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Returning back to the Maplin Microwave Leakage detector for R/C transmitter testing (page 26).

The circuit of the ‘Maplin microwave detector’ is shown in Page 26 Fig3. The detector gives a ‘normal’ half scale deflection with a typical radio control transmitter indicating that the transmitter is OK. The deflection of the meter can be doubled however to give full scale deflection by shorting or replacing both of the resistors used in the circuit with copper wire. The resulting circuit is shown <<<<here in Fig1. The Maplin original circuit (P26) is extremely simple and effective for its intended use. Although the suggested modification would make the device more suitable to the R/C market (Huge by comparison), the mods are difficult to do and an R/C version by the manufacturers would be the answer. The Diode used in the Maplin cannot be traced.

For a DIY R/C transmitter tester, all you need is a meter movement (possibly from an old Tx) and a suitable diode. A graph showing possible diodes is shown in Fig2a for comparison. All diodes worked but the two at the bottom of the graph showed best results. (OA91 and 1N34A) This was because they only need a few millivolts to start detecting the Transmitter output. It is worth mentioning that these diode types are old ‘Germanium’ technology and have been around for years. As such and with no identification, their authenticity cannot guaranteed.

David has now included the Maplin diode trace. It appears to have germanium characteristics but it may well be a special Schottky type.

All of the other possible ‘radio frequency’ small signal diodes gave results but required 50 to 200 millivolts before detection of the test transmitter. This meant that the ‘detector’ had to be closer to the transmitter aerial.

There are now special types of RF detection diodes which should show improved performance detecting 2.4Gig transmitter outputs and we will be looking at samples of these in the near future. The beauty of the circuit is its simplicity and no battery needed.

 Recapping what we have learned about available signal diodes:-
1) Germanium point contact diodes are more sensitive than Silicon Schottky diodes.
2) The response of  Silicon Schottkys can be much improved by a little forward bias (No help for a passive detector ( Fig1) but easily accommodated by offset adjustment when an amplifier is used).
3) Contrary to 1) the response of the Maplin diode (still assumed to be a Silicon Shottky) is outstanding and better than anything else tested.
4) It is possible to produce a simple 2.4GHz detector with similar sensitivity to those we made for 35MHz detection.

Conclusion. In the event of failure to identify the reason for the superiority of the Maplin diode and because of the obsolescence of Germanium technology, biased Silicon Schottkys are probably the way forward to date.


David is working on another original R/C transmitter checker design using a low voltage amplifier chip. His existing transmitter output designs can be seen on Page 22 and 26 of this site.

David’s latest circuit is shown. This uses a compact tuned Quad Aerial as shown.

The interesting bit is the Coil Craft RF choke selected for its resonant frequency of 2.4GHz. The quad aerial RF pickup from an RC transmitter excites the choke into self-resonance via a simple 1.5 turn loop around the choke. This produces a healthy input voltage swing to the OA91 Diode (or other diodes used for test).

Only the positive voltage swings of the excited choke flow through the diode and this begins to fill up the 100n capacitor with microvolts.

This voltage is used for the input to the 7611 op-amp. The amp produces a 15 times voltage output to the indicating meter. This produces a very sensitive pick up of radio control transmitter output level.

A picture of the Quad Aerial is shown for help showing the mechanical construction. The main parts produced from 6 inch of 10 swg copper wire, enamelled if you want. And a 2-8pf beehive capacitor trimmer.

