-button, and a handful of decoupling caps. All of this, a handy box, and some LEDs with built-in current limiting resistors were all handy in my lab… courtesy of the recently (at the time) defunct local Radio Shack – and my irresistible urge to grab everything I could from them in their last days.

Once the design was done it was time to put the mechanical components into the box and see how they all fit. Here is where some on-the-fly creativity was required because one cannot always be sure what parts they have nor how they can be used to solve a particular problem… I mean, I wasn’t building this from a BOM where I could order up precisely what I wanted right?! I had to see what I had around and improvise with that.

As it turns out all of the parts I had handy fit perfectly including some PCB mounted screw terminals that I was able to adapt to the back of the box with a little bit of drilling, sanding, and some small pieces of protoboard.

Next up I designed the layout of the electronics on another piece of proto-board. It’s always a good idea to take this extra step rather than going directly to soldering parts in place – even on something simple like this. The end result almost always turns out better and cleaner for the extra effort.

Then, once I’d put all of the parts in place I made a few measurements (idiot test) to make sure I got it right. A quick look at the box also informed me that I was going to need to make a notch somewhere on the board so that the wires from red LED could get to the other side. The simplest solution to that was to knock off a corner of the board.

Finally I connected everything together and “stuffed” it into the box. I say “stuffed” because, well, it’s a sloppy jumble of wires going everywhere all kind-of crammed into that space. I thought about making it neater, or maybe doing a more sophisticated PCB that would eliminate much of the wiring, but in the end this was a quick-and-dirty job. As such, the extra length of the wires was needed in order to be able to assemble and disassemble the device for testing and/or changes.

If you make the wires too short then there’s no room for getting the circuit board into and out of the box without having to desolder something. The lead length also doesn’t matter too much in this case since it’s all low-voltage DC, and the heat dissipation requirements are vanishingly small – so “stuff” it is.

Make one connection at a time until everything is wired up, make a final test, then it’s stuff-in-a-box. 🙂

Once the lid is on and it’s up and running it’s a pretty solid and fairly professional presentation. The LEDs all work as expected, and the orientation of the toggle switch and the red “do-not-tune” light make the user interface intuitive. With the switch to the left (away from the controls) the red LED is on and the controls are locked. With the switch to the right (toward the controls) the tuner is free to tune and can be reset with the push-button.

I’ve thought about doing something more sophisticated with this… and maybe putting it in a heavier box; but ultimately it does the job, has been reliable, and there really isn’t more to do!

In future, maybe, if I built a feed-point tuner I might like to have it provide information about its’ tuning solution and even provide an analysis of the antenna… or perhaps also take commands to fine-tune the solution or act as a pre-selector … but that’s just me dreaming and NOT what this tuner does. This one is designed to be simple and reliable and it hits those marks very well.

This controller seems to hit those marks too — with just one extra blinkenlight 🙂