 # 13.002 How can I use a thermistor (a resistor which has a variable resistance related to ambient temperature) to read temperature on my Apple II?

```    You can connect a thermistor to a paddle input and supply a voltage to the
other end. Current flows through the thermistor to charge a .022 mfd capacitor
inside the apple2. When the paddle is read, the apple2 discharges the capacitor
and resets a timmer. Then the cap is allowed to charge. When the charge reaches
the trigger level (3.2 volts) the timmer is stopped and the count is read out.

The lower the value of the thermistor, the faster the charge and the lower the
count. Also, the higher the voltage, the faster the charge.  A resistance of
about 120K with a 5 Volt supply will give a count of about 250. You can add an
external capacitor across the paddle input (to ground) to increase the charging
time (if needed)

The formula is   charge=input volts * (1 - exp(-t/RC))

So, you can use a supply voltage and external capitor as required to fit the
resistance value of the thermister (or other resistive component). Probably,
the best way to find the correct values woud be to try a variety of values and
plot the "count" as a function of the variable resistance. Then compare the
plot against the resistance curves for the thermistor (probably not linear),
and work out some conversion formula to use in your program to correlate
"count" to temperature.

By:  Cyrus Roton
```

Continue to: