A Radio Oddity

[Nigel Graham 2Participant @nigelgraham2]

I was reminded of this by an enquiry elsewhere about capacitative effects on a digital tyre-gauge used as a simple DRO…

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Some years ago I had a fairly basic clock / radio with l.e.d.(?) bar-characters, and rather oddly for a modern transistor radio, an aerial consisting of about a foot of thin, insulated, single-core wire dangling from the case.

On evening, working on the desk on which it stood, I noticed there was a small volume of fresh air in front of and above the wireless in which an extended hand would dim the display to about half brightness.

There was no discernible effect on the radio signal. Just the clock display.

I did the obvious – test the rough boundaries of this effect, test for simple shadows, test for my unconsciously moving my sight-line as I moved my arm …

No. There was a small but definite "bubble" in which my hand would cause the display and that alone to dim; and it was an abrupt full/half effect with no gradations.

I was not earthed to anything. The table was wood, on a carpeted wood floor. Touching the instrument's plastic case made no difference to the display, either. I do not know, but it's possible the radio was double-insulated so not earthed. I have not seen any similar effect on anything else.

Anyone any idea what may have been happening?

[Nigel Graham 2Participant @nigelgraham2]

[Anonymous]

My bedside clock-radio does much the same thing when I go near it ….. but that's because it has automatic display brightness and going near it changes the ambient lighting that it perceives.

[Ian PParticipant @ianp]

Just from your description of the radio I took it to be a bedside clock/radio.

If it is then many did/do have self dimming features for the display, so what yours does is quite normal,

Ian P

[SillyOldDufferModerator @sillyoldduffer]

To reduce cost LED displays are usually multiplexed, meaning each digit is turned ON and OFF rapidly in sequence fast enough to fool the eye into believing all the characters are always ON. Multiplexing is driven by an oscillator, and the display can be dimmed varying it's output duty cycle.

So I suspect either an oscillator fault responding to hand capacitance, or – more likely – a deliberate design feature. As I can't think of a good reason for dimming the display temporarily, could be a faulty deliberate design feature!

The circuitry of older electronics often use fiendishly clever ways of getting the most out a limited component count. More than 7 transistors in a radio was a selling point! Sometimes these tricks cause peculiar faults due to components shifting value gradually whilst ageing. Now, because a few million transistors are neither here nor there, digital electronics don't work the same way, and go wrong for other reasons. Now we have software faults, complicated bells and whistles, and no-one fully understands how they work! The technology gap between a crystal set and DAB Radio is enormous.

Dave

[J HancockParticipant @jhancock95746]

You are probably rediscovering the effect which led to the development of the Proximity fuse.

A much ignored invention which transformed the efficiency of AA fire.

[John HinkleyParticipant @johnhinkley26699]

Nigel,

Perhaps you could connect it to a Theremin device to produce a sound/light show?

John

[Ian PParticipant @ianp]

Posted by SillyOldDuffer on 03/04/2021 09:24:09:

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So I suspect either an oscillator fault responding to hand capacitance, or – more likely – a deliberate design feature. As I can't think of a good reason for dimming the display temporarily, could be a faulty deliberate design feature!

…

Dave

Clue to dimming reason is 'Bedside' radio. Some displays at brightness to be visible in a sunlit room would keep one awake all night!

Ian P

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, as Ian P has suggested, auto dimming, I had a cheapish digital alarm clock like that back in the 80's. During daylight or when the room light was on, the digits were bright, turn the room light off and they immediately went dim.

Regards Nick.

[Ian PParticipant @ianp]

Reading Nigel's post again, the description he give of the effect does not fit into the usual auto-dimming feature, in fact his sounds like an auto-brightening!

Step change in brightness is odd one too. Might be worth trying the radio in a dark location and waving a flashlamp around so see if that helps diagnosis.

No connection to Nigel's clock, but a bedside 'projection clock' that came from the Science Museum shop incorporated some sort of proximity device that when you waved your hand over it switched on the projected display for a few seconds. The clock went to WEEE and I never got the chance to examine it closely but the 'sensor' appeared to be just a small (1.5mm) hole in the top surface. I thought it might be a photodetector but since it functioned in total darkness that's unlikely, I now presume it is a simple PIR device.

Ian P

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Ian P, my clock didn't go from full brightness to full dimness, it was on a sliding scale. In good daylight it would be at it's brightest, but would dim down just a little if the curtains were drawn and at dusk it wouldn't be as dim as it was in total darkness. I think I remember it being a LCD display and the bars were green in colour and boasted the latest technology and all that flannel, but it was just an affordable alarm clock to me.

Regards Nick.

[Ian PParticipant @ianp]

Posted by Nicholas Farr on 03/04/2021 11:10:31:

Hi Ian P, my clock didn't go from full brightness to full dimness, it was on a sliding scale. In good daylight it would be at it's brightest, but would dim down just a little if the curtains were drawn and at dusk it wouldn't be as dim as it was in total darkness. I think I remember it being a LCD display and the bars were green in colour and boasted the latest technology and all that flannel, but it was just an affordable alarm clock to me.

Regards Nick.

I would imagine that all auto dimming displays would have an analogue response to changes in ambient light, its the step change than Nigel experiences that is unusual. We dont know if his is a bedside radio so his radio might not have auto dimming.

Ian P

[Nigel Graham 2Participant @nigelgraham2]

It was a bedside radio – well, I didn't place it by my bed, but it was designed for such use, with the alarm function, etc.

I say "was" 'cos it no longer exists. I forget what happened or didn't, only that what was meant to happen, no longer happened.

It had no auto-brightness feature. The display was red and of constant, moderately low brightness to be not too intrusive. Anyway the dimming behaviour it had was not what I'd associate with that function, and I did test to ensure I was not affecting it by placing or removing shadows from it.

I'd have thought if anything you'd want the display bright enough to be visible in day-light (and it was), but not too bright when you are in bed in a dark room. So any difference from an approaching hand would be a slight brightening to help you to read the time without fumbling for small press-buttons. Not a step down in brightness.

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I think Dave may have identified it, by suggesting a fault creating a capacitative proximity detector. It's been puzzling me for years!

Connect it to a Theremin… Now there's an idea, John!

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