Some interesting diode characteristics that were gathered during the project :-

I came to the conclusion that to avoid signals bouncing off me and the vagaries of aerial orientation, the tests would need to be carried out under fixed conditions. I opted for the quad aerial flat on the bench with the aerial of the Futaba 6EX transmitter 1.75 inches above it. This was not an arrangement made for maximum sensitivity but more in the interest of repeatability. With the amplifier gain set to 15 (anything much higher makes zero adjustment a bit too sensitive) and with one of my original 0A91s in circuit, the indication on a 500uA f.s.d. meter was 300uA. I then recorded the indicated current for the various diodes:-

0A91        300uA - bench mark.
0A90       175uA - disappointing
BAT85S  Almost zero indication i.e. useless
1N34?     50uA very disappointing
1N5711   50uA ditto.
1N34A     425uA - better than the bench mark.
MAPLIN DIODE   The fixed test arrangement became unusable as the detected transmission using the Maplin diode produced a violent FSD. An indication of over 500uA could still obtained with the transmitter 1 metre or so from the aerial and which of course was very susceptible to reflected signals etc.  Many times better at detection than any diode tested.


An early version of DC’s Transmitter Checker>>

The version for tests above used a 500uA movement.

Some DMM readings of diodes used :-

BAT 85S    257 Ohms.

0A 90        265 Ohms.

1N34?       292 Ohms.

1N34A       307 Ohms

1N5711     353 Ohms.

1N4148     649 Ohms.

MAPLIN     258 Ohms

To get some idea of the current passing through the diode on test I put another DMM on the 2mA range in series with a 1N5711 on test. The current indicated was 0.995 mA and the total resistance measured 461 Ohms i.e the DMM added 108 Ohms.

DMMs appear check diodes at 1mA. This would not favour the 0A91 and 1N34A diodes because, being point contact devices, they tend to be a bit more resistive than junction or Schottky diodes. This is why I used a maximum of 100uA for the characteristic plots.

Unfortunately there is little to suggest from the forward diode resistance, how each diode will perform as a microwave detector.


Some later versions of the Transmitter Checker 2 are shown here>>


Note that the original Maplin case has had to be made deeper to allow the extra components.


Notes following initial testing of the prototype Checker 2:-

Checker 2 is now working. The range from a 6EX transmitter is giving f.s.d at around a metre. From the results of the bench test I was expecting 2 metres or more and I have pondering why the apparent sensitivity is less than that indicated by the test. The bench test was carried out using a good quality 500uA movement and I wrongly assumed that when the indication was 200uA it would produce full scale on the Maplin 200uA movement. Amazingly considering its prime application in the Maplin Monitor, the Maplin movement has a fairly high resistance of around 700 Ohms and this limits the current which flows from the low impedance amplifier output; remembering that the actual amplifier output voltage at that distance is small. To sum up the range achieved is quite usable but it would be better with a lower resistance movement. Consequently a 1mA movement would probably give much better range; the higher current not being an issue for the amplifier. To prove this I will eventually replace the Maplin case and movement with something ad. hoc. At least it sees the 6EX transmitter!

So far we have a microwave Field Strength Meter that can be used for comparison of transmitter outputs under bench conditions.


<< The picture shows the final prototype of ‘Checker2’ Field Strength Meter (own design by David Caudrey) being used to check the output of a typical R/C transmitter. Notice the larger meter used in this version. Any other 2.4 Gigahertz transmitter should produce similar full scale deflection of the meter if positioned in the same place. Notice that testing must be done on a wooden table and best in field or open garden conditions. (Indoor reflections of microwave (2.4Gig) often disturb the results.)







THE PICTURE TO THE RIGHT SHOWS THE CHECKER 2 BEING USED IN A SMALL KITCHEN. The microwave oven in the corner of the room is switched on, door closed and heating a pint of water. The nearly full scale deflection of the sensitive checker 2 is showing the leakage of the oven that is all around in the small kitchen. It must be said that these ‘hot spots’ vary around the small kitchen but there are plenty of them. (microwave ovens use the same frequency as radio control transmitters)






Returning now to the Passive transmitter output detector at the top of this page for a moment. ( the one that needs no batteries!)


Thomas Budka of RF Diagnostics, LLC pointed out that there was perhaps another way of doing an R/C transmitter output check. He suggested having a look at the RFD102A device which is listed as a ‘low power RF harvesting device’.

