A handheld multi-purpose meter for the HF experimenter

by sv3ora

About the project

During my experiments in HF RF electronics I usually find myself in situations where I need to test some of my circuits with simple test equipment. How simple? Simple enough to do the job, yet quick and easy to build and cost effective. Usually, such equipment is made for the purpose and then dissasembled, or lying around somewhere and then I do not know where it is when I need it again. That is why I have decided to pack such circuits in a single enclosure, to build a simple multi-purpose meter.

Enclosure

Before starting this project, a suitable enclosure must be made. Because this is intended to be a meter with an analogue scale, a sensitive analogue meter is required, and also a suitable enclosure to fit the meter and the controls. This enclosure needs to be handheld as well. There are not many options in my local electronics market for such an enclosure, but even if there were, the whole thing would probably cost quite a lot. The cheapest solution was to buy an old analogue multimeter and keep it's enclosure and meter for the project, then put apart the internal electronics and it's controls. I bought such a multimeter new for 6 Euros from a local electronics shop. You would certainly spend much more if you have to bought the separate components and join them together. Below, is the multimeter I have bought.


This is a classic cheap analogue multimeter. It even includes space for a 9v and a pair of 1.5v batteries inside, so a small PSU could be easily implemented inside the meter.



The internal components were put apart, to leave space for my circuit.



The front panel and rotary switch were also put apart.





The front cover plastic of the old multimeter was cut, so that space is left for the front panel of my custom meter.



A suitable aluminum sheet was cut in size, to be used as a support for the front panel components. Select a thin sheet of aluminum, to be able to cut it easily using a pair of scissors, but thick enough to provide mechanical stability for the front panel components.



Make the appropriate holes onto the aluminum sheet and the plastic of the old meter below it. Then fit the sheet to the meter, using a set of screws.



After finishing with the front panel support, a suitable meter scale has to be made. The old multimeter had a curved meter scale that followed the meter needle. This is ok, but it is more difficult to draw custom curved meter scales and draw the markings onto them in angled positions. Thus, I have chosen to use a linear scale, which is much easier to draw and the markings are not drawn in angles.



Click here to download the scale pattern of the meter. It is a 14Mb bitmap, which you have to print in scale for your meter. For my meter, I opened it with infranview and printed it in custom size of 8.5x6.46. Note, that I have designed the pattern for my own meter, but you may wish to do changes for your meter, that is why I provide you with this pattern as a starting point. After printing the scale, you have to align it to your meter, so that the center of the scale is alligned all the way up with the needle and the needle is correctly aligned at it's extreme movement positions with the scale.

The use of the linear scale has the disadvantage that because of the circular motion of the needle, the linear scale expands towards the ends. However, this does not seem too much of a disadvantage for my purpose, having in mind that the rate of change of the needle motion is not linear anyway. Also, the final step lines that are shown in the preliminary scale below, will probably change positions, depended on the actual calibration of the instrument.



Another point, is that the representation of the circular motion of the meter onto a linear scale, leaves some "empty space" at the top of the scale. I thought of taking advantage of this space , so I plotted three more scales at the top of the meter, which do not start from zero and end at the full scale reading. These could be useful only in circuits where starting and ending positions do not matter, for example in relative measurements or deviation-from-zero measurements.



Because of the representation of the circular motion of the needle onto a linear scale, the needle cannot extend all the way up to the top, at the scale ends. This is not much of a problem, since one can easily virtually extend the needle, as shown above.

Power supply

The first thing I wanted to include inside this meter, was a small integrated power supply. There are many times where I need a power source for my circuits and the only available one, is the big and bulky lab PSU or external heavy batteries. Not only that, but sometimes I need more than one voltage simultaneously. For example a higher power source to feed a QRP transmitter amplifier and also, a tiny power variable voltage source, to tune the varactor of the oscillator section of the transmitter. I usually end up in external potentiometers or variable regulators to do the job. A more permanent, portable and versatile source of power is thus necessary.







onoff1











term1










adj







100k
12v









100k 500r







100k

term4 lm317 3mm 3mm


term2


240r


10k







500r


1uf 5k onoff2








term3

After a bit of thought, I ended up in a small, cheap but versatile PSU design, shown above. The PSU, is used to power external circuits, as well as circuits internal to the meter. The main power source, is a battery internal to the meter. The meter has two positions, one for a 9v battery and one for a pair of 1.5v batteries. I have not yet decided if I will keep these, or replace these with a series AAA holder (to be determined)

The PSU can be used in different configurations:
1. When "onoff1" is on, term1 is at +12v and term3 at 0v and they supply the full battery current.
2. If one conencts the term1 to the gnd of a circuit to be powered, then term3 can be connected to the -12v (useful for PNP circuits).
3. When "adj" is closed the two potentiometers coarse and fine tune the low current positive voltage on term2 (useful for tuning varactors). At the same time, term1 supplies +12v at full current.
4. If "adj" is closed and reverse connections are done, like mentioned in point 2 above, then term2 will present a negative variable voltage and at the same time, term3 will present a full current of -12v.
5. If the GND of the circuit to be powered is connected to term2, then this can be used as a low current split rail PSU, providing term1 with +6v and term3 with -6v. The potentiometers can set the balance of the supply then. I am not sure what is the effect of the 100k and 10k series resistros in that case though.
6. Because higher current with adjustable voltage may also be needed, an LM317 is added. This can work in parallel to the rest of the circuit, providing an extra high power variable voltage source, but not when the split supply configuration is used, unless two independent circuits are to be powered. Also if reverse polarity is used at the right hand section of the circuit, one has to manually take care not to short the two sections grounds together, unless they are electrically isolated. That is why the addition of the onoff2 switch thought to be better at the gnd side of the regulator.

Low current LED indicators, show which section is on/off. There are numerous times where you forget circuits to ON and they drain the battery. So despite that being silly, thought to be very useful indeed.

To be continued...

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