Recent Posts


Another Sunday afternoon project with my son. Of course, as possibly every 4 years old, he loves to act out as fireman and policeman. After a hospital visit we gave him a set of handcuffs but of course nog true policeman is complete without it’s own gun. So we spend some time in the shop and made our own.

The body is made of 18 x 44 mm pinewood leftovers. The top part is circa 150 mm long, the hand grip is ~90 mm. The two parts are just glued together with ordinary PVA wood glue. The body was painted grey with some cheap spray paint. The trigger handle was made from a saddle clip used to mount a PVC tube to the wall. All in all it took us 1 hour or so to assemble and we had some good fun along the way.

I have been held at gun point and arrested countless time by a very brave police officer 🙂

GDO circuits bonanza !

The Gate Dip Oscillator (GDO) is one of the most useful tools for the radio amateur. You can use it to tune LC circuits, as test RF source, check antennas, etc. This GDO circuits bonanza features many previously unpublished GDO circuits from (primarily) Dutch magazines and books.

Bouwen en luisteren – IAN R. SINCLAIR

This book in the “GDO circuits bonanza” contains many simple – yet very useful circuits. To help the constructor a simple layout sketch is given based on vero strip-board. The circuit is build around a single NPN transistor. The transistor (TR1) is set-up in common-base amplifier, the basis is held at a fixed voltage and is connected to AC ground via capacitor C2. This configuration is probably choosen because the input capacitance does not sugger from the Miller effect, which degrades the bandwidth of the common-emittor configuration. This implies that the collector and emitter from the transistor are in phase. Thus a simple small capacitor from collector (output) to emitter (input) is sufficient to provide positive feedback and hence start oscillation. The LC circuit comprised of L1 and VC1 only provides a high resistance and thus (high) amplification at it’s resonance value, which is given as ƒ = 1 / (2 * π * √ L * √ C).

For every component the position in the vero board is given, with the description from which to which copper strip the component should be mounted.

GDO circuits - Bouwen en luisteren - DIP oscillator - pag 58-59GDO circuits - Bouwen en luisteren - DIP oscillator - pag 60-61GDO circuits - Bouwen en luisteren - DIP oscillator - pag 62-63


Theorie en praktijk voor zendamateurs – J.L. Molema

Two other ones in the GDO circuits bonanza. This time from the book “Theorie en praktijk voor zendamateurs” (Theory and practice for radioamateurs), a collection of circuits and little background information on their workings. It features two GDO both based on junction transistors.

Theorie en praktijk – Afb.85

The first circuit is set-up around a PNP transistor, again in common-base configuration. The circuit itself is a little difficult to follow because, unlike current conventions were the positive rail is drawn a top and the ground below, it use the old drawing style were both the supply rail and ground rail are drawn next to each other. Both 3k3 resistors are used to set the base at half the supply rail and 6.8nF cap is used to connect base to ground for AC. The 820Ω resistor and 10nF cap are used for power decoupling the transistor’s collector. The meter read-out is different in a couple of ways. First the signal is taken from the low-impedance emitter. Thus the read-out circuitry will load the LC circuit far less than the design above. Potentially offering greater stability. The second-change is that a transistor is used to amplify the signal, thus a less sensitive meter couple theoretically be used. Finally the meter is connected to a bridge circuit, one leg being the 4k7 resistor and the transistor and the second being the 10k trimmer pot. Although, the circuit recommend using a trimmer here, a normal pot can be used to your advantage as well. With the 50K pot one can then set the sensitivity, with the 10k pot one can the shift the zero-point and thus zoom-in on a small dip.

One thing that should be pointed out, however, is that there is no protection for the meter M. If you are not careful you can easily destroy it when fiddling with the controls.

Coil details are given in the page scans below.


The second circuit is a more traditional design. If you are familiar with the classic Heathkit GD-1B and the Eico 710 models, you’ll probably recognize the circuit immediately. The emitter is held at ground via the 470Ω and 1nF. The LC circuit is connected between basis and collector. Basis and collector are in this configuration 180º out-of-phase. The LC circuit provides another 180º, because the mid-point of the capacitor is connected to ground. Sensitivity is set via the 5k log  potentiometer that changes both the base voltage and the collector. If the DC voltage is increased, the amplitude of the oscillation will also increase.  Th output is sampled from the collector and rectified. Because there is no amplification and more sensitive meter is required.

Theorie en praktijk – Afb.86

Coil details are given in the page scans below.


Ontvangers – F.A.S. Sterrenburg

The next book in the “GDO circuits bonanza ” is probably one of the best books on radio receivers available in the Dutch language. It describes almost every aspect of radio reception: radio propagation, receiver principles, receiver specifications, planning and construction, antennas, receiver inputs, oscillators, IF amplifiers and the list just goes on and on…! It is all written down in an easy to read concise format, with many many examples. I bought his book at a library selling off some surplus books some 20+ years ago and it is still my go-to book for many aspects. It really is a pity it is only available in Dutch!

