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Mr Carlson's Super Probe

Using Standard Through Hole Components on Stripboard / Veroboard





Generally, I don't feature other people's projects on my website, but this is a notable exeption because it's so cool I couldn't help myself. If you've never heard of the 'Mr Carlson's Lab' YouTube channel, then you can discover him by clicking here. Also, to go directly to his original 'Super Probe' project, click here. In his build, he uses surface mounted components on a dedicated PCB, but as this site is all about building stuff the old fashioned way, mainly on stripboard using standard through hole components, that is how I built my version. I hope the information will be useful to those who have a pile of components in their parts bin 'itching' to be put to good use! Of course, all credit goes to Paul Carlson, with this being just my take on his idea




Probe Schematic (Click here to enlarge)


I won't go into too much detail here about how the circuit works as this is covered in Mr Carlson's video, but basically it consists of two parts, the probe itself which contains an extremely high gain amplifier, and a control unit consisting of an audio amplifier, speaker and 9V battery. The probe circuit is quite specific and must be adhered to, though I did substitute the two 4.7uF supply decoupling capacitors for 10uFs which are perfectly fine in this application, only because I couldn't source a subminiature 4.7uF at the time. Another change was the 360pF cap which I substituted for a 330pF as they're a bit more standard here in the UK, again perfectly fine. Paul mentions an optional 10K resistor from the output to ground which is required if the probe is to be fed into a high impedance input, but I included it because I wanted to have the coupling capacitor on the probe output rather than on the LM386 input. And finally, I have omitted the LED because personally I prefer to listen and not watch, but it can be added very easily if required




Probe Stripboard Layout (Click here to enlarge)




Probe Stripboard Cuts


It seems that most people prefer to build their probes in tubular handles, but I came across an amazingly good quality square section extruded aluminium enclosure which is comfortable to hold and matches the control unit. Having said that, my stripboard layout is designed to fit inside the body of a standard de-soldering pump if need be, which required very careful component placement and some very small electrolytic capacitors! The circuit board is mounted inside the enclosure using a 3mm thick double sided sticky pad, after first gluing a strip of plastic onto the metal case to act as an insulator of course! As there is no room for any type of board headers, connections were made using 0.25mm 30AWG wire wrap cable which is thin enough to pass up through any of the spare stripboard holes for a bit of strain relief. The connection points are marked in colour on the layout above (3 go to the toggle switch, 3 to the umbilical cable and 2 to the F socket). Grounding to the case is made via the F socket earthing tag which is explained in more detail later




Completed Probe Board

Unlike the probe, you can be quite flexible with the design of the control unit, as it's really just a 'bog standard' audio amplifier. I used the trusty old LM386 as I had one available, and also because it has a gain of 200 which means that the extra preamp transistor at the input of the original circuit can be omitted. Incidentally, the original hand drawn circuit is visible in Paul's video, which can be easily paused to take notes. Audio can be fed to either the inverting or non-inverting input of the LM386, with the unused input being connected to ground. I chose the inverting input simply because I was able to create a neater looking board layout. Note that an IC socket is used with pin 3 snipped off and a blob of solder added between pin 4 and the remainder of pin 3 on the actual socket (shown greyed out). This trick allows the positive supply to pass underneath the IC to pin 6. In the original circuit, the volume control track is effectively the emitter load resistor of the probe, but I found that this caused a 'shushing' sound when the shaft was turned due to DC on the wiper. Placing a 1uF capacitor before the pot prevents this, and here it's added in the probe rather than in the control unit


Amplifier Schematic
Amplifier Stripboard Layout
 

Amplifier Stripboard Cuts


Completed LM386 Board
The controls are mounted on the front panel of the enclosure along with a 5 pin DIN input socket. The volume control pot is a 10K linear type which incorporates the power switch, and a red LED indicates that the unit is on. The 5K pot (also linear) forms a potential divider across the 9V battery which provides a variable voltage supply to the probe and acts as the sensitivity control. Paul's original circuit runs from a 5V supply, but I'm using 9V from a PP3 battery. The LM386 works well on 9V and by keeping the sensitivity control to just over half way, the probe will get its required 5V. Turning the sensitivity control fully clockwise will give 9V to the probe which won't do it any harm, though the transistors may turn fully on and saturate. Having voltage available on the DIN socket is quite handy, as it means that other accessories and projects requiring power can be connected


Control Panel Schematic




Completed Control Unit


And what about the actual sensing 'antenna'? Well, the main criteria for this is that it must be completly screened all the way along its length except for just a tiny amount of wire (about 8mm) protruding from the end. For mine I used an F connector and a short length of satellite double screened TV coax with an end cap for insulation. Simple, cheap and it can be swapped out easily. For the umbilical cable going to the control unit I used Comas ATM balanced mic cable which exits the enclosure through a flex cord grip as used on lamp fittings. I find these very useful for all sorts of projects, but a 10mm nut needs to be added. To house everything in I used an extruded aluminium enclosure from Amazon which was the perfect size, though drilling out the holes for the loudspeaker did require some extra care to get right! Mounting the speaker downward facing isn't a problem as the whole thing seems to act as a sound box and it's not intended to be Hi-Fi anyway, espectially with a 50mm paper cone driver!




Size Reference



Yes it can fit inside a desoldering pump!



The Final Enclosure



The Sensing Antenna



Through hole rules... Sometimes!