D. MOHAN KUMAR
S.C.
DWIVEDI
H
ere is an ultra-sensitive fire sensor that activates an alarm when it detects fire. Thermistor based fire alarms have a drawback; the alarm turns on only if the fire heats the thermis- tor in close vicinity. In this circuit, a sensitive PIN diode is used as a fire sensor for a longer-range fire detection.It detects visible light and infrared (IR) in the range of 430nm - 1100nm. So visible light and IR from the fire can easily activate the sensor to trigger the alarm. It also detects sparks in the mains wiring and, if these persist, it gives a warning alarm.
It is an ideal protective device for showrooms, lockers, record rooms and so on. Author’s proto- type is shown in Fig. 1.
PIN diode BPW34 (Fig. 2) is used in the circuit as light and IR sensor. BPW34 is a 2-pin photodi- ode with anode (A) and cathode (K). The anode end can easily be identified from the top-view flat surface of the photodiode. A small solder point to which a thin wire is connected is the anode and the other one is the cathode terminal. BPW34 is a tiny PIN photodi- ode or mini solar cell with radiant sensitive surface that generates 350mV DC open-circuit voltage when exposed to 900nm light. It is sensitive to natural sunlight and also to light from fire. So it is ideal for use as a light sensor.
BPW34 photodiode can be used in zero-bias as well as reverse-bias states. Its resistance decreases when light falls on it.
Circuit and working
Circuit diagram of the PIN diode based fire sensor is shown in Fig. 3. It is built around 9V battery, PIN diode BPW34 (D1), op-amp CA3140 (IC1), counter CD4060 (IC2), transistors BC547 (T1 and T2), a piezo buzzer (PZ1) and a few other components.
In the circuit, PIN photodiode BPW34 is connected to the invert- ing and non-inverting inputs of op-amp IC1 in reverse-biased mode to feed photo current into the input of op-amp. CA3140 is a 4.5MHz Bi- MOs op-amp with MOSFET inputs and bipolar output.
Gate-protected MOSFET (PMOS) transistors in the input circuit provide very high input impedance, typically around 1.5T ohms. The IC requires very low input current, as low as 10pA, to change output status to high or low.
In the circuit, IC1 is used as a transimpedance amplifier to act as a current-to-voltage converter. IC1 amplifies and converts the photo current generated in the PIN diode to the corresponding voltage in its output. The non-inverting input is
connected to the ground and anode of photodiode, while the inverting input gets photo current from the PIN diode.
PARTS LIST
Semiconductors:
IC1 - CA3140 op-amp IC2 - CD4060 counter T1, T2 - BC547 npn transistor LED1-LED3 - 5mm LED
D1 - BPW34 PIN photodiode
Resistors (all 1/4-watt, ±5% carbon):
R1, R5, R6 - 1-mega-ohm R2, R3 - 1-kilo-ohm R4, R7, R8 - 100-ohm Capacitor: C1 - 0.22µF ceramic disk Miscellaneous: BATT.1 - 9V battery PZ1 - Piezo buzzer
Fig. 1: Author’s prototype
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Large-value feedback resistor R1 sets the gain of the transimpedance amplifier since it is in inverting con- figuration. Connection of non-in- verting input to ground provides low impedance load for the photodiode,
D. Mohan Kumar was associate professor at Government College for Women, Thiruvananthapuram, Kerala
Fig. 3: Circuit diagram of the PIN diode based fire sensor
Fig. 4: PCB layout of the PIN diode fire alarm
Fig. 5: Component layout of the PCB
which keeps the photodi- ode voltage low.
The photodiode operates in the photo- voltaic mode with no external bias. Feedback of the op-amp keeps the photodiode current equal to the feedback current through R1. So the input offset voltage due to the photodiode is very low in this self-biased photovol- taic mode. This permits a large gain without any large-output offset volt- age. This configuration is selected to get large gain in low-light conditions.
