How do I use an op-amp in a new Bit design?

Thanks RyanI (@StuckInSynth) I will have to find that LVM321.

The closest opamp I have on hand is the nte928m, It works with a +5 supply, and has a rail-to-“almost rail” output, zero to 3.8 volts. I had noticed the littleBits all use 321s, but have not been able to get my hands on one, nor any other single-supply & rail-to-rail amplifier (do others exist?), so I’ve compromised my designs so far to use the 928. For things like lighting LEDs, it works fine, since the voltage needs to be dropped for the LEDs anyway.

But some applications require the full 5 volts however, so the 321 will be needed. For example I spec’ed the fan for my co2 sensor to run on 5 volts to give 0.2 L/min through the detector. It was difficult to go as low as 5 volts. So if I put an opamp that drops the output below 5 volts, I’ll need to find (and calibrate!) another fan (which I don’t want to do. 321 to the rescue!)

… to be continued.

Yo, @syedBits, my initial bitlab prototype does not need to be littleBits compliant, right? It just has to work? And it can be huge, compared to the size of a typical littleBit?

@chris101You can draft a project and we’ll be happy to give you feedback!

Thanks @syedBits, I have an update that is getting much closer to a prototype Bit here. I would certainly appreciate any comments, ideas and especially critiques you may have!

So you can see my application for the NTE928N op-amp in the write-up for my people counter project. It’s different than the protective use the 321 is often put to in bits. I still need to get some 321 chips to work with, but none of my usual vendors carry them.

Where would I be able to find them available, or are there other ‘rail to rail’ amplifiers that would work?

@chris101 You can substitute with other op amps for now. I had 3 modules go up for voting that all had 741 op amps in them. If a module gets the 1,000 votes, then you can redesign and test with the exact components littleBits uses.

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Thanks Jude. That is what I ended up doing with the co2 project - I used NTE928N dual op-amp (it’s the same as the LM358). I liked the 928 because it output is rail-to-“almost rail”. That means it goes to zero, but not quite to 5 volts. Since the maximum output of the sensor I have is 4 volts, this is a pretty good match.

However for applications that measure the voltage, like my bargraph, or Ryan’s number bit, it is imperative that the output follow the input all the way from 0 volts up to 5 volts. None of the op-amps I have now can do that, or maybe I am not doing something right?

Here are my experiments with the three op-amps I have.

Experimental Setup

LM324N

NTE928M (LM358 equivalent)

NTE941 (LM741 equivalent)

When I get a LV321 (or another rail-to-rail device) I will do the same test and post it back in this thread. Or if I am wrong about how a rail-to-rail device works, somebody please correct me!

the LV321 has a CMOS output stage. LM324 and 741 have transistor output stages. CMOS is basically a variable resistor so can connect output to either rail as long as current not too large. 324 has darlington pair outputs which means two Vbe drops atleast which is around 1.2 volts. Section 8.2.1.3 Application Curve in TI Datasheet for LM324 shows a similiar graph. http://www.ti.com/lit/ds/symlink/lm124-n.pdf Nice thing about datasheets is that they typically have a standard format…

Thanks Peter (@codewizard58),

As an amateur electronics technician, I am a complete noob, and your post really sent me back to school. :smiley:

I had seen that graph in the datasheet, but I did not know how to estimate the voltage drop, and the ‘Vmax’ line on the graph was not labeled, so I did the experiments to find out what the max voltage would be with the op-amps I have. Do I understand from your comment:

to mean that any CMOS op-amp (with a +5 volt/GND power supply) would be capable of +5 volt output?

Some Op amps have FET front ends for High impedence. The 321 has a CMOS output stage. I have not found a circuit diagram for it. CMOS usually uses P and N fet devices. Both LM324 and LMV321 would be considered totem pole output configurations.

Is ‘totem pole’ output the same as a push-pull amplifier, like the old, sweet-sounding, tube amplifiers? I thought that was for power - does it also affect the maximum output voltage? Because, except where I need amplification (like in the people counter circuit), littleBits uses LV321’s to follow the voltage of the input for circuit protection.

I think it looks great @chris101 feel free to submit and we’ll get back to you with feedback asap.

Thanks @syedBits! I am on track to do just that. I hope to have time to work out the remaining logic issues over the next couple of weeks, as school is out from today until June 1.

That’s awesome Chris!

I think I found it: op-amp nirvana!

I just got several OPA4342 op-amp chips. They are regular, “through hole” DIP style chips, so they will work on a breadboard, chip-sockets, or solder them into perf-board. Single (or dual) supply, +5 volts and ground work just fine. It’s a 14 pin quad package, with the same pinout as the popular LM324, so you can just drop it into an existing circuit.

Oh, did I mention that they are RAIL TO RAIL!!!

Here is the datasheet. I just tested them, they work as expected: 0.0v to 5.0v.

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@chris101, do you know if a 4 pin rail to rail op amp? I really like 4 pin chips.

Hey @JackANDJude - the 4342 is the only single supply, rail to rail opamp in a DIP through hole package I have been able to directly purchase. But after I found them, I quit looking. There are a lot of choices of dual and single chips though. Here is a list from digikey (I have not bought from them, but hey are referenced at aruino forum a lot):

http://www.digikey.com/product-search/en?vendor=0&keywords=opamp+dip+rail

I had a chance to look through that list, and this one looks good:

http://www.digikey.com/product-detail/en/MCP6241-E%2FP/MCP6241-E%2FP-ND/683249

It doesn’t hurt that it’s made just down the street from where I live either. :smile:

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I never found the Microchip device, but I came across these op amps from Adafruit:

TLV2462, Rail to Rail input and output, 80ma, 2.7-6v, 6.4MHz op-amp

2462s are dual amp chips should be good for building new littleBits projects. I just ordered a few, so I’ll let you-all know how they work.

(By the way, I’m still liking the 4342s - I used them is this and this project., but these TI chips are are smaller and cheaper when a circuit only needs 2 amplifiers.)

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Did you try the TLV2462 yet, @chris101? I had bought the LV321 SMD, but the truth is I much prefer through hole. :wink:

Hello @JackANDJude, hello @chris101,

I used the TLV2462 on my breadboard bit. It works quite ok. However, when I tried to use it as an inverting amplifier, it starts to swing under certain conditions. I haven’t had the time to investigate this further.
Currently I only use it as a buffer in the voltage follower configuration.