This application features a low-power, internal-sensor, temperature-to-period converter (MAX6576). Drawing only 140 μA of current from a supply of 2.7V to 5.5 V, it comes in a 6-pin SOT package and reads temperatures between "40īC and +125īC. Its output is a square wave with period proportional to the absolute temperature of the IC's die temperature, in degrees Kelvin (1īK = 1īC).
The proportionality factor can be hardwired as 10, 40, 160, or 640 μs/īC. For this application, the factor is set for the lowest operating frequency (which gives the highest averaging and lowest noise), with the resulting period equal to 192ms at room temperature (i.e, about 5.2Hz at 25īC = 298īK). That period is longer than the time constant of the IC, which is the limit for maximum rate of temperature slew detectable by the device. At the maximum operating temperature (125īC) the signal period is 255ms, and at minimum (-40īC) the period is 149ms.
Figure 1. This IR transmitter produces a short, 10 μs IR pulse for each positive-going transition produced by the temperature-to-period converter (MAX6576).
Click to Enlarge
The IR-transmitter circuit (Figure 1) generates a short, high-intensity IR pulse of about 10 μs for each positive-going transition of the IC's square-wave output. The circuit is very simple, using one fourth of a 74HC132 quad dual-input Schmitt trigger as a positive-edge differentiator, with the remaining three gates connected in parallel as a driver for the IR LED. The IR LED is a standard type found in handheld remote-control units.
Total supply current for the circuit when measured near room temperature is below 140 μA. Powered by a CR3032 lithium battery (3mm thick by 23mm diameter), the IR-linked sensor is capable of continuous operation for six months. Two CR3032s in parallel provide continuous operation for a whole year. Standard lithium primary batteries like the CR3032 are limited, however, by their 60C maximum operating temperature.
A poly-carbon monofluoride lithium high-temperature battery from Panasonic (the BR2477A, 7.7mm thick by 24.7mm diameter) can power the sensor and IR transmitter for a year while operating between "40C and +125C. Because the battery is larger than all other components in the sensor unit combined, it alone virtually defines the unit's overall size.
The IR receiver and signal processor
The purpose of the receiver/signal processor is to receive IR pulses generated by the temperature-sensor IR transmitter, and produce from them a digital output and an analog output. The digital output, simply a standard logic pulse recovered from the IR signal, is useful as an interrupt for a microcontroller. You can then recover original temperature data from this signal with the help of a peripheral timer in the microcontroller and a simple scaling algorithm.
The analog output is a dc voltage proportional to the IR signal period, determined by the MAX6576 output, which in turn is proportional to the absolute temperature. The analog output allows a DMM or DVM scaled for direct temperature readings to read temperature at the location of the IR-linked sensor. You can build a stand-alone unit using an all-in-one DVM IC like the MAX1495, which drives LCD displays.
Figure 2. This IR receiver converts IR pulses received from the Figure 1 circuit to an analog output proportional to temperature (the IR sensor is the diode device at upper left).
Click to Enlarge
The receiver/signal processor unit consists of four circuit blocks: the amplifier/filter, the signal recovery/timing generator, the linear ramp generator, and the buffered sample-and-hold (Figure 2). The amplifier/filter includes two stages of low-noise amplification, whose differentiation and integration constants are optimized for minimum noise. The amplifier/filter input connects to an IR sensor of the type used in IrDA links and in the remote control units for TV and other appliances.
A Schmitt-trigger one-shot and two cascaded digital differentiators do the signal recovery and timing. The scope shot of Figure 3 shows the ~1ms recovered pulse (SIGNAL_PULSE, which is also the digital output), and the two sequential 50īs pulses triggered by SIGNAL_PULSE: PULSE1 and PULSE2.
Figure 3. Each SIGNAL_PULSE recovered from an IR transmission produces two pulses as shown for controlling the output sample-hold circuit.
Click to Enlarge
The linear ramp (Figure 4) is generated by a constant current provided by the resistor RRAMP and a bootstrapped reference circuit charging the capacitor CRAMP. The ramp is buffered by the same op amp that bootstraps the reference IC. Because the ramp slope is a function of the capacitor value and current trough the resistor, tolerances on the resistor, the capacitor, and the reference-output voltage impose the need for an adjustment. Implemented with a potentiometer in series with RRAMP, this adjustment trims the accuracy of the analog temperature readout.
Figure 4. IR pulses from the Figure 1 transmitter produce a linear ramp that charges the sample-hold capacitor.
Click to Enlarge
A dual 4:1 analog multiplexer (MAX4618) and op amp (MAX4236) form a sample-hold amplifier (S&H). When triggered by PULSE1, the S&H stores the value of ramp voltage as the voltage on S&H capacitor CSH at the end of PULSE1. PULSE2 then resets the ramp to zero by appropriate control of the multiplexer. This analog output is available at the unity-gain-buffer output of the S&H.
Because the period of the recovered input signal (SIGNAL_PULSE) is proportional to the temperature of the remote sensor's location, the ramp voltage present at the arrival of a second SIGNAL_PULSE is also proportional to temperature. That voltage, memorized by the S&H circuit, forms an analog output stable to three significant digits. The range of the IR link with full ambient illumination is about 20 feet.
You can extend the useful range of the IR link by adding a transistor to provide greater current drive to the IR LED. Increasing the receiver gain can also extend the range, because the noise floor is not a limitation for the component values shown. The circuit of Figure 2 was constructed as a prototype for demonstration. For a more permanent system, you should add a signal monitor with signal-loss indicator. That function can be implemented with a single IC (a watchdog timer driven by PULSE1).