IR Circuit - Reassurance... or a Telling-off!

[Hippogriff]

Hi all,

I am broadening my horizons, looking at trying to decipher IR codes via a TSOP34836 that I hope can go into a PIC at some point. At the moment I've just set up a simple circuit with the TSOP34836, hooked-up my new Saleae Logic and pointed a Sky+ remote at it - bingo, it recorded different pulses for different buttons, seems to be working and I'm very pleased.

The red button on the Sky+ remote, apparently...

**broken link removed**

As I always seem to do, I started wondering what I could remove from the sample circuit presented in the TSOP34836 datasheet, so I've now removed the 0.1µF capacitor and I have no resistor either.

Everything still works the same. The question I have - for those familiar with this stuff - is what could I be doing wrong here? Right now I can't see what could go wrong, but I'm sure the designers of the simple sample circuit put the resistor and capacitor in there for a reason. Do you reckon I'm OK working without them?

 

[Sceadwian]

They're for noise prevention, put that in a real circuit without the cap or resistor and you'll have problems. Why would you want to remove them? The size for those components is negligible. Trust the engineers that make the devices and always use the recommended components.

 

[Hippogriff]

Thanks for that. Noise prevention does sound like a good reason, but I'd like to see what the effect actually is as I build the thing into a circuit - it's only going to be on veroboard in the end anyway. I do like to ask questions about this stuff and try things out rather than just accept what's there. I did start with all the bits, but as I removed things and saw that it still worked in exactly the same manner, I then questioned why they were there... after all - it does say "recommended" as opposed to "mandatory".

 

[Sceadwian]

Try the chip without the recommended components in a micro controller or other circuit that has any kind of decent switching speed or if you're switching any kind of even light current, you'll see why they are there almost immediately, you'll get all sorts of false triggers. It may even tweak out in the presence of florescent lights.

Play if you'd like, but I would not consider those two particular components optional in any circuit where space/weight/cost wasn't at such a hyper critical point to manage, if that were even the case you wouldn't even used such a module there are far smaller ones.

 

[Hippogriff]

Space, weight or cost isn't the issue in anything I do, but numbers of components is - fewer components = less soldering, which I am not a great fan of. Anyway, that's good advice - I shall be connecting the TSOP up to a PIC (probably a 12F683, maybe a 16F676) soon and then I'll be trying to recognise the buttons pressed on the remote I want to use. I shall report back if I start to observe the kinds of things you describe... if so, back to the extra recommended components.

Just out of curiosity - as I'm only a learner - what smaller options are there for the TSOP stuff? I mean, to me, this is a pretty small thingamajig.

 

[Sceadwian]

I can't name any off the top of my head I'm afraid. I have one about 1/4 that size tucked away in a box somewhere though.

 

[Hippogriff]

Very quick update... I have the circuit running (on breadboard) and can now distinguish between all my remote control signals. I have now tried this both with and without the resistor and capacitor that is suggested (recommended?) in the TSOP datasheet and there appears to be not an iota of difference in how things work.

Therefore, I want to listen to the experts, but my own testing is telling me it has no effect. Should I only worry about this if my circuit is going to get much more complicated? If it's relatively simple, which it most certainly is, am I less prone to noise and false triggers and suchlike?

 

[carbonzit]

With all due respect to the regulars here, I share some of your misgivings about "extra components" and don't think you've been getting particularly good advice here.

Which is not to say that those replying to you have been wrong: just that they haven't done a very good job of explaining just why you ought to have that "extra" capacitor and resistor.

Looking at the datasheet for this device (Vishay), they say:

R1 + C1 recommended to suppress power supply disturbances.

Notice they say "recommended", not "required". Now what are those for? As your respondents have alluded to, but not said explicitly, the cazapitor is for power-supply decoupling: basically, isolating the IR sensor from any funny stuff on the power line. This is standard practice with most microelectronics, which, as you've found out empirically, isn't always required.

Now if this is part of a larger circuit, which it undoubtedly will be, then that circuit (PIC, f'rinstance) should include its own supply decoupling capacitors, which should probably be sufficient to protect this as well. And I'm a little surprised not to see a smaller cap, like 0.1 uF, commonly used for such purposes. If you look at most commercially-produced boards, there's usually at least one of these little guys next to any chip to protect it.

