reversing the resistance of a thermistor?? experienced assistance

[ulysses]

hey guys

i am working on a circuit add on to the TB MKII to modify the resistance of to the collector of Q3 based on temperature.

i have a thermistor that varies resistance based on heat.

at 10 degrees celcius it has 160k resistance
at 40 degrees celcius it ahs 60k resistance

i need to vary the resistance to about a 5k swing around 8.2k. i breifly figured out that i can get some swing by putting a 9k parallel resistor in parallel with this thermistor..

problem is, the thermistor swings the wrong way. i need less resistance when its cold and more resistance when its hot. is there an easy way to swing the resistance of the thermisor the other way?

i was thinking about 555 circuit to swing an overpowererd resistor (in series with a hi brite led) into action (to warm up the ge transistor) when the voltage of the collector is less than 4.5 volts. i know this will waste battery power on the led.. but i dont care!! i want my tb mkII to sound good all the time.. i get tired of having to adjust it to stop it sounding wooly!!

cheers
ulysses

[soulsonic]

You need a different kind of thermistor. What you have is called a "Negative Temperature Coefficient" thermistor. You need a "Positive Temperature Coefficient" one.

[Paul Perry (Frostwave)]

One complicating factor is that the commonest form of NTC (neg temp coeff) thermistors are baseed on a semiconductive material that varies very strongly wiht temp, while the PTC ones are often metal alloy based and much milder in their action.

Even so, it should be possible - with some heavy design work or experimenting - to use a NTC in the application, think of it this way, you want to have the increase in temp to increase a current that fights against the leakage.