circuit ideas

94 April 2013 | ElEctronics For You www.EFYmAg.com

D. Mohan KuMar

Sensitive LPG Leakage alarm a few discrete components.

GS1 is a six-pin gas sensor that can detect very small traces of LPG in the air and has a swift response time. However, it has very less sensitivity to alcohol and smoke. The sensor’s out-put is in the form of resistance.

As indicated in Fig. 1, the pins of GSI are H, A and B, two each on either side. H pins are for the heater with no

Here is an ultra-sensitive LPG sensor that generates loud beeps when it senses any gas

leakage. It detects vapours of lique-fied petroleum gas anywhere between 200 and 10,000 ppm and drives a pi-ezobuzzer to catch attention for imme-diate action. The buzzer beeps until the concentration of gas in the air decreases to a safe level. The circuit uses an MQ6 gas sensor, which is designed to sense LPG, propane and isobutane gases.

Circuit and workingFig. 1 shows the circuit of the LPG sensor. The circuit is built around 5V voltage regulator 7805 (IC1), gas sensor MQ6 (GS1), counter IC 4060 (IC2) and

polarity. Input pins A or B and output pins A or B can be connected either way round.

The coil heater inside the sensor can be easily heated with 5V DC. If pin A is connected to 5V DC through variable resistor VR1, use pin B as the output or vice versa. Both A and B pins can be shorted. In short, H pins are connected to positive and negative rails, A or B pin to 5V DC, and B or A for output.

The resistance value of GSI is dif-ferent for various kinds and concen-tration of gases. So when using this sensor, sensitivity arrangement is very important. For accurate detection, it is necessary to calibrate the sensor for

1000 ppm of LPG concentration in the air with load resistance of about 20 kilo-ohms. (In the datasheet, the load resistance range of MQ6 is mentioned as 10 kilo-ohms to 47 kilo-ohms.)

Preset VR1 is used to adjust the sensitivity of the sensor to a par-ticular gas concentration. Output from the sensor is connected to the base of transistor T1, which acts as a switch to trigger the alarm gen-erator built around IC2.

IC2 is a binary counter IC that oscillates using capacitor C2 and resistor R5. Transistor T1 controls the reset pin (pin 12) of IC2. When

sani theo

Test PointsTest point Details

TP0 0V, GND

TP1 12V

TP2 5V

TP3 0V when sensor is exposed to LPG and LED1 blinks

TP4 5V square pulse

LED1BATT.112V

C2 0.22u

R5 100K

R41K

R71K

R1100E

R21K

R6 1M

R310K

GND

TP1

TP3

TP0

T1BC547

T2BC547

PZ1PIEZO

16 VDD

12 MR

11 RS

9 CEXT

10 REXT

8 GND

6Q6

14Q7

13Q8

15Q9

1Q11

2Q12

4Q5

5Q4

7Q3

3Q13

IC2

4060

BUZZER

GS1MQ6SENSOR

VR14.7K

1

2

3IC17805

C1100u,25V

A AH

B H B

TP2

TP4

R820K

Fig. 1: Circuit of the sensitive LPG sensor

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circuit ideas

95www.EFYmAg.com ElEctronics For You | April 2013

Working of the circuit is simple. When the sensor detects LPG in the air, its output becomes high and transistor T1 conducts to make reset pin of IC2 low. This triggers IC2 to oscillate, which is indicated by LED1. After a few seconds, the buzzer starts beeping to indicate gas leakage.

The circuit works off 12V DC from a battery (BATT.1) or you can use an adaptor. IC1 provides regu-lated 5V DC supply for the sensor and IC2.

Construction and testingAn actual-size, single-side PCB for sensitive LPG sensor is shown in Fig. 2 and its component layout in Fig. 3. After assembling the circuit on a PCB, enclose it in a suitable case with an opening to allow the gas to enter. Place the unit near the LPG cylinder or gas stove within a distance of one metre. Vary preset VR1 to adjust the

sensitivity of the sensor.To test the circuit, check 12V at

test point TP1 with respect to TP0 to

verify the correct power supply. Place the unit near the gas stove burner and turn on the burner for a few seconds without igniting. Then, turn ’the burner ‘off’ and adjust VR1 until you see LED1 glowing. TP3 should be low at this moment.

The author is an associate professor at Govern-ment College for Women, Thiruvananthapuram, Kerala

Fig. 2: An actual-size, single-side PCB for sensitive LPG sensor

Fig. 3: Component layout for the PCB

the reset pin is high IC2 does not oscil-late, and when this pin goes low IC2 starts oscillating.

Parts ListSemiconductors:IC1 - 7805, 5V regulator IC2 - 4060 ripple-carry binary counter T1, T2 - BC547 npn transistorLED1 - 5mm red LED Resistors (all ¼-watt, ±5% carbon):R1 - 100-ohmR2, R4, R7 - 1-kilo-ohmR3 - 10-kilo-ohmR5 - 100-kilo-ohmR6 - 1-mega-ohm R8 - 20-kilo-ohmVR1 - 4.7-kilo-ohm presetCapacitors:C1 - 100µF, 25V electrolyticC2 - 0.22µF ceramic diskMiscellaneous:BATT.1 - 12V batteryPZ1 - PiezobuzzerGS1 - 6-pin MQ6 LPG sensor