Among the Amateur Radio fraternity, a device such as the one described here would otherwise be known as a Field Strength Meter (FSM). Its main purpose would be to check that RF power is being radiated from a transmitting antenna. Some FSMs are tuneable across a wide range of frequencies, which allows the device to work with different transmitters and antennae - usually from HF to VHF and beyond. A difference with this unit is that it is only sensitive to those signals in the R/C section of the radio spectrum. The on-board trimmer allows you to tweak for maximum signal strength of your frequency of choice, whether it be 27MHz AM, or 40MHz FM.

In order to read the received signal strength, such a unit would ordinarily employ a mechanical analogue meter. The unit described here differs in that it uses four super-bright LEDs. The benefit of this is that it allows you to read the display from a distance - even in bright sunlight.

Image shows an earlier version. The unit described below
now has 'power-on' LED and the toroid re-located.

A recent search on the Web returned several such circuits, but the one that caught my eye was found at the Norcim website. Their original circuit employs a pre-wound Toko-type inductor. I found it pretty difficult trying to locate Toko's in the UK, so searched for whatever could be found in the junque-box. A quick rummage revealed a T50-6 toroid (yellow), and a small 22pF trimmer capacitor. So instead of fixed capacitor and variable inductor, we now have variable capacitor and fixed inductor.
Once the signal is rectified through diode OA47, it enters potential-divider resistors, R1 to R5. These values have been calculated that each LED will glow at 3dB increments. The 360-ohm resistor (R3) is a non-E12 value. And since the JB was devoid of such a value, I soldered a 330-ohm and 27-ohm in series.
With the values given for the trimmer and inductor, you should be able to tune any UK transmitter on any of the UK R/C bands. For our colonial cousins running on 72MHz, you'll probably have to remove one winding from the toroid. Or better still, wind 23 turns on a T50-10 toroid (black).

"A correctly-tuned R/C transmitter will radiate enough RF energy that four LEDS will glow from a distance of ten metres.".

Or words to that effect. But it depends on the length of the antenna. Norcim specify a 500mm whip. The antenna I'm using falls short of that number by about half... which means reduced sensitivity.
I stuck it in there because (1) it was small enough to fit the enclosure, (2) I didn't mind the reduced sensitivity, (3) it was free (robbed from an old cordless phone) and (4) mainly because it was nicely made.

Preparing the board:

Since a bog-standard cast-alu enclosure was already in the JB, and since it weighed-in at 112mm H x 62mm W x 31mm D, the board layout and size had to be made that it would fit these dimensions -- with room to spare that it didn't clash with the antenna... or on/off switch... battery...

Begin by cutting a piece of veroboard, 22 holes x 28 tracks (fig.1), and make 12 breaks at the following locations...

• H16
• M11 + M15
• N15
• P11
• Q11
• R11
• S4 + S15
• T11 + T15
• W6

...then prune both upper corners in order that they avoid the lid retainers.

Preparing the enclosure:

Prior to soldering components, use the bare board as a template for drilling the enclosure. Take a coloured Sharpie pen, then with the copper side towards you, dab a spot of ink at the following locations...

• Q3
• H11
• E18
• L18
• S18
• Y18

At this stage the antenna is attached. The LEDs had to be spaced from the board with enough clearance that the section of antenna inside the enclosure could not hinder or contact with the trimmer. T'was found by trial and error that 25mm is a good distance. It allows each LED to protrude through the front panel by 2mm or so, while leaving enough all-round clearance of the antenna.

Stuffing the board:

The components can be soldered in any order. I started with the 12 wire links, then worked from left to right.
Because some resistors are mounted vertically, it's impossible to draw their colours. So to clarify, resistor locations are...

R1 at G8 and P8    (100K     = Brown Black Yellow)
R2 at K20 and P20  (470-ohm  = Yellow Violet Brown)
R3 at P16 and R16  (360-ohm  = Orange Blue Brown)
R4 at R20 and U20  (100-ohm  = Brown Black Brown)
R5 at R8 and X8    (22-ohm   = Red Red Black)
R6 at F9 and M9    (270-ohm  = Red Violet Brown)
R7 at M14 and M16  (270-ohm  = Red Violet Brown)
R8 at T14 and T16  (270-ohm  = Red Violet Brown)
R9 at T9 and Z9    (270-ohm  = Red Violet Brown)
R10 at H14 and H18 (1000-ohm = Brown Black Red)

Antenna connection at W3.
Battery "-" at X15; battery "+" at Y15.
OA47 diode cathode at S5; anode at W5.
Toroid at W4 and X2
(secured in place with a dab of hot-melt glue).