Of course being a book of 1978 many new techniques in receiver design, such as H-mode mixers, DDS VFOs, SDR and new modulation techniques as WSPR, are not covered. But then you cannot run, if you cannot walk, can you? So, I would definitely recommended this book to any youngster planning on building a receiver.

In chapter 15 “Meetinstrumenten” (Measuring instruments) many principles and some simple to build tools are described including two dippers. The first one is build around a EM87 tuning indicator tube, very cleverly using the triode portion of the tube as the oscillator. Oscillator and indicator are all in one tube. Although the author, very politely, doesn’t mention it, the circuit is from it’s own hand. It was originally published in the Dutch magazine Radio Electronica 1974, page 49. The author goes to great length to describe how to build the circuit, enclosure, which coils to use and how to calibrate the circuit.

Dipper with EM87

After the previous circuits, the oscillator configuration itself should be becoming familiar: an LC circuit connected between a tube’s grid (gate) and plate (drain), the central part of the capacitor grounded to provide the necessary 180º phase shift. The second half of the tube (the actual indicator) is then used to display the amplitude of the oscillation. In the original article you can find a great deal of information about the construction and the coils used.

The second dipper described in the book is totally different. It is a transistor dipper, again in common-base configuration. However, now a couple of diodes are used to rectify the output and stabilize the amplitude of the oscillator signal. This agc signal is then also used to indicate the dip.

The use of a negative power supply looks maybe a bit strange but it is actually quit clever. In this way one half of the LC circuit can be connected to the ground, minimizing hand effects. I am not sure if the actual rectification circuit around D1 and D2 is drawn entirely correct. It seems strange to put D1 in anti-parallel with T2…

You can find more information in the page scans below..


Kleine zenders en oscillatoren – Herbert Brosch

This booklet is written by German amateur Herbert Brosch (now SK), who has compiled a collection of many simple circuits about oscillators and small transmitters. The GDO feaured in this book is classic Collpits based GDO with a J-FET and uses a sensitive meter to display the rectified gate current. Not even a basic 100nF buffer capacitor is included in the schematic….

Kleine Zenders en oscillatoren – dipmeter

You can find more information in the scans below:

Although I am not a great fan of this book, it features some nice and unorthodox circuits, like this Push-pull oscillator:

Push-pull oscillator

Test probes with super fine tip

These test probes wereld made several years ago and have proven to be very useful in many applications. The small tip allows you to probe even the tiniest SMT components.  Because the tip is really, really sharp (don’t ask me how I know… ouch) it allows you to breach residue of solder flux or even penetrate the outer isolation of a cable.

Best thing, they are very cheap and easy to make yourself:

  • Two fineliners (Hema: this or this ) a €0.80 = €1.60
  • Two 4 mm banana test leads (I would recommend about 1 m) a €7.95 = €15.90
  • Two needles from your wife’s sewing box : free
  • Two Heat shrink tubing 8 mm Ø, 20 mm as a strain relief €1.00
  • Some bits and pieces that you have likely lying around (solder, super glue, two component epoxy, etc)
  • Optional 10 mm heat shrink tubing in the desired color.
  • So grand total is about €20

If you check the picture below, you’ll see that the few step necessary to make these probes are mostly self explanatory.


Remove the plastic stop at the back (the part that you are normally chewing on) of the fineliner and use some needle nosed pliers or a pincet to extract the cotton ink reservoir. Be careful with the ink, it can really create a mess! From the other side remove the small compressed felt tip. At one end of the cable, snip of the banana plug and measure the diameter of the cable. Drill a hole through the plastic stop, slightly larger than the cable diameter. Slide the narrow heat shrink tubing over the cable, so you can’t forget it later (don’t ask me…) and put the cable through the stop. Strip the cable over a length of ~5 mm. Insert the rear end of the needle in the cable, so it is nicely centered and apply solder to create a solid joint.
Now insert the assembly through the fineliner’s body all the way to the other end. Insert the needle in the tip until it protrudes about 5 mm . Use a smal amount of two component epoxy to fix the assembly. Now, you’re almost done!
Put the hole tip and cable assembly back into the fineliner’s body, as well as the back stop. To create your strain relief, apply a small amount of super glue on the stop as well as on the cable near the stop. Shove the heat shrink tubing back, so it covers the cap and use a heat gun or lighter to shrink the tubing.

Optionally you can now add the large colored heat shrink tubing over your SMT probes. I liked this for it covers the logo of the original manufacturer of the fineliner and the texture of the tubing gives a better grip on the probe than the original flat surface. If you want you can keep the metal clip on the cap, so you can hang your probes from a rail. Since the cap fits quit tight to the body, I decided to remove the clip and cover the part with some tubing as well.