Must admit that we could not find this listed in our usual Farnell or RS distributers, or other main suppliers that we use for small quantities of electronic devices to try, so we got one or two direct from the manufacturers themselves.

On receiving the RFD102A devices for test,


1) they are small ! 

2) they are perfectly engineered.

3) they are not as small as the usual SM components that we have negotiated, so with care, they are suitable for the DIY electronics enthusiast.


On first glance the RFD102A may look a little complicated with all of its copper solder pads. But fear not for our use, only three pads are used.




The first picture above shows the RFD102A connected directly to the Maplin meter movement. Notice the 2.4Gigahertz dipole aerial wires spot soldered to the top copper pads. These were made out of 15Amp fuse wire from the hardware shop. Each one has been snipped afterwards to 28mm long for correct ¼ wave length of 2.4GHz

The second picture shows the relative deflections of the RF meters used. The left meter is a standard Maplin type Microwave Leak detector. The right meter is the modified Maplin type meter using the RFD102A to directly drive the meter movement. Clearly the RFD102A is producing greater deflection of the needle under this test condition. But remember, the modified meter aerial is positioned parallel to the Tx aerial giving an advantage.

It must be noted however that orientation of both modified and unmodified detectors used in the above test did produce various conflicting results.

Conclusions however did show that the RFD102A produced greater deflection of the meter than any other diode we tested, other than the genuine Maplin diode used by the Microwave Leakage Detector. The genuine diode still produced marginally more deflection per distance than the RFD102A version.


A very effective R/C transmitter output checker, can be produced however using the RFD102A…..

The RFD102A can directly drive a Light Emitting Diode (LED)!  All other diodes tested were totally incapable of doing this without an external battery.


With this in mind, detection of the R/C transmission was switched to illuminating an LED and the prototype ‘Passive Transmitter Tester’ is shown above. As can be seen, only three components are used. The RFD102A, 15amp fuse wire, and an LED. The open box shows the simple construction. For interest, the LED was pinched from a redundant garden solar lamp. The smart box was provided by S.L.M. Engineers Cheltenham.

It is pocketable. Easy to use. Needs no batteries. Indicates at 15 cm from transmitter aerial. Will last for years.







Space Engineers, Don Mathes and Doug Spreng, over 50 years ago, developed a unique electric motor Servo. The output arm followed perfectly the width of repetitive input Pulse. This was a historic invention by these two Guys. The servos that we are all still using with our new 2.4MHz Systems still work in the same way.

THE FOLLOWING CIRCUIT PRODUCES REPETITIVE INPUT PULSES that can be proportionally varied in width via a control knob to drive any RC servo clockwise and anti-clockwise to test smoothness of operation and any dithering or dead spots.

Any RC battery pack can be plugged-in on the left hand input of the circuit and the servo to be tested plugged-in to the right hand of the circuit. A practical layout is shown later using standard battery and servo type plugs.


   The circuit is a text book transistor Multivibrator using two transistors Q1 and Q2 cycling at around 60 cycles per second. The 47K variable resistor (R5) proportionally adjusts the output pulse width between 1 millisecond to 2 millisecond wide. This is the required standard for RC servos to go from one side to the other.

C3 is an extra component which slows down the circuit. This allows it to be happy with higher current servos which snatch at the battery supply and trigger the circuits timing.

If substituting the transistors pick high gain types (250+) at low milliamps.








The circuit is shown above which can be assembled on Veroboard. The LT Spice trace shows the circuit producing the positive going servo pulses at around 60 per second. The trace shows both minimum and maximum pulse width superimposed. The 47K control pot provides smooth variation of the servo between these two positions.






Possible circuit layout using Veroboard. (the pot could be included on the main board?)


Assembly in S.L.M box.

Notes- With the layout above, the battery can be plugged in to either of the angled plugs and the servo into the remaining plug.

             Do not use flux or chemicals on the veroboard. (it will change the timings of the circuit). Use only electronic cored solder.













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