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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, again perfectly fine. I had to add the optional 10K resistor from the output to ground which is required if the probe is to be fed into a high impedance input, or in my case, an amplifier with a capacitor on its input




Probe Stripboard Layout (Click here to enlarge)


The circuit board is grounded to the enclosure through a 3mm eyelet tag connected to the emitter of the second transistor, shown as a green dot on the layout above. Incidentally, any common general purpose NPN transistors can be used, so I used my favourite BC337 as I have lots of them. It's probably best not to use transistors of too high gain as they may be a bit much in this circuit. And also if possible, try to source the smallest components you can find, which will help if you want to fit the board into a small enclosure




Probe Stripboard Cuts


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 easily be 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 the board layout looked neater. Note that an IC socket is used and pin 3 is snipped off and a blob of solder added between pins 3 and 4 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 if I connected the probe directly to the volume pot I could hear 'shushing' caused by DC on the wiper as the shaft is turned. Placing a 1uF before the pot prevents this. The volume control and input capacitor are mounted 'off board' on the front panel


Amplifier Schematic
Amplifier Stripboard Layout
 

Amplifier Stripboard Cuts
The controls are mounted on the front panel of the enclosure along with a 5 pin DIN input socket (pins 4 and 5 are not used and so make convenient mounting tags for the two elecrolytics). 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 devices or projects requiring power can be connected. The PP3 battery holder was mounted on the rear panel of the enclosure for easy access


Control Panel Schematic




Control Unit Internals


Super Probe Internals
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 10mm) 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. The probe enclosure is a small Eddystone diecast box... OK not the prettiest thing in the world but it's easy to work with and it does mean the whole thing is screened. The cable (I used Van Damme Classic XKE) exits the enclosure through a flex cord grip as used on lamp fittings. I find these very useful for all sorts of projects, but you will have to add a 10mm nut


Underside View
The enclosure used was from a 'dead' project that I repurposed. Luckily, most of the previously drilled holes were in the right position though I did have to drill some extras for the loudspeaker. Mounting the speaker downward facing isn't a problem as the whole thing seems to act as a sound box. It's not intended to be Hi-Fi anyway, espectially with a 50mm paper coned driver!