Normally, in ambient light condition, photocur- rent from the PIN diode is very low; it keeps output of IC1 low. When the PIN diode detects visible light or IR from fire, its photo current increases and transimpedance ampli- fier IC1 converts this current to corresponding output voltage. High output from IC1 activates transistor T1 and LED1 glows. This indicates that the circuit has detected fire. When T1 conducts, it takes reset pin
7 8 6 5 1 4 3 2 IC1 CA3140 R2 1K T1 BC547 R1 1M R3 1K LED1 16 V 12 RESET 11 9 10 8 Vss 6 Q7 14 Q8 13 Q9 15 Q10 1 Q12 2 Q13 4 Q6 5 Q5 7 Q4 3 Q14 DD 0 0 0 1 0 _ 0 IC2 CD4060 R4 100E C1 0.22u R5 1M R6 1M T2 BC547 R8 100E R7 100E PZ1 PIEZO BUZZER D1 BPW34 LED3 LED2 BATT.1 9V DC D1 = BPW34 PIN PHOTODIODE GND 12 of IC2 to ground potential and CD4060 starts oscillating. IC2 is a binary counter with ten outputs that turn high one by one when it oscillates due to C1 and R6. Oscillation of IC2 is indicated by the blinking of LED2. When output Q6 (pin 4) of IC2 turns high after 15 seconds, T2 conducts and activates pi- ezo buzzer PZ1, and LED3 also glows. The alarm repeats again after 15 seconds if fire persists.
You can also turn on an AC alarm that produces a loud sound by replacing PZ1 with a relay circuitry (not shown here). The AC alarm is activated through contacts of the relay used for this purpose.
Construction and testing
An actual-size, single-side PCB for the PIN diode based fire sensor is shown in Fig. 4 and its component layout in Fig. 5. Enclose the PCB in a small box in such a way that you can connect PIN diode BPW34 eas- ily at the rear side of the box. Install the PIN diode in a suitable place and cover it such that normal light/ sunlight does not fall on it.
Testing the circuit is simple. Nor- mally, when there is no fire flame near the PIN diode, the piezo buzzer does not sound. When a fire flame is sensed by the PIN diode, piezo buzzer sounds an alarm. Its detec- tion range is around two metres. It can also detect sparks in the mains wiring due to short-circuit.
SOMNATH BERA
W
e can measure tempera- ture and humidity inside the fridge using a normal temperature-humidity indicator but relative humidity (RH) could be inaccurate in that case. The moment the fridge door is opened, RH will shoot up due to ingress or egress of moisture in the surroundings.The small sniffer device, de- scribed in this article, picks up tem- perature and humidity from inside the fridge and transmits on an RF link to a nearby receiver unit. The receiver unit checks the received code, identifies the right sniffer device and displays live temperature and humidity. Author’s prototype is shown in Fig. 1.
Circuit and working
Circuit diagram of the transmitter unit is shown in Fig. 2. It is built around ATmega328P microcon- troller (MCU) (IC1) with Arduino Uno bootloader, AM2302 digital temperature and humidity sensor connected as SENSOR1 to CON1, 433MHz transmitter (TX1), 5V zener ZD1, 3.3V zener ZD2 and a few other components.
Low power consumption of the transmitter is the essence for long operating hours of the gadget. For that, the conventional regulator is replaced by 5V and 3.3V zener diodes, with 330-ohm resistor R3 and 100-ohm resistor R4 in series to reduce current consumption.
Circuit diagram of the receiver unit is shown in Fig. 3. It is built around another ATmega328P MCU (IC2) with Arduino Uno bootloader,
voltage regulator 7805 (IC3), 16x2 LCD character module display (LCD1), 433MHz receiver (RX1) and a few other components. The receiver checks the code word sent by the trans- mitter unit and displays tempera- ture and humidity on the LCD.
If the trans- mitter stalls or its power supply gets interrupted, there is no way for the receiver to know whether the incoming
Fig. 1: Author’s prototype Fig. 2: Circuit diagram of the transmitter unit 2 PD0/RXD 3 PD1/TXD 4 PD2 6 PD4 8 GND 10 PB7/XTAL2 1 PC6/RESET 5 PD3 7 Vcc 11 PD5 9 PB6/XTAL1 12 PD6 13 PD7 14 PB0 27 PC4/SDA 26 PC3 25 PC2 23 PC0 21 AVREF 19 PB5/SCK 28 PC5/SCL 24 PC1 22 AGND 18 PB4/MISO 20 AVcc 17 PB3/MOSI 16 PB2 15 PB1 IC1 ATMEGA328P XTAL1 16MHz C1 22p C2 22p SENSOR1AM2302 1 GND 2 DATA 3 Vcc 4 ANT TX1 RF TX ANT1 BATT1 9V TP1 R2 10K GND VCC S1 C5 100n R3 330E ZD1 5V 100uC4 25V ZD2 3.3V C3 100u R4 100E DATA TP2 R1 10K 25V CON1 FOR