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I want to make pressure dependant resistor.

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I wonder if anyone could help,
i'm trying to make a pressure sensitive unit,
i dont mean air pressure,
i want the resistance to change when i squeeze it.

Like a pressure pad, but much bigger.
I intend it to vary the current to a 12v headlamp.
From dim to bright, or better.

I have tried powdered carbon from battery middles,
it jams up.
I tried including powdered rubber mixed in with
powdered carbon, got nowhere with that.
I tried layers of wet paper between layers of metal,
that stunk bad and didnt work.

If anybody has any suggestions, i would be happy to
try them.

I would think its an easy thing to make.

It would have to be capable of being squeezed and
changing its resistance, then when its released,
changing its resistance back again,

I will be trying powdered up sponge mixed with
powdered up carbon rods from battery middles,
between plates of metal.

Somebody might already know the best materials to use.

Regards, John
Load Cell is pressure sensitive transducers which you can find in most of the digital weigh scales.
but i dont want something feeble.

As i said,
I want it to be able turn a car headlamp up and down.
Thats why i'm trying to make one.
You want to build a circuit which can give a variable voltage for car bulb,and this voltage depend from human weight?
Could you maybe use a foot control from an old sewing machine. You could probably get a used one from a sewing machine repair store cheap.
Thank you all for your replies.

I will be trying a mix of 1 part tyre rubber filings,
4 parts stainless steel filings,
10 parts fine carbon filings from battery middles.

The whole thoroughly mixed
and tested in an old glass fuse case
with a metal rod for me to press it down.

This test item is to get a working mixture
of fairly low resistance.

When i get a working mixture,
i will make it big enough to handle a headlamp.

I have had no success yet.

SEBI, it doesnt have to be 'human weight', in
fact when i get it working, it will be operated
by a bar from something else,
but if you know something that will operate from
a human weight and control a headlamp
then i would like to know.

GENE, thanks but i'm looking for a single item
with two wires attached that will go in series
with a load, and control the current from a 12
volt source depending on the pressure applied.
Rather like an old electrical oil gauge would
work a meter on a motor car dashboard.

Regards, John

I'm having some encouraging results
with 1 part battery carbon filings
mixed with
3 parts hardwood filings.

1000 ohms to about 4 ohms,
from pressing hard with my finger,
to releasing then back again slowly.

But this is only a small unit,
on an ohm-meter.

The finished unit will be expected to run hot,
so i cant use wood filings.
or rubber tyre filings.

I will be trying chalk or tile filings next,
mixed with carbon filings.

Any comments or suggestions welcome,
Regards, John
do you know those flat piezo speakers?
well, connect one to a ohmmeter and then mesure its resistance.
then preee with your finger on it
what do you see?
Hi bogdanfirst,

Well i haven't tried that,
but i doubt that i could put much of a load
in series with it.

I wonder what the old oil pressure indicators
had in them?
The ones that work an electrical gauge,
not the ones that just work a light.

I'm looking for something that can vary the
current to a 12 volt headlamp.

I know that the oil pressure indicators wont
do that, but if i can find whats in them i
might be able to make a big one.

Hi bogdanfirst,

Because i want all the parts of this unit to be
serviceable and repairable with only basic hand
tools and from materials that are easily obtainable.

You could say that a power transistor is readily
available, i have given it a lot of thought and i
want to make this unit without any transistors at all.

I will be trying charcoal next.

Maybe the filings i'm using are too fine,
cos it keeps jamming into a lump!

Regards, John
ALL parts serviceable???

So supposing you get this recipe of iron filings, rubber, wood and fairy dust sorted and it works perfectly, how is that going to be more serviceable than a standard piezo speaker and a standard transistor?
Hi xpi0t0s,

Your first post, welcome to this group.
I want to make this unit really basic.
As simple as possible.

A mixture of carbon and possibly slate
(with a sprinkle of fairy dust)
may not look much like a 2N3055, but i
think it might handle a bit more wattage.

