Advice to identify a component and possible work around for use with PICAXE.

JPU

Senior Member
Hi

I have recently began working on a project to convert a generic foot pedal to be wireless and connect to a wireless receiver which is connected to a PICAXE which will control a brushless motors controller.

I have decided to use the ERF modules as the wireless link and I have written some very simple code which at the moment allows the user to adjust a POT connected to 08M2. The READADC command reads the voltage from the POT and then PICAXE outputs the data to an ERF. The second ERF receives the data, sends it to another 08M2 which then outputs the corresponding value via sertxd to the PE terminal. This all works well and as a test program I am happy at the moment.

The next stage in my project was to strip the foot pedal I purchased of Ebay. Its a generic 240V sewing machine foot pedal.

I was hoping to adapt some of the components within and attach my test circuit to the pedal however I have come up against a hurdle right at the start!. (Please bear in mind that I am very novice at electronics so please stick with me)

The Foot pedal seems simple but I was expecting a POT within in, however it has some sort of resistor controlled by the pedal which only has two terminals. When I measure the resistance across the terminals I get 0 - 2K ohms depending on how far in the peddle is pushed. I thought I would be able to apply a voltage (5V) to this component and then use the readADC to measure the output. However this is not the case as the voltage taken from the component is either 0V or 3.8V depending on the position of the pedal and there seems to be no gradual increase or decrease as I would expect with a POT with 3 terminals.

I have included some pics to help and my questions are:

1) What is this component called, that is the large white mass at the ctr of the pic.
2) Is there anyway I can get this to work given the components at hand and my limited knowledge of electronics.

Thanks very much for your help and I appreciate you sticking with me as this is a bit long winded.
 

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JPU

Senior Member
This is a third picture given some more details of the other component joining the two terminals of the "THING".

Thanks
 

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westaust55

Moderator
The two terminal device is likely a potentiometer with one wire to the body of the resistance and the second wire to the wiper.
Another term often used for a two terminal variable resistance is "rheostat".

Rheostats are typically higher power rated than potentiometers and liking at the photos I see it is rated at 90 Watts.
 

JPU

Senior Member
Hi

Thanks, Here is the link to the ebay page, however it doesn't say much about it. Do you think there is a way I can use this with the low voltages I have (5V). I guess this has been designed for 240V but is there a work around?

Thanks
 

Hemi345

Senior Member
Could you connect one terminal to your +V source, the other one to a leg on the picaxe and from that same leg, use like a 2K resistor to Gnd?
 

Buzby

Senior Member
I agree with westie, it's probably a rheostat.

Try connect one terminal to 0v, and the other to 5v through a resistor, say maybe 1k ohm.

Then use readadc to read the voltage at the junction of the resistor and the rheostat.
 

StigOfTheDump

Senior Member
As mentioned in the above 2 posts. With the 2k resistor you would only get 2.5v range on the ADC. With the 1k resistor you would get a bigger range 3.3v but would eat batteries faster. You would also need to consider having a microswitch at the unpressed end to disconnect it when not in use, as it will still be drawing current (possibly even more than when on, depending on configuration). Check my maths please.
 

JPU

Senior Member
Thats fantastic. I followed Buzby and Hemis instruction and now I can get a variable output. I measured the voltage (5V in) and I used a 1K resistor.

However, the range is 2.1V - 3.8V. So when you depress the pedal as soon as the current flows the lowest voltage is 2.1V through to the maximum voltage which can be seen, 3.8V. Could I improve this range? What is the purpose of the green component which shorts the two terminals?

Thanks guys...
 
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JPU

Senior Member
Here are some more pics of the setup and component. It has two grey shaft protruding from it. They get compressed and the resistance changes..

I have fitted a 680ohm resistor here to experiment although the voltage output has decreased the range is roughly the same..
 

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hippy

Technical Support
Staff member
What is the purpose of the green component which shorts the two terminals?
It's a capacitor and probably serving as a suppressor should the rheostat create any noise while moving from one wire winding to the next.

I would guess it's basically live through rheostat, into motor, and then back to neutral for the sewing machine.

Used as a variable resistor into a PICAXE ADC you can probably remove it. It's unlikely to cause any issues if left in circuit and may be beneficial left in place.
 

hippy

Technical Support
Staff member
Wouldn't an accelerometer only indicate when the pedal is being moved rather than give an absolute indication of how far it has been depressed ?
 

Goeytex

Senior Member
While it can measure angles, should the foot pedal be moved to a location with a different base angle, I would think that it might misbehave. What happens if it gets kicked about accidentally?

I'd stick with a variable resistor for this application.
 

Buzby

Senior Member
Admittedly, the rheostat is probably the best way for this application.

But how many different ways could it be done ?

I'll suggest a few for starters.

1, Infra red distance sensor, to measure distance from the base to the pedal.
2, Curved vane and LDR, as the vane moves it lets more light onto the LDR.
3, Quantum Tunneling effect compression sensors.
4, A tube with a plunger, resonant frequency indicates position of plunger.

There's more than one way to skin a cat.
 

JPU

Senior Member
Well, all I can say is that it wont be happening anyway soon at my house!! Two blown ERFs, gutted, my fault as you would expect, shorted! So then I decided to press on with testing the code and somehow managed to blow the one AXE027 cable that I have, whilst connecting the serial upload pin on my baord so I could check results in the terminal. I ran a wire from the serial output of the PIC connected to the foot pedal to pin 6 on the pic which would receive and interpret the data. There was a small ark and life no more.

I have run the test mentioned in the PE on the AXE027 cable and the voltages seem OK but it wont download any data. I have tried several other chips!

Other than by giving up, how can I avoid this when my new cable arrives, should I have used a resistor somewhere in the link..


PS, believe it or not it was all working at one point!!!
 

bluejets

Senior Member
The component in the foot pedal will be a "carbon stack".

One stack each side of the central shaft, seriesed by a bridging actuator at the operator end.

A collection of small carbon discs much like carbon brush material.

As the op says, when compressed the resistance changes.

More suited to controlling the 240v series motor directly rather than through electronics I would imagine.

As hippy says the green component is a suppressor.

Hope this helps.
 

westaust55

Moderator
The green "thing" looks likely to be a 250 Volt capacitor (to help filter out noise) or maybe a surge limiter (so voltage cannot exceed 250 Volts).
 
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