Ancient RF teardown: crude but effective

Ancient RF teardown: crude but effective

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By eeNews Europe

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 by Telectron, also called a clicker, see Figure 1.

It has no date markings, but I estimate it is 30 to 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, I assume it’s a 230MHz unit. The package is 10 cm by 5.5 cm by 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: 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.

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. Power comes directly, without regulation, from a standard 9V 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?

Bill Schweber, is an electronics engineer and author who has written for EE Times, was analog editor at EDN and prior to that worked in marketing communications for Analog Design and was also editor of its technical journal.

This article first appeared on EE Times’ Planet Analog website.

Related links and articles:

Go active or stay passive?

Let’s aim above 100GHz

Analog optical fiber forges RF link

Book presents an analog wonderland

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