Constructing a circuit that would trigger my cordless house phone intercom circuit?

KeepItSimpleStupid · Jul 11, 2021

That's funny.

Drill some of the vias out and cut the foil with a razer blade.

Remember, your meter will likely add resistance depending on the range.

Nigel Goodwin · Jul 12, 2021

Buk said:
must be plated through vias ; so it's back to the ~~drawing board~~ scrap box.

I refer you to post #25

If you haven't got tobacco tins, use bean tin lids, or flatten the sides.

Buk · Jul 12, 2021

Attempt 2: Glued aluminium foil to both sides of a piece thin (0.5mm) stiff and relatively heatproof plastic. Soldered wires to both sides.

Good continuity between cable and foil both sides; and total isolation side to side.

But when I insert it into the battery holder and connect the meter set to the 2000 µAdc range, the phone does not power up?

Nigel Goodwin · Jul 12, 2021

Buk said:
Attempt 2: Glued aluminium foil to both sides of a piece thin (0.5mm) stiff and relatively heatproof plastic. Soldered wires to both sides.

View attachment 132416

Good continuity between cable and foil both sides; and total isolation side to side.

Well the 'quality' of the soldering certainly suggests you used aluminium foil - which was why I suggested tin plate

But when I insert it into the battery holder and connect the meter set to the 2000 µAdc range, the phone does not power up?

2000uA is only 2mA, which is too low a range, and the voltage drop across the meter is likely to stop it working.

Stick a croc clip lead across the two bare copper wires you can see in the picture, to ensure that the phone works then (proving the adaptor you've made is working).

Buk · Jul 12, 2021

Nigel Goodwin said:
Well the 'quality' of the soldering certainly suggests you used aluminium foil

Hmph.

Is that such a bad job?
Most of what might look like excess solder are just wrinkles in the foil.

Have you ever tried soldering copper to kitchen foil?

Nigel Goodwin said:
which was why I suggested tin plate

I did look at that, but with two layers of bean can, the battery wouldn't go in the holder.

Nigel Goodwin said:
Stick a croc clip lead across the two bare copper wires

Why didn't I think of that!

Nigel Goodwin · Jul 12, 2021

Buk said:
Hmph.

View attachment 132421
Is that such a bad job?
Most of what might look like excess solder are just wrinkles in the foil.

Have you ever tried soldering copper to kitchen foil?

No - aluminium doesn't solder (with normal tin/lead solder) - so you don't ever try to solder it. You can get special aluminium solder, but I've no idea if that would solder to copper or not?.

And yes, that looks horrible - the solder around the wire is floating above the aluminium - there's no actual joint.

with two layers of bean can, the battery wouldn't go in the holder

Take the beans out first!

How thick are your bean cans?.

Les Jones · Jul 12, 2021

In post #14 the picture shows the red and black power wires soldered to the battery holder. Why not just unsolder one of those wires and connect your current meter between the end of the wire and the battery holder connection from which it was unsoldered ?

Les.

gophert · Jul 12, 2021

Nigel Goodwin said:
Take the beans out first!

How thick are your bean cans?.

Good one!

Buk · Jul 12, 2021

Les Jones said:
In post #14 the picture shows the red and black power wires soldered to the battery holder. Why not just unsolder one of those wires and connect your current meter between the end of the wire and the battery holder connection from which it was unsoldered ?

Les.

In part because it's neigh impossible to operate the phone when in pieces, and there's no convenient hole to root the wire through when together.

But mostly because SWMBO would linch me if I &$%%&^% the thing up.

Buk · Jul 12, 2021

Hm. Close the circuit direct and the phone works.

Put the meter on Vdc and it measures 2.29V.
Put it on Adc (whatever range) and nothing. On the meter, or the phone.

I can only suspect that the NiMH charging circuit is detecting the increased resistance and switching the battery out of the circuit.

Another interesting thing is that the only chip I can identify -- an FM24C16B FRAM chip visible on the circuit boad just below the silver can labelled cs 220 4V

-- is speced to run at 4.5v - 5.5v. Presumably, this is where the 40 number memory and redial etc. are stored. These can be viewed and edited when on battery power. Which to me suggests that there must be a boost converter in there somewhere?

Also, do you guys know what the black blob (with a letter K) that covers/hides the main chip is? It is literally rock hard. I tried to cut into it with a penknife, and it blunted the tip, without leaving a mark on the blob.

gophert · Jul 12, 2021

Buk said:
Also, do you guys know what the black blob (with a letter K) that covers/hides the main chip is? It is literally rock hard. I tried to cut into it with a penknife, and it blunted the tip, without leaving a mark on the blob.

This is starting to look like an electronics version of this...

Nigel Goodwin · Jul 12, 2021

Buk said:
Another interesting thing is that the only chip I can identify -- an FM24C16B FRAM chip visible on the circuit boad just below the silver can labelled cs 220 4V

FRAM chips aren't TTL, they don't need 5V.

Also, do you guys know what the black blob (with a letter K) that covers/hides the main chip is? It is literally rock hard. I tried to cut into it with a penknife, and it blunted the tip, without leaving a mark on the blob.

It's a COB (Chip On Board), nothing you should be concerned with, you can't buy it, and you can't replace it - the entire PCB is effectively the chip, with the silicon wafer covered in epoxy.

Buk · Jul 12, 2021

Nigel Goodwin said:
FRAM chips aren't TTL, they don't need 5V.

See for your self: VDDPower supply: Min:4.5V Typ: 5.0V Max:5.5V

(Are you guessing again?)

Nigel Goodwin said:
It's a COB (Chip On Board), nothing you should be concerned with, you can't buy it, and you can't replace it - the entire PCB is effectively the chip, with the silicon wafer covered in epoxy.

Thanks.

rjenkinsgb · Jul 12, 2021

Buk said:
Also, do you guys know what the black blob (with a letter K) that covers/hides the main chip is? It is literally rock hard.

To clarify Nigel's answer, something like this (obviously not the same PCB).
A bare silicon die, bonded directly to the PCB rather than bonded in to a carrier package and encapsulated in that, like a conventional IC.

The epoxy glob is to protect the bond wires.

Buk · Jul 12, 2021

gophert said:
...

Nothing useful; as always.

Buk · Jul 12, 2021

rjenkinsgb said:
To clarify Nigel's answer, something like this (obviously not the same PCB).
A bare silicon die, bonded directly to the PCB rather than bonded in to a carrier package and encapsulated in that, like a conventional IC.

The epoxy glob is to protect the bond wires.

View attachment 132426

Thanks. Never encountered one of those before.

Nigel Goodwin · Jul 12, 2021

Buk said:
See for your self: VDDPower supply: Min:4.5V Typ: 5.0V Max:5.5V

Not the ones I use

Buk · Jul 12, 2021

Buk said:
Put the meter on Vdc and it measures 2.29V.
Put it on Adc (whatever range) and nothing. On the meter, or the phone.

I can only suspect that the NiMH charging circuit is detecting the increased resistance and switching the battery out of the circuit.

Is this a reasonable conclusion?

gophert · Jul 12, 2021

Are you keeping your test leads connected to the same solder pads for both VDC and ADC?
Do you have extra fuses for your meter?

Nigel Goodwin · Jul 12, 2021

Buk said:
Is this a reasonable conclusion?

No, the charging circuit will almost certainly be crude, nasty, and really horrible - probably just a single resistor.

I explained the likely reason in post #44, you had the meter set to much too small a range, so much too high a resistance. However, this assuming the 'soldered' connections to the foil are working (which is dubious) and that the foil itself is making good electrical contact.

Constructing a circuit that would trigger my cordless house phone intercom circuit?

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