why would a calculator have an RF antenna?

Thread Starter

Vignelli

Joined Nov 13, 2023
20
I was replacing the battery in a calculator.

calcwtf-1.jpg

wait a minute, it's got an antenna circuit in it?

diode, capacitor, "RF"

calcwtf-2.jpg

why would a calculator have an antenna circuit?

calcwtf-3.jpg

anybody got an explanation?
 

Ya’akov

Joined Jan 27, 2019
8,973
Welcome to AAC.

It‘s not an antenna. The diode is power related. The split circular pad is the type designed to be used as jumper with a solder bridge, The RF doesn‘t mean radio frequency.

Is there something on the back of the case that uses the two square pads as contacts, like a piezo buzzer, or the battery holder?
 

Thread Starter

Vignelli

Joined Nov 13, 2023
20
no, nothing inside or outside the back of the case. featureless plastic slab. screw it on, that's it.

what else might RF stand for?
 

Ya’akov

Joined Jan 27, 2019
8,973
no, nothing inside or outside the back of the case. featureless plastic slab. screw it on, that's it.

what else might RF stand for?
I’m not sure. I do know that’s not radio frequency related, though. I think it is an alternative mounting pointing for a different or additional diode.
 

WBahn

Joined Mar 31, 2012
29,865
It's very possible that the same board is used in different calculators and the distinction is made by whether or not that spot is populated.

The first thing that comes to mind when I see RF on a circuit board is a reference designator for something like a feedback resistor. I don't know if that makes sense for that circuit or not.
 

Alec_t

Joined Sep 17, 2013
14,220
what else might RF stand for?
Considering there's a pcb hole there, most likely for heat dissipation, my money says it's a Resettable Fuse.
But I'm surprised a calculator would need one, except perhaps for reversed supply protection?
Edit:
And why a spark-gap across the diode?
 
Last edited:

strantor

Joined Oct 3, 2010
6,778
Considering there's a pcb hole there, most likely for heat dissipation, my money says it's a Resettable Fuse.
But I'm surprised a calculator would need one, except perhaps for reversed supply protection?
Could be that solar powered calculator needs reverse polarity protection in case the light shines backwards. But since this is a cold war era calculator, my money says this calculator had (or didn't have) a remote listening device in it, depending on where you bought it. This one must be from Iceland or something as the RF component has been deemed not worth installing.

/s
 

Externet

Joined Nov 29, 2005
2,182
The diode nests in a pcb hole to give clearance to the lid and prevent breakage as other components are less thick.
The spark gap is not; it is solder pads to short-circuit the anode and cathode traces with or without the diode in place. The square pads 'RF' is a provision for an additional component to enable a feature/function or for a test probe pair aligned to them fitting in the pcb hole during manufacture.
 

MisterBill2

Joined Jan 23, 2018
17,814
If the PCB is used for a number of markets, RF might be for "Required Fuse", because some parts of the world are vastly paranoid about fault currents.
Or it may be an abbreviation for words in a non-USA language. And looking at the rest of the picture, it may also mean "remove function."
 

sparky 1

Joined Nov 3, 2018
755
a pcb designer may have been told because a charger might be used and there might be a product safety compliance so they left an open pad and room for a very small fuse. the nomenclature used for a pcb part name abbreviated RF Ribbon Fuse. Unlike the schematic symbol for fuse the engineer could measure the cut out to determine the correct size solder in mini fuse given the board height clearance. The calculator company would understand ribbon fuse and hand cranks.
The acceptance of chip on board and history is interesting but less logical today, the link below you can see that traditional calculators drew current but consumers may have wanted to run there new calculator on a walwart because of the conceived battery life issue from the past.
In order to ease consumers into buying more batteries Radio shack introduced a battery card you needed a full row of punched stars to get free battery.
The Calculator-on-a-Chip (vintagecalculators.com)
 
Last edited:

Externet

Joined Nov 29, 2005
2,182
Never seen a solar powered calculator in need of a fuse.
The square pads in question connect only to the chip on board. Perhaps tells the chip to use the decimal comma or decimal period or things like that for different world markets.
Knowing the chip part number should clear the doubt at its data sheet. Those other etched numbers could tell something if relate what processor chip is meant to be installed in that pcb.
 
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