The LDR changes its resistance with the amount of light shining on it.

A phototransistor allows more current to flow through it, so the configuration for a phototransistor is different. I believe that each phototransistor needs a resistor and is connected from Vcc to ground. Since there is some variation in the resistor and the phototransistors, it would be difficult to have the same amount of current through each one for the same amount of light.

Makes sense.

I believe another big difference is that phototransistors respond much more quickly than LDR devices, but I could be outdated in that belief.

This will be a factor because I want to shine a laser on the sensers to tweak the motors.

 

Are two power supplies required for the negative inputs of an op-amp? Can negative and positive voltages be derrived from a single battery source simultaneously?

 

Are two power supplies required for the negative inputs of an op-amp? Can negative and positive voltages be derrived from a single battery source simultaneously?

Yes. There are a number of ways to approach this depending on the details of what you are trying to accomplish.

The simplest is probably just to use a voltage divider to create of reference voltage midway between the battery terminals and then buffer that with an opamp to establish your "ground". As with all approaches, there are practical considerations that limit the situations in which this is a viable approach.

At the other end of the spectrum you could use switching supplies to generate all of your power supply nodes.

 

Am I stuck dividing the voltage though? I was reading somewhere that if I stuck a lead between two stacked double AA batteries and completed the circuit in both directions, I could reference the middle point as ground (something like this). But it seems I'd only get -1.5v and +1.5v instead of -3v and +3v (the point of running batteries in series). It seems like I might as well have 4 batteries (2 supplies) if I'm forced to divide the voltage up.

Anyway, I'll read into the voltage divider method thank you very much

 

If you tap the point between two batteries, then you have two power supplies and not one. That is a perfectly fine, and common, way of getting positive/negative supply rails. But it does not qualify as an answer to your question, which was specific to getting the rails without using multiple supplies.

You are correct that you would need to use 4 cells to get ±3V rails this way. One of the downsides to this approach is that the batteries do not drain evenly and so your rails do not degrade symmetrically.

 

If you tap the point between two batteries, then you have two power supplies and not one. That is a perfectly fine, and common, way of getting positive/negative supply rails. But it does not qualify as an answer to your question, which was specific to getting the rails without using multiple supplies.

You are correct that you would need to use 4 cells to get ±3V rails this way. One of the downsides to this approach is that the batteries do not drain evenly and so your rails do not degrade symmetrically.

Rail symmetry is not generally a requirement for op amps.

 

No, it is not, but for many circuits, including many that use opamps, if the rails are not symmetric the circuit does not operate properly. If nothing else, there are frequently portions of a circuit that only use one rail. So if one of the rails dies and the other rail is good, part of the circuit functions and part of it doesn't. This can lead to difficult to identify problems, especially at the user level, because the user believes the circuit should be working (gee, how can the batteries be dead if the power indicator is on?).

 

If you tap the point between two batteries, then you have two power supplies and not one. That is a perfectly fine, and common, way of getting positive/negative supply rails. But it does not qualify as an answer to your question, which was specific to getting the rails without using multiple supplies.

You are correct that you would need to use 4 cells to get ±3V rails this way. One of the downsides to this approach is that the batteries do not drain evenly and so your rails do not degrade symmetrically.

Makes sense about uneven drainage.

So with the voltage divider method are you also stuck with lower voltages. I haven't found any evidence that it's possible yet. All the examples show the voltages divvyied up.

 

I don't see a need for two power supplies. Why are you thinking you need them?

 

Maybe a charge pump is what I'm looking for?

 

I don't see a need for two power supplies. Why are you thinking you need them?

I'm not, I definitely don't want two power supplies. I just need to obtain my negative charge from one supply to feed a comparator and I think I'm thoroughly confused.

 

Very, interesting. I kinda of see what your talking about, but I need to study this more. I thought it was very odd that they used different resistor values.

I'm just now starting to wrap my mind around op-amps and I'm starting to see how and why you need two in order to open transistors at opposite corners. This is very cool.

Another cool thing is that using a comparator circuit overcomes any issues caused by LDR slow response. The comparator switches each side of the H-Bridge rapidly, so the transistors on each side of the bridge are not both on for a significant amount of time. Both on at the same time = a short = trouble!

 

To be honest, what really confuses me is the idea that current could run two directions over the same wire simultaneously. And maybe it can't. I'm not sure. My experiments indicate that it can't because all of my components have anode/cathode symbols which seem to suggest that you can only hook them up one way and that current conventionally runs from positive to negative (it's the only way I can get an led to light up).


What I have read though, and looking at the pinouts is a comparator requires a negative voltage to output when the comparator determines the input - voltage is higher than the input + voltage. I'm starting to think I only need a small negative voltage though (which I should get from a voltage divider?), because ultimately it's
feeding into an hbridge which iteself should allow plenty of current through the collectors and emitters.

Is there a voltage divider in that schema?

 

You don't need two power supplies.
What voltage do you want to apply to your motor?

 

What I have read though, and looking at the pinouts is a comparator requires a negative voltage to output when the comparator determines the input - voltage is higher than the input + voltage. I'm starting to think I only need a small negative voltage though (which I should get from a voltage divider?), because ultimately it's feeding into an hbridge which iteself should allow plenty of current through the collectors and emitters.

There's a misunderstanding. The -ve input to the comparator means that that input is normally more negative than the +ve input (even if the inputs are +100V and +103V). In this situation the comparator has a low output signal. When the -ve input has a more positive voltage than the +ve input, the comparator has a high output signal. The comparator will happyly operate when the 2 rails are less than 15V apart (depending on the part) and the inputs are between the rails. So for the values given above, the rails might be +105V and +95V.

 

I want 3 volts or as close to that as I can get.

 

+1
The comparator output switches between the voltage on V+ and the reference on the V- terminal.
V- can be at ground reference. It doesn't have to be negative (below ground reference).

 

There's a misunderstanding. The -ve input to the comparator means that that input is normally more negative than the +ve input (even if the inputs are +100V and +103V). In this situation the comparator has a low output signal. When the -ve input has a more positive voltage than the +ve input, the comparator has a high output signal. The comparator will happyly operate when the 2 rails are less than 15V apart (depending on the part) and the inputs are between the rails. So for the values given above, the rails might be +105V and +95V.

When (-ve)>(+ve), the output will be low, not high as you stated.

EDIT: Maybe I don't understand what you mean by -ve and +ve. I thought you were referring to the -input and +input comparator pins.

 

Also, is their a recommend modeling software (using linux) where I could draw this type of thing?

geda, oregano, eagle, electric, and spice are a few that i can think of that may serve the purpose but there are a lot more


long time linux user

 

When (-ve)>(+ve), the output will be low, not high as you stated.

EDIT: Maybe I don't understand what you mean by -ve and +ve. I thought you were referring to the -input and +input comparator pins.

You might be right, I didn't check the datasheet. What a blunder.