Sensor
We wanted a sensor that we could use as either an ambient air temperature probe within the smoker or as a meat probe, looking forward to other projects in which we’d extend the calibrated range of measurements beyond the 210–240°F range.We happen to have a wireless barbeque thermometer with an appropriate probe, so we designed our circuits around it.We’ve also seen probes as part of oven thermometers and other equipment at Bed Bath and Beyond, and at Super Target, so look around.
The probe (Figure 7) is a negative temperature coefficient (NTC) thermistor encased appropriately for food use and with a high-temperature cable.
FIGURE 7: Probe
We used the sigma-delta analog-to-digital converter (ADC) capability of the Javelin Stamp processor to read the probe, connecting them together in the circuit shown in Figure 8-9. The probe itself is in series with a 10K Ohm resistor, as in Figure 4; the junction of the two is the measurement point for the ADC. The sigma delta converter works by summing the two voltages fed to the integrating capacitor in Figure 8. One voltage is the output of the resistive divider formed by the thermistor and fixed resistor; the other is the direct 0 or 5-volt output of a pin on the Javelin Stamp. The software in the Javelin Stamp modulates the pulses on that output pin to cause the input pin, connected directly to the integrating capacitor, to settle at the 2.5 volt logic threshold.
FIGURE 8: Temperature probe measurement
The result of the sigma delta ADC software processing is a count from 0 to 255, corresponding to the duty cycle on the output pin. A value of zero corresponds to zero volts from the divider; a value of 255 corresponds to 5 volts.