Output voltage of LM35 series IC temperature sensors follows linearly (@10mV/°C) the centigrade temperature of its surroundings, taking 0mV at 0°C temperature. The ADC continuously scans its eight input lines. The scanning process is governed by a 3-bit binary up counter built around CD4029 (IC6).
The counter places a continuously-changing 3-bit binary number on A-B-C input lines of the ADC (pins 25, 24 and 23). Scanning rate is dependent upon the clock constructed around timer NE555 (IC2), and is 8Hz, approximately. Hence, each of the eight sensors is allowed to send data to the ADC for approximately one-eighth of a second, irrespective of whether all sensors are connected or not.
Here, IC3 is configured in continuous operational mode. So, whenever a particular sensor is addressed, output lines of the ADC reflect the present analogue output status of the sensor. Output of the ADC goes to data input lines of special encoders HT12E (IC4 and IC5); higher nibble to IC4 and lower nibble to IC5, respectively.
As TE input (pin 14) of encoders is permanently grounded (logic 0), the encoders are configured to produce encoded data continuously. Data is available at pin 17 of IC4 and IC5. These two encoded digital outputs are alternately steered to TX1 (TX-433MHz), a UHF RF transmitter module, to modulate UHF carrier wave generated by the module. Selection of encoder output is done in the following manner:
Whenever IC2 output pulse goes high, pin 10 of AND gate N3 of IC7 (a quad 2-input TTL AND gate) also goes high, allowing output of IC5 to be steered to TX1 through diode D4. At the same time, due to the presence of transistor inverter T1, pin 13 of gate N4 of IC7 goes low, inhibiting output of IC4 to reach TX1 through gate N4. As soon as the clock pulse returns to logic 0, output of IC4 gets its passage to TX1 through gate N4 of IC7.
So, in essence, analogue data of a sensor is converted and the resultant 8-bit digital data is sent to the remote end using ASK/OOK modulation, in a complete clock cycle of IC2.
Modulated signal is radiated into space through a wire (approximately 35cm long), acting as an antenna, connected at the antenna point of the module.
Receiver. Fig. 3 shows the receiving unit of the wireless addressable digital thermometer. RX1, a 433MHz RF receiver module, is used to receive and demodulate ASK-modulated RF signal transmitted by TX1 of the transmitter unit. Demodulated output is a train of rectangular pulses comprising a 4-bit data nibble and destined for a particular decoder as explained earlier.
Transistor BC547 (T2) is used as a pulse amplifier to amplify the signal output from RX1 and, hence, raises the pulse height to CMOS compatible logic -1 (>3.5V at 5V). This compatible output is then fed to CMOS NAND gate 4011 (IC19). NAND gate N1 helps to get pulses of perfect rectangular-wave shape. Output of gate N1 of IC19 is fed to decoders HT12D (IC20 and IC21). Address lines of the decoders are preset to receive data from encoders IC4 and IC5, respectively.
LEDs (LED5 and LED6) connected at their VT outputs (pin 17) flicker to indicate reception of valid data. Decoding speed (as measured at pin 15) is 200kHz (approximately). Decoded data is then fed to IC22, a DAC (0808). Analogue current output of the DAC (pin 4) is loaded with resistor R26. Voltage developed across it is fed to a digital multimeter, which shows the temperature on mV scale.
A thumbwheel switch (TWS1) is used to change the preset address of the decoders (IC20 and IC21). The switch changes the last three LSB of the address. For example, to get data from the sensor (IC13) connected at IN4 input of the ADC, the number to be set on TWS1 is 4.
Power supplies. The thermometer needs power supplies both for its transmitter and receiver units. Whereas the transmitter unit needs only a +5V regulated power supply built around X1, D1, D2 and IC8 (Fig. 2), the receiver unit needs a dual +5V and -5V supplies built around X2, BR1, IC17 and IC18 (Fig. 3). Both circuits work from 230V AC, 50Hz connected to the primaries of transformers X1 and X2, respectively.
Power supply circuits are self-explanatory. Three-terminal positive voltage regulator IC 7805 is used both in the transmitter and the receiver units to get regulated +5V supplies. On the other hand, a three-terminal negative-voltage regulator IC 7905 is used to get -5V supply required for DAC0808 of the receiver unit.