This reminds me of how it's not a good idea to just slavishly recite "you need [such=and-such] in your circuit" without a good explanation. I'm currently converting an old ATX computer power supply into a bench supply, and almost all of the how-to pages on the web tell me that I have to include a resistor across the 5V supply in order to load the supply for it to work properly. Turns out this is totally untrue, at least in the case of my particular unit (and some other pages do point out that such a load is not always required, depending on the supply).

I guess what I'm saying is that it would be nice that if people say "you need x, y and z", they would at least make an attempt to explain why. I also am somewhat of a "rebel" in this way, I guess, because I don't like to put things in there without knowing why, or if they really aren't needed. (Of course, it's probably not a good idea to go pulling stuff out willy-nilly without knowing why either ...)

 

[Sceadwian]

carbonzit, I have used modules of the same type in a circuit before that would produce nothing but complete garbage glitched output without the recommended resistor and capactor, I have seen several posts over the years with people having the SAME problem in digital circuits without the IR module being decoupled. If you just want to drop this circuit into a foreign environment you use the recommended components unless you KNOW why. What they do (reduce noise) was explained, the why however was not, it smoothes the power supply ripple which those types of modules are very sensitive to.

I hear you about the ATX supply, I've never had to use a load resistor before on any ATX supply I've used, but the reason is sound, stability at low to no load is not guaranteed and transients with heavy loads suddenly applied can be worse (meaning badly out of spec) if a load resistor is not used. Please note CAN be worse, not 'the world will end if you don't use this resistor'. You'd have to monitor the voltage using a decent scope when it's loaded/unloaded to see the stability issues that might be there, for the bulk majority of things that people use these types of bench supplies for the transients that might be occurring effect nothing, it's important to note for people that may be expecting this to work properly without the warning.

 

[ronv]

The difference is the manufacture has experience with millions of the sensors - you have some experience with a few. There is probably an engineer or two at the manufacture that could give you 10 or 20 applications where you must have the filter. Try this one. Fire up a 555 timer at the same frequency as the sensor. (no decoupling please. It works fine without it) See if the range is still as good.

 

[KeepItSimpleStupid]

The lab and the real world are different. Watch what happens when you put that IR sensor in a room with varying light levels, a Plasma or LCD TV and sunlight and you have compact CFL's in the room too. Some cable boxes won't work under these conditions: dark room, LCD TV because they were designed before CFLs and LCD TV's were popular.

What about you putting junk on the power line? Radio emissions from the processor clock?

Do you have all this stuff in your LAB?

 

[Nigel Goodwin]

Very quick update... I have the circuit running (on breadboard) and can now distinguish between all my remote control signals. I have now tried this both with and without the resistor and capacitor that is suggested (recommended?) in the TSOP datasheet and there appears to be not an iota of difference in how things work.

Therefore, I want to listen to the experts, but my own testing is telling me it has no effect. Should I only worry about this if my circuit is going to get much more complicated? If it's relatively simple, which it most certainly is, am I less prone to noise and false triggers and suchlike?

Add the compopnents, they are really pretty essential, and you shouldn't go around randomly removing parts if you don't know what you're doing. They are for stability, without them there's a very real chance that the circuit will be unstable, usually only under certain circumstances. With those components it will be reliable and work all the time, without them it may very well not - I've certainly seen circuits not work when people have foolishly left them out.

 

[Hippogriff]

Do you have all this stuff in your LAB?

I do have GU10 halogen lights and a plasma TV in my 'lab.', yeah... but that's only 'cos I don't have a lab., it's my dining room table. For what it is, it's being tested in near enough 'real world' situations, I was just reporting back that there's no difference with or without those components after I'd been told there may be a difference... it's worth remembering that I'm a just-getting-started hobbyist.

Oh, and my dining room has a window as well. ;-)

 

[Sceadwian]

How do you have it wired up? The parasitics in a breadboard might be enough to keep it stable, might be accidentally decoupled through some other part of the circuit as well. It's kind of a moot point if what you're doing is working, but considering the cost and size of the elements you want to remove simple leaving them in (since you already had them there earlier) would make a lot more sense. An ounce of prevention is worth a pound of cure.