Voltage comparator using LM741 not switching at desired voltage

I'm relatively new to circuit design and as a mechanical engineer have mostly a mechanical background. However, I am trying to design a circuit and having some trouble.

The circuit needs to ground a neutral switch wire when a certain voltage is measured from a potentiometer. The potentiometer is in an accelerator pedal and outputs a voltage from 1V to 3.5V. When the voltage reaches 1.2V I would like the circuit to ground the neutral wire coming from the car ECU.

I'm attempting to do this using a LM741 op am, L7805 voltage regulators, a voltage divider, and a transistor.

The issues I am encountering are as follows:
1. I'm getting 1.2V as desired into the non-inverting input (pin 3) but the IC isn't actually switching to high until ~2.5V is reached.

2. Once the output voltage switches to low at pin 6 it isn't ~0V like I am expecting considering V- (pin 4) is wired to ground.

3. I'm regulating the supply voltage into V+ (pin 7) because the alternator voltage varies and from testing this seems to affect the performance of the chip.

4. I haven't chosen a transistor yet because I am not getting the output signal I am expecting. I'd love to hear you opinions on using a transistor here or if there is a better option.

I've attached an image of my circuit. Thanks for your help!

Hi. Just a quick reply, I might try and post more in a day or two.

You have made a lot of errors here which are very easy for newbies to electronics (I can recall making some of them myself :-)

1. The 741 is an op-amp. If you are needing a comparator, use a comparator, not an op-amp. Some suitable ones are the LM311 (single) and the LM393 (dual). My preference would be the LM393 but either should work for you.

2. The 741 is an old op-amp and likes a supply voltage of +/- 15V or more. You should look for a component that is specified to work at 5V supply (both the above comparators will). The reasons for this will be clearer in 3. and 4.

3. The 741 inputs will only work reliably to within a few volts of the supply lines. In your application you are putting your inputs within 1 or 2 volts of the negative supply, and the 741 probably won't be happy with this. If you look at the datasheets for the comparators, it states that the inputs will work right down to the negative supply rail.

4. Similarly the 741 outputs will only go to within a few volts of the supply rails. with a 5V supply you may get all sorts of unintended and incorrect outputs (as you have done). The comparators are better in this respect. The LM311 has a transistor on the output with the collector and emitter on pins, wo you can use it to pull-up or pull down a load. Depending what you're trying to drive, this transistor may be suitable for you to use directly.

5. If you are using a 5V regulator (7805) for the supply, you can use that to supply your voltage divider for the reference voltage as well. No need to have a second one.

6. Your circuit diagram will have 5 x 1 / 1.33 = 3.75 volts on pin 3 as a reference. I assume you just got them the wrong way round as that will give you 5 x 0.33 / 1.33 = 1.128 volts. Which is about what you say you're getting :-)

Have a think about the above and have a look at the referenced data sheets, I hope this will make things a bit clearer. Cheers // David

Sorry, in point 6, to be clear
"I assume you got the resistors the wrong way round...