Need help on very basic stuff

Thank you, this is great stuff. I already bought one of those ir receivers at said store, but I will need a 555 now to get the emitter to pulse.

About input voltage... I noticed that my amp readings came out right, but I always had more voltage than I expected. Should I build this:

instead? Also, on the receiver side, I would like to use the power of the rs232 serial interface and send a signal to that very special pin.

This is exciting. Another one: Why does using 20kOhm before the detector work better than using about (9-5)/0.05=80?

Thanks for the help and the resources. I am currently checking out National Semiconductor.

There is a lot more to this when you go for distance. The beam should be modulated so that the detector circuit can easily see the difference between your beam and background radiation. This is most easily done with an LM555 integrated circuit operation in astable mode. A transmitter built around an analog oscillator will likely drift so the receiver needs to track the changes. You can do that with an LM567 but you should know that designing the circuit is not for a beginner. There needs to be more power for the transmitter and maybe a lens to focus the invisible beam at the receiver.

That being said, there are Ir receiver modules available, even at Rat Shack. You will need to build your 555 transmitter circuit to operate near 40kHz for the little detector to lock on. Don't forget the input voltage these units need, you will need to read on the use of the 78xx series of voltage regulators. The LM7805 is one very common device. Note its input voltage requirements and its ability to dissipate the heat it will generate when operating. Don't worry, they are rugged / abuse tolerant and easy to use.

Have fun and enjoy the hobby. Most of the info you will need to read is on the National Semiconductor (and many other) web site.

How do, Manoo. Since you're at Radio Shack already, wander over to the counter and ask about the Electronic Sensor Lab, Catalog #: 28-278 which costs $49.99. It's a good intro to sensors in general, and has several opto experiments, including the emitter-detector pair you're talking about. The best feature of the kit is the manual, written by Forrest M. Mims III. This kit is actually a pretty good intro to the subject of electronics in general for the complete newbie.

You're going to have a bit of trouble getting a sensor range of a couple of feet with the emitter-detector pair you mentioned. Your light intensity is not sufficient at that range to cause much change in the output of the detector. Hence Lord Garth's suggestion of modulating the light output of the emitter at 40KHz, and then setting up a detector to select that frequency and amplify it. Even then, it's going to be a bit of a tough go. It may help to use additional optics to focus the light output of the emitter on the detector a couple of feet away.

Either way, though, I would guess your goal is a little beyond your reach without quite a bit more technical knowledge. I'd suggest buying the Electronic Sensor Lab, and also getting a copy of Mr. Mims' book, "Getting Started in Electronics" (not available at Radio Shack, but you can get it from many libraries or amazon.com ). He frequently references Radio Shack part numbers, which can be extremely helpful, too.

With the level of electronics knowledge you have, it's actually amazing you got your emitter-detector pair to actually do something. An intelligent person like yourself shouldn't have much of a problem getting up to speed in electronics with a bit of help.

If you're in a hurry, though, some of the infrared/opto circuits on this page may be of some help:

Good luck Chris

Thank you, Chris and Rich. I noticed that even though I had some classes on the topic back in College, I didn't retain the slightest bit of practical knowledge about the topic. Therefore, you can imagine how thrilled I was when my visible LED would actually turn on and off when I put my finger through the beam.

Since I actually AM in a hurry, (and I like experimenting better than reading), please let me know what you all think about this circuit, generously published by David Cook:

If I can make my Voltage lower to +5V DC steady, (with an LM7805), can't I just build above circuit and voila? Or what major problems would I face?

And on the other end, I use the (276-640) 38kHz IR Receiver Module from Radio Shack, possibly in combination with another LM7805 for voltage regulation and maybe a transistor (as switch) for supplying the result to the rs232 pin? Or can I use the power of the rs232 directly? I remember having seen circuits on the topic, but I think there is some danger involved.

The electronics kit sounds like a good idea, but on the other hand, you know how many parts I can burn out for 50 bucks? :-)

Thank you all again for your help. I'll keep you posted about my progress, if that's ok... Manoo.

The circuit in the link looks like it would do the job for the emitter. There aren't any docs on the RS detector (usually they just have connections to Vcc and GND, with the third pin as output which may require a pullup resistor). There should be a basic hookup diagram on the back of the package. Line up your LED and detector, then start pulling the detector away until the signal just disappears, tweak the pot until the signal comes back on, then stretch it some more. Repeat until you can't get a signal. That should be the appropriate tuned frequency for the detector.

Good luck Chris

Depending on your intended use, you might find that you'll get better results using one a PIR (passive infrared) sensor

Most of the heavy lifting has already been done; all you need to do, typically, is provide it power and ground and a line to monitor the output.

Radio Shack has one, p/n 49-1327. However, where they were once famous for having great on-line documentation for their parts, they are now rather infamous for not having docs. Hard to tell what connector you need or what the output is like.

There are other choices. MPJA has one, that looks like it would be easy to use, for $10 at

Ok. I am still not sure I am doing it right, but it seems to work. I have a configuration where I use a 555 timer in astable mode, cranking its frequency to what I hope is somewhere between 38 and 40 kHz. On the other side, I have the Rat Shack IR receiver and a couple of transistors and resistors and when I wave my hand between the two, my LED on the receiver lights up.

I noticed, that it mainly picks up on a signal change, rather than the emitter itsself, that's why I liked someone's suggestion (forgot who it was) to use a second 555 timer to turn the emitter circuit on and off in a range of about 1 to 10 khZ.

Getting that LED to light up tho, tells me its time to move on to the next problem: How do I get that flash to translate to a signal to the serial interface on my computer? (rs232). The wiring should be real easy, but I can't find a simple circuit out there on the internet.

Does anyone have a good resource? Thank you Manuel

Chris wrote:

I actually did it! Thank you for your contributions all.

I built a sender emitting IR at good-know-what frequency (with a 555 astable) and a 38kHz receiver from radio-shack that responds to my hand moving at a certain minimal speed through the beam. The signal on the output goes into the receive-pin on the rs232, the handshake is looped-back and ground goes to exactly there.

When running a port diagnostics program, I receive a '0' when my hand breaks the beam.

I already downloaded a library for reading the COM-Port in c#. I might end up having to set an interrupt trap, but from here on out, I feel much more comfortable.

Thanks again Manuel