That’s all there is to it. If you assemble all you’re parts and tools up front (something that is always nice to do anyway) you can easily be finishes in an hour and end up with some really nice and useful test probes.

Have fun making your own!



PS 1 – I noticed there are some (cheap) super glue brands that cure so fast they are useless for this application. In order not to ruin you’re almost finished probe, do a quick test.

PS 2 When making the first set of probes years ago, I found that there are two types of needles: those made of steel and those made of aluminium. The latter being impossible to solder because of the thin layer of aluminium oxide at the outer layer of the needle. If you’re not sure with which variety you’re dealing with, use a magnet to check it out.


SMT DIP adapter PCB from TI


If you like prototyping but find it’s a nuisance to mess around with small, all too common, SMT packages, the Texas Instruments SMT DIP adapter kit might be something for you.

The DIP adapter kit supports six of the most popular industry-standard packages, including:

  • D and U (SOIC-8)
  • PW (TSSOP-8)
  • DGK (MSOP-8, VSSOP-8)
  • DBV (SOT23-6, SOT23-5 and SOT23-3)
  • DCK (SC70-6 and SC70-5)
  • DRL (SOT563-6)

There are six coupon boards per supported package type,  so in total 30 adapter PCBs. It comes complete with terminal strips. Best of all it costs only $10.00 USD!

You can find more information or order the SMT DIP adapter kit HERE

I don’t have any stock or interest in TI, nor do I get paid, I just found it a neat and cheap solution.

Do-it-yourself SMT prototyping board

PA3COR SMT Prototyping board

Frequently, I need to do some SMT prototyping. Partly because some components are only available in an SMT enclosure (like the MCP4725, a nice small 12 bits DAC), partly because it needs to be small and sometimes it is Saturday 5 past 5 and the only type of a critical part available is SMT. Two weeks ago I saw on SMD shop a SMT prototyping board.

It features pads in 0.05″x0.05″ square pattern and every fourth pad is connected to the groundplane on the bottomside.
Looks quit nice, with the only drawback a price tag of 8.22€ for 100 X 80 mm board. (yes I am a skeap skate dutchie). Nowadays, you can order 10 pieces of 50 mm x 50 mm PCB for around €10 in China. So, I figured, why not design your own?

Below you see the result. the first figure is the top layer

PA3COR SMT Prototyping board - top view

The pads are laid out in a 0.05″ pattern an do have a size of 0.042″ by 0.042″. Every fourth pad there is a hole to the bottom layer. The holes are non-plated-through. If you want to connect them to ground just stick a small wire in and solder it at both ends.

And here is the bottom layer, which is usually used for ground layer.

PA3COR SMT Prototyping board - bottom view

Have fun! If you would like to receive the gerber files, let me now!

Update : you can download the Gerber files for the SMT prototyping board HERE !

HP-456A Current probe – repair

This HP-456A current probe is a real beauty. Released as a new product by the Hewlett-Packard Company in 1960, the 456A was HP’s first solid-state, stand-alone, clip-on current probe. Its elegantly designed amplifier uses two— then “state-of-the art”—PNP germanium transistors. The Original Probe In 1960, The Hewlett-Packard Journal (July-August, Vol. 11) proudly announced: “This new probe measures current over the full range of the frequencies most commonly used in typical work—25~ to 20 megacycles—and over an amplitude range from below 0.5 mA to 1 A rms.

HP-456A product

Below you’ll find the circuit diagram of the HP-456A probe.

HP-456A circuit diagram

Maybe the concept of the circuit is unclear at first but actually the idea behind it is really simple and effective. (Especially considering the fact that this performance was achieved, with what would be considered nowadays, mediocre components at best.)
Q1 functions as the input transistor in common base configuration. As you will know in a CB setup, the base is hold at a fixed voltage (0V in this case), the emitter is used as the input and the collector is used as the output. Main characteristics (and in this advantages) are the low input resistance and the high frequency. The latter is caused by the fact that the base is at a fixed voltage effect and the Miller effect is eliminated. Also because the emitter current equals the collector current, there is no current amplification.
Q2 is the output amplifier and is set up in a common emitter configuration (emitter hold at a fixed voltage).
The combination of the two transistors works as op-amp in current to voltage topology (eq. transimpedance configuration.) See below of sketch of the amplifier configuration used in the HP-456A :

transimpedance a,plifier

Rf is implemented as R9 parallel tot R10. Beautiful isn’t it?

The reason the setup didn’t work in the one I bought is that the cable actual current probe was broken on were it connected to the PCB in the amplifier case. (in the circuit near C2 and C3). After fixing this it works like a charm. Not bad for a 25+ years old device!

I you have any questions let me know!