Hello there, I would like to do the Gas detector project from the circuit below. I want to replace Q1 BC557 with an LM358 op-amp. How can I connect the LM358 and 10k potentiometer to this circuit below? Do I need R1, or should I remove it? Thanks.

 

hi johny,
The BC557 is used to invert the signal from Sensor to the 555.
Can you measure the amplitude/level of the signal coming out of the Sensor?
It should be possible to use the LM358 to replace the BC557 as the inverter.
E

 

As there are 2 outputs from the gas sensor with one marked D0 and the other marked A0 I suspect that D0 is digital and A0 is analogue. I think D0 just switches state a some preset level so adding a pot and opamp would not let you change the level that the LED switches on. The part number of the sensor or the datasheet would tell us if this theory is correct.
Les.

 

Your question is not at all clear. It sounds like what you want to do is change the circuit so that the detection level for the gas is adjustable. It that correct, or do you want an adjustable ON time for the LED?

Please post a schematic that is your best guess at what you want, so we can provide some steering.

ak

 

Or do you, as you wrote, want to substitute an LM358 for the BC557?

 

Using the 358 as a comparator could certainly work. Probably either with the digital out OR the analog out.