Why twenty-seven? It was the project #27 Telemetering Transmitter from one of my favorite books the 1964 book 103 Simple Transistor Projects, by Tom Kneitel. (Pages 32, 33) These project books came out in the 1960s and were a loose collection of circuits, mostly radio/CB circuits with little or no construction details or theory of operation. They generally had a short description, schematic, and parts-list. These books also typically had a list of "acceptable transistor substitutions" that were probably a holdover from the tube days. This project was one of my pre-teen self's bucket-list projects. There were at least 3 other projects in this particular book I always wanted to build.
Although I am not exactly sure how I planned to send this device aloft. But I do remember trying to build it up with a CB radio crystal and a few random transistors and some vague idea of several toy helium balloons stuffed in a laundry bag! Probably it would have never worked even if I could have got this circuit to work without any test equipment or knowledge of how the circuit worked.
Information is not even transmitted digitally from this device either. Unlike today we would slap in an Arduino and an off-the-shelf RF data module, like a RFM69HCW Transceiver breakout, and sending tracking with a cheap GPS module, mix it all together with a bunch of libraries to drive it.
The telemetry here is AM modulated RF carrier (it could easily be FM modulated with a varicap diode in the RF oscillator circuit) The frequency of the modulation is controlled by a thermistor. This type of tone telemetry is like the early satellites and rockets, to transmit the telemetry data to earth stations.
The original project #27 goes into some detail about the FCC requirements, ca. 1964. He even recommends the transmitter is inspected by a certified person to comply with the then requirements.
...and it even had a little certificate tag to sign and attach to the device, or "a reasonable facsimile" proving it was worthy of transmitting within in the limits allowed. Also, to protect it from the weather by carefully wrapping it in plastic bag with its 9-inch No.18 buss wire antenna "naturally" protruding. Putting it all in a small, padded box for attachment to the balloon.
"...Put your name and address on the box"
I suppose if you want to find out where it ended up. There are sites with balloon flight prediction that i suppose will get you close. I think in this day and age it may be better to include something like this statement:
"Scientific Instrument, no explosive or harmful substances contained please notify if found..."
This also makes it look official:
"...Perforated board as the chassis"
For construction we were also advised to keep it as compact as possible and some details about putting it on 11- or 10-meter bands with appropriate licenses and antenna length.
Instead, I created two functional versions, called MkI and MkIII. The Mk1 is pretty much a copy of the original version. The Mk3 may actually be in compliance to operate as a telemetering device on the current 27-MHz band. The Mk3 version also features the same simple Temperature-to-Frequency converter it also now includes a way to send altitude information encoded in the length of the tone burst (Pulse modulated) and even a bright LED beacon also attached to the circuit. (This circuit was loosely based on Project #100)
They are without any Germanium or PNP transistors.
The files for the new versions of the are here in my GitHub
I thought I had some P-N-P Ge Transistors in my parts box, but it turned out they were mostly old Si devices. I found the original transistors on - line but at a cost that did not make it practical to try or even want to risk these antique transistors to this experiment... Even buying a vintage 3rd overtone crystal on e-bay was somewhat costly. The parts on this list are still available. That strange 4-legged Ge transistor was apparently used often in RF circuits, and apparently...
"...Great for your Rangemaster clone and other fuzzbox / distortion projects, these Mullard made OC71 / CV5712 germanium transistors are getting harder and harder to find"
I found the OC170 for $12.95 +$4.80 Shipping. Also, the other transistors here: 2N107 - e-bay were for $25.00 for a pair, (used) and about another $8 - $10 for a used crystal so basically around $50 USD in parts needed for this original design!
Instead, I created two functional versions, called MkI and MkIII. The Mk1 is pretty much a copy of the original version. The Mk3 may actually be in compliance to operate as a telemetering device on the current 27-MHz band. The Mk3 version also features the same simple Temperature-to-Frequency converter it also now includes a way to send altitude information encoded in the length of the tone burst (Pulse modulated) and even a bright LED beacon also attached to the circuit. (This circuit was loosely based on Project #100)
They are without any Germanium or PNP transistors.
The files for the new versions of the are here in my GitHub
Basically, the first version was a simple overtone crystal oscillator modulated with a multivibrator circuit - just like the original. The crystal was taken from a toy "27MHZ" remote controller. ($free) It turns out the frequency is in the ISM telemetry part of that band.