Solder the LEDs that they're at a height of 25mm (1 inch) from the board. Their cathodes (denoted by a 'flat' on the body) should all be facing towards the bottom of the board.
Since the LM660 is static-sensitive - and the most expensive component - I opted to use a 14-pin DIL socket. Solder pin #1 at T13 and pin #14 at T10.

'Twould be a sad world without veroboard, but there's no denying that it's not as versatile, or tidy-looking, as a dedicated PCB. Building with veroboard is sometimes seen as the poor man's alternative. But sometimes a simple circuit does not justify the hassle of etching a one-off board.
On the plus side, it can be made to look a tad more elegant by using a parts-placement overlay. With this, all you have to do is stuff the board where the components are already shown. Another advantage is that it reduces the likelihood of soldering components in the wrong locations.

The image shown on the right is obviously too large in the real world. It has to be reduced in size first. So once you've copied it to your drive, and in order to have the printout match the actual board dimensions, you need to set your printer reduction to 54%.
When you get your first printout, hold it, and the board, against the light, then align the holes nearest the corners. If you're lucky they should be pretty much spot on. If not, try adjusting your printer setting just one per-cent plus/minus either side. It's a case of trial-and-error for each individual printer. But starting with 54% will put you in the right numbers.
Once you're armed with the correct setting, make a final printout on gloss photo paper. Roughly trim it to size, but leave approx 5mm around the perimeter. Paste the board and the printout with Pritt-Stick, then align / stick them together. Try not to get glue on the printed side, because it sticks like you-know-what and looks pug-ugly.
Trim off the excess overhang.

Pushing them skinny legs of resistors, etc, through plain-paper overlay is easy. Pushing them through gloss paper isn't. Jab a needle through there first.

Image show where the on/off toggle switch has been shoe-horned next to
the antenna. This particular battery has been in use for well over a year.

Setting up is dead easy. But before inserting the I.C., connect a 9-volt battery and confirm that the green LED is glowing. Then check for power at pin 4 (pos.) and pin 11 (neg.) If all is well, disconnect the battery, then insert the I.C. (pin 1 at location T13). Solder a short length of wire from the antenna to location W3.
The unit should work right from the start, but if it doesn't, check the polarity of each LED. Check the polarity of the diode and the 100uF capacitor. Check also for unwanted solder blobs and/or dry joints on the track side.

Now for the fun part...

Switch on your R/C transmitter, fully extend the antenna, then place it a few metres from the tester. Now switch on the tester, fully extend its antenna, then - with a small flat-blade screwdriver - tweak VC1 trimmer until all four red LEDs are glowing. If you use the specified 500mm antenna you should be able to tweak the incoming signal that all four LEDs are glowing with a distance of 10 metres between the unit and the transmitter.
I'm using a shortened antenna, so the overall range is slightly less. This is not a problem since the main reason I built the device was for testing and aligning homemade transmitters at close proximity, on the bench.

All graphics on this page are drawn with the standard Paint program that comes bundled with every version of Micro$oft Windoze. The image on the right - although drawn and saved using MS Paint BMP format - has been converted to GIF format. (GIF images are quick to download; BMP images aren't). So you'll have to right-click, then select 'Save Picture As...', and save it back to BMP format. When it's loaded on your drive, open it with MS Paint (or your favourite graphics editor), and add your own custom graphics. But bear in mind not to move or adjust the five circled cross-wires. They give the exact locations of the trimmer and 5 LEDs and serve as a tempate for drilling the enclosure front panel. You can, however, add your own text and graphics to just about any else where you please.

The decal on the right is obviously too large. So, like the parts-placement overlay, this too has to be reduced. You'll have to set your printer to 54% reduction in order to obtain the correct scale.

"I was so impressed, I bought the idea of sticking one inside a swanky box.".
(Not Victor Kiam).



It's true, this is a pretty sensitive unit. And although the shorter antenna does incur a small signal loss, the sensitivity is still more than adequate for bench-testing.