picaxe variable resistance

[toby gross]

I am working on a temperature project I have got a 1.5k ohm resistor between these two wires which go to a heather unit and the heather goes full blast, I would like to control the resistance between these two wires from 100 ohms to about 1500 ohms what's the best possible way? At 100 ohms between the wires the heather goes really slow and at 1500 ohms it's at full speed. I've got the picaxe reading a temp sensor so with a desired value I could have the picaxe maintain the correct temperature with increaseing and decreaseing the value of the resistance between these two wires.
Thanks for your responds, Toby

 

[Jeremy Leach]

Well one option would be to combine an LED with an LDR. The LED could be powered from one picaxe output and be driven by a PWM signal from the picaxe. Changing the PWM duty (in picaxe Basic) would vary the LED 'brightness'. If this light shines onto an LDR then the varying brightness will vary the LDR resistance. You might have to experiment to get the range of resistance you require (choice of LDR and perhaps using a parallel resistance across the LDR). There have been other discussions about this, and Rick Harris has mentioned LED and LDR combinations that he's made with his school pupils and sealing in tubes.

However I think there might also be other options depending on your circuit. Picaxe outputs can be used to combine resitance values 'digitally' and the people here will be able to talk about this if you can give a bit more information about the circuit. Any chance of uploading a picture?

 

[boriz]

How much current?

 

[Dippy]

Absolutely Boriz.
Before anyone can make a sound suggestion the specification is required.

What Jeremy said is prefectly sound for small signals. In fact you can buy these LED/LDR optocouplers ready-made sealed devices.
Note, they vary with temperature quite significantly, well they did when I used them.
But usng this method would be nicest too from the isolation aspect.

No doubt someone will mention digital pots. Well, possibley but their voltage and current is limited.

So, Toby, you will have to dig your Multimeter out and check some parameters. volatge across that resistor and the current through it all at max and min.

 

[Andrew Cowan]

How big is the resistor (physically)? If it is bigger than your standard 1/4W job, then that indicates high currents (or voltages). That would mean an LDR/digipot couldn't be used.

A

 

[Dippy]

True potentially (haha). But we need figures not guesses.

 

[toby gross]

variable resistance

Now I've got a 1/4 watt resistor between the wires, when I take a volt meter there is 2 volts on one side of the resistor and .100 volts on the other side of the resistor.
Thanks, Toby

 

[toby gross]

Could I use the pwm command for this application?

 

[eclectic]

@toby

I think that before people can answer,

you need to supply all the information that was
requested in posts 2, 3 and 4.

e

 

[Dippy]

Toby, me old mate, as eclectic says we need all the info that was asked for.

This is almost like me asking you why my car won't start, but I've looked at the light bulbs.

So far, our mental circuit diagram is a 1/4Watt (how do you know?) resistor between two wires which join on to something, somewhere in a heater.
Mains volts?

You have measured 2V one side and 0.1V the other side of the resistor.
With respect to what?
You put the red lead lead on either end of the resistor and the black lead where?
If mains are you sure you are measuring DC?

ec , Westy and Andrew have newer models of crystal ball, I'm confident they can sort this out...

To help them determine the level of explanation, can you indicate your electronics experience? Schoolboy? Absolute Novice? Experienced Hobbyist?

 

[jglenn]

Do the old photoresistors, selenium, or whatever they are (we can't get them here that I know of) respond to LED light? Red or white?

You can use a FET if the bias circuit is completed. Sometimes requires a - voltage. There should be one here:

http://www.national.com/an/AN/AN-32.pdf

 

[cvrwy]

I have a feeling I'm going to regret this......you fellows were going along so nicely on this discussion.........

What is a heather? Must be a UK thing.........

 

[premelec]

I've presumed "heater" was meant - but what do I know?

 

[inglewoodpete]

I thought it was a blue-flowering plant that grows wild in Brittain. Read about it in "The Famous Five".

 

[Dippy]

Oh, maybe it is a voltage controlled heather bush ?

I had the 'auto spell check' set on my Crystal Ball and it said 'heater'.

I would imagine the selenium cells would respond. I'm (nearly) certain one of my old photo light-meters used them.
But as we can't get hold of them (easily) then , well, er...ummm is there a huge amount of point persuing that?

I'm sure there are several ways to do this, but the spec and circuit schematic are needed to provide decent suggestions. Without drawings/figures it's almost like playing "What's My Line?".

Anyway, in a few hours time Toby will be poviding us with a schematic and some measurements.... which reminds me, I must make an appointment with the dentist today.

 

[premelec]

I've used the selenium cells in times past - they generate a voltage - the modern equivalent is the silicon photovoltaic cell and if you want linear light input output response you run them as close to short circuit as possible and measure their current [by voltage across a small resistor...].

The usual LDR is something like cadmium sulfide and it works fine to change resistance with an LED in my experience - you can make a slow speed opto isolator or variable resistor to control current to lamps to illuminate that beautiful heather...