This is the PWB version, 3D models. Nobody even dreamed, we could do something like this to look at a project before it was built back in 1964? It is roughly the size of a 1.5V AAA cell holder.
FCC Rules. These FCC rules are kind of confusing. I am not sure why Tom was so concerned. Most rules govern manufactured devices. I doubt if the men in the black vans will show up if you are trying to act like the devices authorized on these bands and don't try to manufacture and sell these. A quick search of the FCC rules section 15.205 does not even mention these bands. I did find this section it seems to cover the label requirements for devices for sell:
§ 15.305 Equipment authorization requirement.
PCS devices operating under this subpart shall be certified by the Commission under the procedures in subpart J of part 2 of this chapter before marketing. The application for certification must contain sufficient information to demonstrate compliance with the requirements of this subpart.
I did see some current requirements for Part 15 telemetry transmitting devices that more power is apparently now allowed - but - thought I saw a limit now on the length of time a "tone" can be transmitted, the original transmitter and the Mk1 both transmit a tone continuously so they would probably not meet this. (can't seem to find that section)
But of course, none of these transmitters are FCC type accepted for manufacture -- but for experimental purposes on the ISM band it should be OK. Truely experimental devices can legally still be on the 10-meter band - if you have amateur radio license - there still are some identification requirements. In any case, it should follow the part 15 requirement to not interfere with any other user on the bands.
On 11 meters, data links/telemetry can operate between 26957 - 27283 kHzPart 95 of the FCC rules allow use of the six 26 MHz/27 MHz RC frequencies for high power control, supervision, data link, telemetry networks and "attention getting" devices. Mean (average) power limit is 4 watts on 26.995 MHz, 27.045 MHz, 27.095 MHz, 27.145 MHz, 27.195 MHz and 25 watts on 27.255 MHz The majority of high-power systems use 27.255 MHz - however some do transmit 4 watts on 26.995 MHz
The Mk3 version. It attempts to limit the time that the tone is transmitted (saves on battery life too) and to attempt to clean up the output signal to hopefully better comply with the rules. The original transmitter seems to rely on low power and the antenna length to limit the spurious emissions surely caused by a 3rd overtone crystal oscillator.
The timing circuit was implemented as another two-transistor multivibrator. Since it a lot like: "#100 Flashing Light" project -- I also added a high efficiency LED that produces a bright strobe beacon as well so it can be tracked and possibly help find it. Since the timing was arbitrarily set, I realized that that could be, in fact, controlled and a second channel of information could be transmitted by this pulse length instead of a fixed resistor or potentiometer.
Oh, and yes, Anything worth doing is worth over doing! I added 4 additional transistors. The second 2 transistor multivibrator (drawn in a more recognizable form) one transistor as a keying switch, the 4th transistor is a modulator. Also added are some simple low pass and trap filter to clean up the output.
I then came up with a 1960's technology version of an altimeter to control the tone burst length based on air pressure. I would consist of simple sealed tube of air with a plunger attached to a small slide potentiometer. I will add details of this device later. (The Mk2 Transmitter was a never completed high(er) power version.)
Mk3 Transmitter (top)
The original was intended to be powered only by 1.5V cell. Three volts should be OK to meet power output requirements. I think a modern LiPo would probably survive the extreme cold this balloon-borne radiosonde would encounter. There is now plenty of room for two cells as shown on the bottom view. Of course, weight is a prime consideration for balloon payloads as well so LiPo may be best, with no holder. Here is a 3D peek at the bottom to show scale - that is a dual AAA battery holder.
Mk3 (Bottom)
"...Calibrating them with a known temperature on the ground before sending them aloft"
The thermistor used was not the original Philco 33-1343-3 or GLOBAR 22-922 specified. I got new 20 ea. 10K thermistors, new for a few $ online. The nice thing is they are small and react to temperature changes very rapidly.
Selection of the capacitor sets the frequency range. f = 1/1.38*Rth*C Where: C= C1 = C3, Rth = R5. In this design, select the value of C such that the frequency is well in the audio range at lower temperatures to allow the modulating frequency to pass thru audio circuits of the receiver.
OK now that we know all about what it is it is time to build and test one. Part 2 will be the construction details and some testing. Maybe there will even be a Part 3 if there is ever a flight test?!
...Stay Tuned!
