Infra Red reciever - resistor deleted?

[radiogareth]

I note that in the latest on-line manual, (p.113+114 Manual 2) the 33OR resistor that used to exist in the positive feed to the Sensor has been removed. Before I change my PCB, could Technical comment on any effect this may have - or not, I guess!! ??

Thanks....

 

[geoff07]

Take a look at the Vishay data sheet for the TSOP range of similar units. The resistor is there to 'improve robustness against electrical overstress' whatever that means. In practice it is optional, but I had one overheat for unexplained reasons when used without so I do use the resistors, 'just in case'.

 

[John West]

A lot of devices will operate over a wide range of voltages and currents, but will run cooler and work just as well, (and live longer,) with a lower voltage than the maximum allowed.

 

[Goeytex]

I think that the diagram in the Picaxe Manual is for general understanding and should not be taken as an absolute design specification. The actual design and component values should be selected based upon the manufacturer's datasheet of the actual device used, not by what is given the Picaxe Manual. The Picaxe does not know what brand and model of the actual device being used.

For example: The Picaxe manual shows using an external 4K7 pullup resistor. However the Vishay Datasheet for TSSOP receivers show these devices to have an internal pullup of 33K. The external pullup is not generally needed with these receivers. Adding the 4K7 resistor creates a parallel circuit where the pullup value becomes about 4.1K. This results in an output current of about 1.2ma. The Picaxe input only needs a few microamps. So the internal pullup is all that is needed UNLESS there is a specific reason that more current is necessary.

Attached is a link to the circuit recommended by Vishay for robustness and protection from voltage spikes. See Figure 2

http://www.vishay.com/docs/80067/appoverv.pdf

 

[Dippy]

"The actual design and component values should be selected based upon the manufacturer's datasheet of the actual device used, not by what is given the Picaxe Manual."
- I totally agree. A sentence along those lines should be in 36pt writing in the Manual as newbies will take things totally literally (cut'n'paste) - I'm sure we all have at some time.

 

[radiogareth]

Fair enough, it stays then The 4K7 can stay too, as its pads are needed in another role if I choose to use a potentiometer into the A/D in place of the IR signal (14M2). Although I tend to agree (refer to the data sheets) as 'promoters' of the PICAXE I think the circuits should be 'solid' rather than just a 'guide', with or without a warning. People respect the Picaxe brand image, and as I indicated, the older manual did indeed have a 330R in there.

Thanks for the inputs...

 

[Goeytex]

Although I tend to agree (refer to the data sheets) as 'promoters' of the PICAXE I think the circuits should be 'solid' rather than just a 'guide', with or without a warning. People respect the Picaxe brand image, and as I indicated, the older manual did indeed have a 330R in there.

In many cases there is no possible way that the folks at Rev Ed can provide more than guides or suggested circuits since they have no way of knowing what specific device will be adopted by the end user. I don't see in the Picaxe Manual where any specific device was specified for the IR receiver. With no specified device I think it is a mistake for the diagram to be considered more than an example.

However, if a specific device is indicated in the Picaxe Manual ... ie, Brand/Model/Part_Number, then the folks at Rev ED should be responsible to provide a solid diagram. Even then there may be conditions where the circuit might need to be modified regardless of the Datasheet or Picaxe Manual recommendations.

 

[g6ejd]

I would keep the R in, it can only be good for the circuit design.

 

[eclectic]

Just for info.,
http://www.techsupplies.co.uk/epages/Store.sf/en_GB/?ObjectPath=/Shops/Store.TechSupplies/Products/LED020



Datasheet page 4 circuit


http://www.picaxe.com/docs/led020.pdf



e

 

[Technical]

The circuits in the manuals are generally for guidance as suggested above.

Importantly we try to avoid confusion and keep things as simple as possible where possible for beginners, and that particular resistor was taken out quite simply because it did confuse quite a lot of people!
In most hobbyist type circuits it is not essential as it is not necessary for correct operation. But leaving it in does no harm at all and so check the datasheet for your particular part.