added new simple circuit to project

smoke sensor MQ2 used DC supply to operate,the output from sensor cannot be entered to PLC Siemens Logo because that things used  AC supply to operate, so that I need to convert output from MQ2 to AC form. I need to combine the comparator circuit with relay to the output of MQ2, then the input for Siemens Logo can be operate normally in AC form.

above is the circuit for the comparator design. the output from the comparator circuit will be connected to relay and the output from the relay will connect to the input for the PLC Logo. To construct this circuit, it needs amplifier OP07 with the variable resistor and 1k ohm resistor.

Voltage comparator is a circuit which compares two voltages and switches the output to either high or low state depending upon which voltage is higher. A voltage comparator based on opamp is shown here. Fig1 shows a voltage comparator in inverting mode

In non inverting comparator the reference voltage is applied to the inverting input and the voltage to be compared is applied to the non inverting input. Whenever the voltage to be compared (Vin) goes above the reference voltage , the output of the opamp swings to positive saturation (V+) and vice versa. Actually what happens is that, the difference between Vin and Vref, (Vin – Vref) will be a positive value and is amplified to infinity by the opamp. Since there is no feedback resistor Rf, the opamp is in open loop mode and so the voltage gain (Av) will be close to infinity. So the output voltage swings to the maximum possible value ie; V+. Remember the equation Av = 1 + (Rf/R1). When the Vin goes below Vref, the reverse occurs.

So that, the output from the comparator circuit can be supported by the AC voltage because the voltage gain  close to infinity.