Anyway if this unit does actually work
it will be something i have been pondering
for a long time now.

The car headlamp is just a dummy load of
about the right amount, its not actually to
control a headlamp.

Ive given up on the charcoal, cos its combustable,
i will be trying fine sand and carbon next.
Pixie dust optional.

Any practical suggestions?

What you are looking for sounds like a lamp dimmer to me. You can get a toggle type at Wally World for about 7 bucks. If a return-when-released action is required, you could fit the toggle with a spring. I fear that if you continue to persist in a chemical solution the mixture will ultimately compact itself and cease to return to the off-state - unless this is intended to be a one-shot device.

Perhaps it would help if you clearly described what the load is and what current you hope to control. Also, maybe you could be clearer as to the arrangement of the activation device (step-on, two finger control, plunger, drive over switch, etc.)
Hi Gene,

I would not be at all surprised if a device is already
available to do exactly what i want.

I assume you have read this thread,
and you know i want to vary the current through a car
headlamp using some kind of pressure sensitive device.

By vary, i dont mean switch on or off,
i mean increase or decrease smoothly.

I do not know of a lamp dimmer for 12 volt operation,
unless you mean a Headlamp Dipper Switch,
That would not control current smoothly,
it switches it around between lamps.

I dont know what 'Wally World' is, but my guess is
that its a motor car accessory shop,
and you mean a headlamp dip switch.

I have been drying out some fine sand and making a
unit to hold it with a plunger, a bit like a pump.

I have bolted on washers to the stationary part
and to the moving part, and connected wires to them.
The mixture will go in it later, after ive ground up
some carbon for it. Its about an inch diameter and
about two inches long.

I expect the compression to take about a millimetre,
but i dont know yet.
I want it to vary between from roughly 800 ohms down
to about 4 or 5 ohms or maybe better.
It would have to be lower than 1 ohm to light a 60 watt
headlamp even poorly.

But for test purposes i would be happy with lower
single figures.

It is not intended to be a one-shot device, it is
intended to return to the high resistance state after

The activation device will be a bar from a nearby
assembly, the current i want to control would be as
much as i can get away with, i expect about 2 Amps
would make the unit quite warm, when i get suitable
materials i expect it to run hot.

Eventually i hope to make units controlling 10 Amps,
These would be somewhat larger, and hot running.

Meanwhile, i just want to make a fairly small version.

I had considered sliding contacts on solid carbon parts,
but this was not practical.

The nearest thing i have found readily available is the
old pressure sensors fitted to some cars.
But i havent been able to get hold of one yet.
As mentioned before, i mean the electrical sensors that
work a gauge not a light.

I may try breaking the carbon up rather than using
filings, maybe a pile of bits like sand wont clump up
into a lump.

Cheers, John

Maybe you could use alot of those slide style 'pots' with springs on their input and connect them all in paralell. I dont think you will get what you want by mixing ground up materials. Car headlamp is about 55watts right ? So your trying to make a 55watt 'sprung' variable resistor ?

If it were my choice I would go with the transistors 8)

Good luck

John - what is your desired travel distance of the plunger as the resistance increases from 0 to 800 ohms?
Hi Gene,

Its been quite small with previous units.
I expect a millimetre or two.

I dont mind much so long as its not too far.

Hi Y'all,

Yesterday taken up with spin drier problem.
Will be back to my pressure dependant
resistor as soon as possible,

Still trying.
The mix of carbon powder from battery middles,
is sort of clumping up and becoming a lump,
so i am now introducing powdered graphite
in small amounts to try to stop this.

I now have a cylinder about 3/4 inch dia with
about an inch and a half of powdered mix.
The cylinder is plastic, with a plunger and a
washer on top and a washer below.
The two washers are the connections.

I am also wondering if i could use a pile of
washers separated by pads of blotting paper
soaked in some kind of electrolyte.

This is more difficult than i imagined

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