[Course Design] Pointer type gas alarm 47415Y

In the era when MCU was too expensive, analog circuits and pointer tables were used. (MP-4 sensor) realizes gas alarm.

The main function is to detect the methane content in the air, such as > 1000PPM, sound and light alarm, and relay action.

USB power supply or (12V-24V) power supply, 4-20ma output mode can be provided when powered by 24V.

The MP-4 sensor requires a heating power supply and a power supply on the sensor side. It needs to be warmed up for at least 60 seconds.

1) Sensor disconnection detection function.

U2.1 forms a forward amplifier to amplify the sensor heating current, R14 3R current sensing resistor, normally around 50ma, Q1 is turned on, and AL1 outputs low level. Otherwise, it outputs high level.

U2.2 forms a voltage comparator, and a normal sensor has an output of about 0.02V in clean air (actual measurement). Normally the comparator outputs low level. If the sensor side is disconnected (sensor output 0V), the output will be high level.

2) Sensor alarm

U5.1 forms a differential amplifier, subtracts the zero point (R51 adjustment), and magnifies it ten times to display the pointer meter.

U5.2 forms a voltage comparator, the sensor value is greater than the set value (R50 adjustment), and the output is high level.

3) Power on and warm up

R5 and CP3 form an RC charging circuit, and U1.1 forms a voltage comparator, which outputs a low level after about 60 seconds of power-on.

4) Buzzer driver

U1.2 forms an oscillation circuit and generates a square wave to drive the passive buzzer. The frequency is determined by R34, C11 and C12. Q3 can turn off oscillation and output low level. Q5 chooses to add C12 to change the oscillation frequency.

5) Relay drive

Q6 Q4 form a self-locking driver, and SW3 can choose whether to self-lock.

6) 4-20mA output

U8.1 forms a current source, and U8.2 forms a current amplifier. When the sensor fails, Q9 pulls the input low so that the output is zero.

U7 DC-DC circuit generates 24V to 5V for power supply. SW4 selects USB or external power supply.

7) Test button instructions

After pressing the test button SW1, a 30K resistor is connected in parallel to the sensor, and the voltage should be about 150mV (ignoring the sensor resistance, theoretical value). The pointer meter is around 1.5V. The buzzer sounds, the relay closes, and AL3 lights up.

The current output is about 11.5mA. 150mV amplifies 1.5V 10 times, to U8, (0.8V + 1.5V)/2=1.15V. converted to 11.5mA.

Can be used to test whether the back-end circuit is normal.

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The circuit board debugging is completed and there are two flying wires. The verification function is normal.

The circuit has been modified and the schematic and PCB have been updated.

The video has been updated https://www.bilibili.com/video/bv1sK4y1R7r9

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Supplementary instructions on using tweezers to short-circuit the resistor in the video:

1) Short-circuit the sensor current sampling R11, which is equivalent to the sensor heating group being open-circuited, there is no current, the voltage is zero, the amplifier output is zero, and a low-frequency alarm occurs.

2) Short-circuit sensor output R17, which is equivalent to sensor output zero and low-frequency alarm.

3) Short-circuit the power-on preheating resistor R5 to quickly charge CAP3, increase the voltage, and turn off the alarm.

In addition, the measured oscillator circuit can work normally. After the oscillator works for about 60 seconds, the lowest voltage of -0.4V will indeed be generated on C12, but it will not breakdown Q5.

--------------------------------Supplementary circuit function description----------------- ----------------------------------

1) The 2.5V reference voltage (VREF) is generated by TL431 for use by sensors and comparators.

2) Sensor circuit, pin 1 and pin 2 are heating electrodes. Pins 3 and 4 are measuring electrodes.

AVCC is powered by 5V, and R14 is the heating electrode current detection resistor. When the sensor is powered on, the current is approximately 50mA, and approximately 150mV is generated on R14.

VREF supplies power to the measuring electrode. After R17 divides the voltage, pin 4 of the sensor outputs voltage.

After pressing the test button SW1, a 30K resistor is connected in parallel to the sensor, and the voltage should be about 150mV (ignoring the sensor resistance, theoretical value).

3) Power-on preheating circuit. When powering on, R5 charges CAP3, and the voltage on CAP3 gradually increases. After about 60 seconds, the voltage on CAP3 is greater than 2.5V.

