Advertisement

Analog Angle Blog

“Ancient RF”: crude but effective

We now view the convenience of sophisticated home-automation systems with their smartphone-enabled, Internet-connected functions such as garage door control as “routine.” It wasn't always so, of course, but smart engineers worked some design magic with far fewer options, yet still did clever work.

This became clear to me when I came across an old garage-door remote control, also called a “clicker” by Telectron, Figure 1 . Though it has no date markings, but I estimate it is about 30-40 years old, based on the design and asking some questions of family members. There's no indication of frequency, but due to the “Model No. T80–RF 230” designation on the back, and some similar units I found on the web such as this and this, I assume it's a 230-MHz unit. The package is 10 cm × 5.5 cm × 2.5 cm thick and has only one user control, a large square push-button which actives a small switch on the PC board. It's the genuine embodiment of simplicity of operation, that's for sure.

Figure 1

This garage-door opener is a model of simplicity in design and use; it has just one user button, with a large arrow molded into it.

This garage-door opener is a model of simplicity in design and use; it has just one user button, with a large arrow molded into it.

Compared to today's RF units which operate from 900 MHz for a basic dedicated unit to 2.4 GHz and higher for a smartphone connection, it is both easy and tempting to dismiss the design of this RF unit as trivial. However, the available parts were larger, design and layout tools were non-existent, and units like these had to be very, very inexpensive.

The design, Figure 2 , uses a simple one-transistor LC-resonant tank to set the frequency (no crystal); this can be adjusted as needed since the capacitor is a variable trimming type consisting of five interdigitated metal semicircles each about 8 mm in diameter, and 6 mm long overall. I didn’t see a separate inductor, so I think the large wire loop serves the dual purpose of being the LC inductor and the antenna—a nice touch.

Figure 2

The design and layout are typical of mass-market low-end consumer RF circuits of several decades ago, with few active and passives and a wide-open layout and PC board tracks, using through-hole components on a single-sided board.

The design and layout are typical of mass-market low-end consumer RF circuits of several decades ago, with few active and passives and a wide-open layout and PC board tracks, using through-hole components on a single-sided board.

There are seven small resistors, a settable DIP switch with 9 poles (I assume to set some sort of unique “code”) and a few bypass capacitors. The only IC is a UM3750, a basic encoder/decoder, now obsolete (of course) but fortunately archived on the web, see here. Power comes directly, without regulation, from a standard 9-V battery which probably lasted several years, as I am fairly sure there is no quiescent or idle current drain to worry about; thus, when the unit was not in use (the user button not pressed) it was a hard “off” rather than a soft one.

What about the microcontroller? There is none, and there is no firmware, either. This is really a mostly analog circuit with some digital management (even though the case boasts it as “digital”). Even the PC board is a throwback: single-sided, and not FR-4 material (and I don’t think it is low-cost, punched phenolic, but I am not sure). The PC traces are enormously wide and generous by today's standards—you could certainly stuff, assemble, and solder this board by hand with little difficulty, and that's likely how it was made. Reverse-engineering this unit to generate its schematic by following the PC board traces would take about 15 minutes, I estimate. All in all, it's a very simple, effective, low-cost execution to meet the application requirements.

Are you impressed by the basic elegance of this design, and how neatly it was executed? Overall, did these now-obsolete products and their designers have it easier or harder than today's RF designers with respect to application requirements, cost pressures, functionality, resources, regulatory standards, design tools, and test equipment? In some ways, do you wish those days were still available for single-person, almost fun-like projects?

Related

Soft-on-off-is-designer-s-dilemma–user-s-headache

4 comments on ““Ancient RF”: crude but effective

  1. jonharris0
    July 21, 2015

    Very cool view of the past design of a simple everyday item, a garage door opener.  How simple it seems compared to the fancy, complex designs used today.  I'd venture to guess the security concerns have grown over the years forcing the more complex designs we see today…plus folks always seem to want to do more with the “Look what this thing can do!” mentality.  Thanks for the neat insight!

  2. pcsalex
    July 22, 2015

    Bill,

    just a few weeks ego I was involved in  the analysis of an even older garage door opener circuit — the  unit stil works with a tube transmitter! and a receiver made all with tubes.

    That unit has a “modern” power supply one transistor –2nN270– makes in an invertet circuit the “high voltage” from a 3V battery, which is also used to heat the filament of the oscillator/transmitter tube, which is a limited life-time 100hours type subminiature tube.

    The tube oscillastes at two diffrent frequencies –in the same time –the RF is around 240MHz, and the modulation frequency  which is in the higher audio range — due  to the size of the small transformer, which is used as the “tank circuit” for the low frequency oscillator.

    After replacing the battery in the “remote controller” –which works for atleast 50m– the unit is working as good as it did some –atleast –fifty years ego….

  3. RomainFR
    July 24, 2015

    Hi,

    Totally agree with you mostly about the “ego” of people these days. They all want to show that they can do way better and way complex that before, hate that mentality actually in most of case.

     

    Regards

  4. zeeglen
    September 29, 2015

    One does have to wonder about crystal-less RF frequency stability over temperature – can the receiver (presumably regenerative) in a heated garage still track a transmitter that is at 40 below zero in January?  And does the FCC demand better frequency stability and harmonic suppression these days than back then?

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.