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Old 12th Apr 2023, 7:14 pm   #1
buggies
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Default Old electronic (kitchen) scales forum?

Short version:- I have searched this section of the forum with few results - similarly not much success with Google. Is anyone aware of a forum/website with special interest in "vintage" (35 years) electronic scales?

Longer:- I came across our previous scales yesterday and decided to see if/how they worked. They were bought in 1987/88, probably from Argos, as Google found them in a catalogue of that period for £29.99.
Model is Salter Trimscale 2000 - made in Switzerland.
Eventually managed to get the thing apart despite the one-way plastic clips and found that the measurement is done by means of a moving plate capacitor rather than the strain gauge I expected. I have noted the chips used and taken some photographs if anyone is interested. It needs 9 volts 100mA and does function although after re-assembly there is a large measurement error.
It will probably just go back on the top shelf...
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Old 12th Apr 2023, 8:20 pm   #2
Lucien Nunes
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Default Re: Old electronic (kitchen) scales forum?

I don't know about a forum with a specific interest but it's rather interesting, not least because I wasn't aware of how those functioned either. It makes for a fascinating comparison with a radio tuning gang or Theremin.

I'll hijack this opportunity to note that this relatively unusual application of a familiar principle is a perfect example of the value of MEET - our all-under-one-roof museum and learning resource for vintage electronics. At the moment everything is compartmentalised: People with an interest in scales might tend to discuss them on a vintage kitchen appliances forum, radios on a radio forum, Theremins on an electronic music forum etc, and museum provision is very much likewise. Yet they all depend on the same principle - variable capacitance. The average person is never going to visit a dozen different specialised museums to try and get a grasp of how that works, and the kitchen people never get to discover that the same method was used to play music, etc. etc.

Anyway thank you for showing how it works, now I need to find one to go in our musical instrument section!
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Old 12th Apr 2023, 10:22 pm   #3
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Default Re: Old electronic (kitchen) scales forum?

Hi Lucien,

I am tempted to risk taking the thing apart again and reverse engineering the hardware. The plastic is quite brittle and the parts just clip together with one-way ramps.
Initial inspection shows the capacitor has three plates. The upper and middle plate are fixed and connect to two inputs of a Texas JFET op-amp TL062CP. The bottom plate is fastened to the top part of the case and connects to the output of the op-amp. I have not traced any further into the circuit.

For general info:-
The main chip is an NEC D7520C PMOS 4 bit processor (with a massive 48 nibble RAM and mask-programmed ROM). This drives the four 7 segment LEDs directly. Othe semiconductors include Toshiba TC4520BP, Philips HEF4069UBP, LM78L05, 4x BC237B and a BC558B which I think is used as a clock oscillator with a Toko coil.
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Old 13th Apr 2023, 12:19 pm   #4
The Philpott
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Default Re: Old electronic (kitchen) scales forum?

Jeez. As a species we do love to over-complicate things, don't we!?

I've just taken delivery of a 'digital water heater urn' (not ordered by myself) c/w LCD display and mickey mouse push-buttons.
It's being returned in favour of a Buffalo boiler with rotary temperature control and a proper rocker switch..Keeping it simple.

Dave
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Old 13th Apr 2023, 2:48 pm   #5
Graham G3ZVT
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Default Re: Old electronic (kitchen) scales forum?

George, if you use the search term "capacitive load cell", you will find countless articles about this tech.
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Old 13th Apr 2023, 3:31 pm   #6
elanman99
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Default Re: Old electronic (kitchen) scales forum?

From your capacitor plates description I take it that there is some movement between them as weight is applied. What is the device that opposes the load, is it a spring or some sprung structure moulded into the casing?

Are we assuming that the load varies the capacitance which is part of an oscillator circuit. I also assume that its not essential for the capacitive transducer to have a linear relationship to the load as there is processor involved.

Ian
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Old 14th Apr 2023, 6:01 pm   #7
G6fylneil
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Default Re: Old electronic (kitchen) scales forum?

I bought my mother some electronic scales in the early '80s. they used a capacitive system. They were made by Avery, then part of the GEC empire so we got a discount on them.
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