U1.1 forms a voltage comparator. When the reverse input terminal is smaller than the non-directional input terminal (2.5V), the op amp output is high. AL0 high level, alarm.

4) Sensor heating electrode disconnection detection. When power is on, R14 generates about 150mV, U2.1 co-directional amplifier amplifies it 25 times, and the op amp outputs about 3.75V, which is output in the reverse direction by Q1. When the sensor heating electrode is disconnected, R14 goes to 0V, the op amp outputs 0V, Q1 reversely outputs 5V, AL1 is high level, and the alarm occurs.

5) Sensor measurement electrode disconnection detection. When the sensor works normally, the output voltage of sensor pin 4 is greater than 10mV.

When the wire is disconnected, there is no voltage at pin 4 of the sensor. The U2.2 voltage comparator is used. When the reverse input terminal is smaller than the non-directional input terminal (6.4mV after resistor division), the op amp output is high and AL2 alarms.

6) The sensor voltage is converted into a pointer voltmeter. The sensor's 4-pin output voltage is input into a differential amplifier composed of op amp U5.1, subtracts the voltage at potentiometer R51 (minus the sensor's zero point voltage, manually adjusted), amplifies it 10 times, and then outputs it to the pointer meter voltmeter to display SOUT.

7) Sensor alarm. When methane is detected, the output voltage of pin 4 of the sensor becomes high. After being output by the operational amplifier U5.1 (SOUT signal), it enters the voltage comparator composed of U5.2.

When the non-inverting input terminal is greater than the inverting input terminal (the voltage of potentiometer R50, the alarm threshold is adjusted by R50), the op amp output is high and AL3 alarms.

8) Relay driver. When any one of AL0, AL1, AL2, AL3 (an OR gate composed of diodes) outputs a high level, the input Q4 drives the relay to close.

Self-locking, when Q4 is turned on, Q6 is also turned on at the same time, and the high level of Q6 collector is input to the base of Q4, resulting in self-locking. When SW2 is pressed, the base of Q4 is forced to low level, releasing the self-locking state.

SW3 selects self-locking or non-self-locking.

9) Buzzer drive circuit. U1.2 forms an oscillation circuit and generates a square wave to drive the passive buzzer. The frequency is determined by R34, C11 and C12. Q3 can turn off oscillation. Q5 chooses to add C12 to reduce the oscillation frequency.

When any one of AL0, AL1, AL2, AL3 (composed of diodes or gates) outputs a high level, the oscillator will start working.

When any one of AL0, AL1, and AL2 (an OR gate composed of diodes) outputs a high level, Q5 is turned on, which will reduce the oscillation frequency and generate a low-frequency alarm sound. Indicates sensor failure.

When only AL3 outputs high level, the oscillator starts to work, Q5 is not conductive, and a high-frequency alarm sound is generated. Prompt for methane exceeding the standard alarm.

10) 4-20mA output circuit. 4-20mA output is a common industrial output method and is effective when powered by 24V. The SOUT signal input U8.1 forms a current source (follower) with an adder. R89 and R90 divide the voltage to generate 0.8V, which is added to the SOUT signal and divided by 2.

When the sensor SOUT signal is 0V, the Q8 emitter outputs 0.4V (0.4mA flows through R87). It can be seen from the figure that the currents of R76 and R87 are the same, and their voltages are in a mirror image relationship.

U8.2 and PMOS Q7 form a voltage follower, and the voltage on (R92 connected in parallel with R73) is the same as the voltage on R76. R7 ratio (R92 in parallel with R73) is 10 times.

Therefore, the current flowing through (R92 in parallel with R73) is 10 times that of R76 (the output is 4mA when the SOUT signal is 0V).

The D16 voltage regulator tube pulls down the voltage so that the input voltage is within the operating range of the op amp (otherwise a rail-to-rail op amp is required).

When AL0, AL1, AL2 (formed by diodes or gate), Q9 is turned on when the sensor fails, pulling the input low to 0V, so that the output is 0mA. (0mA generally indicates sensor failure)

11) 5V power supply circuit, USB supplies 5V. When 24V is powered, U7 steps down to generate 5V power supply. The switch selects the power supply mode.

12) 24V power supply circuit and indicator light. After LC filtering, it is supplied to the 4-20mA circuit.