frequency detection without oscilloscope

[Glass-Giant]

I'm building an IR transmitter and receiver.  I'm a newbie, working from untested plans, and am still making quite a few mistakes.

The transmitter is based on a 555 in astable mode.  I would like to make sure it works and is at least in the same order of magnitude as the desired frequency before making the receiver, but I don't have an oscilloscope.  The transmitter works in the 30-60Khz range.  So I can't verify it visually with a visible led substituted for the ir led. 

I was wondering if there would be a way to connect it to a speaker to generate a tone.  As I understand it, the upper range of human hearing is about 20Khz, so I was wondering if there would be a way to only allow every 2nd or 3rd or 4th pulse through.  I could then compare the tone to a known tone (assuming I can find a wav of the specific tone).

Or maybe there's an easier way?

 

[Kevin Weddle]

An IR transceiver is meant to communicate by means of high and low pulses. I think it is for two reasons. One may be that the data is already digitized. Another may be that, depending on your requirements, the transducing effects don't give you a pristine signal.

Could somebody explain here how a CCD works. It is not high on my list, but interesting nonetheless.

If I were to guess, each CCD pixel would have three wavelength sensitive areas. One for red, green, and blue. The color to be displayed would reflect light of these three colors in varying intensity. Anybody know for sure, just let us know.

 

[Codyhtml]

hey use a pc based scope there are some in the downloads section

 

[audioguru2]

My portable Digital Multimeter has a very accurate frequency counter built-in. ;D

 

[Glass-Giant]

If the pulses are square waves, you can feed it through a decade or ripple counter to reduce the frequency.

The signal is from a 555, so I think that means it's square-wave.  A good suggestion.  I'll keep it in mind.

hey use a pc based scope there are some in the downloads section

From what I understand, the highest frequency which a sound card scope can identify is based on it's sampling rate.  I've got 3 computers - 1 good laptop, 1 crappy PC and an old junked 233.  I wouldn't risk the laptop on something I made myself (the probe).  The crappy PC is, unfortunately, the main computer I use (I hate working on laptops), so ditto.  I'm guessing the old 233's soundcard wouldn't go up to the 30-40KHz range, as CD-quality sound is 44Khz.  I don't really know.  But I guess I could use a sound card scope with the decade counter to give me much better results than by ear...  It's kind of funny - this little project is going to lead to a bunch of other little projects (555 tester (done), bench power supply, parallel port pin status indicator (half done), sound card oscilloscope probe).

 

[Glass-Giant]

My portable Digital Multimeter has a very accurate frequency counter built-in. ;D

Um, the smiley seems to indicate a joke or a gloat.  At the risk of sounding stupid, does it really have one like I would expect (a readout that says XXXKHz) or are you indicating that all you need to determine frequency is to know the resistance/capacitance values? 

Please explain to the dumb newb (using small words, if possible). 

 

[Codyhtml]

your not a dumb newb your actually quite smart especialy for starting out (besides your passing interrest years ago) but it probably seems like your dumb but i used to think the same about me a while ago but then i started learning much more any way i'm not quite sure what audioguru ment if he was serious or joking mabe he clicked the wrong smiley?
o and what i did t test a pc based progect was tested it on an old computer then after it didn't blow out my old computer i had the guts to try it on my good computer mabe try that (ps mabe you need to get a new desktop  )

 

[Glass-Giant]

o and what i did t test a pc based progect was tested it on an old computer then after it didn't blow out my old computer i had the guts to try it on my good computer mabe try that (ps mabe you need to get a new desktop  )

That's a pretty good idea.  I'll see if I can dig up the old set of instructions for a probe... 

It's so frustrating not knowing if my transmitter or receiver isn't working.  I just calculated the frequency using the ACTUAL values of the resistors and capacitor, as measured by the DMM.  The cap was pretty far off (0.65, instead of the 1nF I though it was).  I think I read the code wrong.  Anyway, the calculated frequency came out to be anywhere from 72Hz to 41KHz (with ~50% duty cycle), depending on the value of the pot (0-11.4KHz).  I'm using a TSOP 1738 (38KHz) for testing the receiver.  I'm assuming the 555 is reliable enough not to drift 3KHz.  When I substitute a 1uF cap for the .65nF I can actually see the visible LED changing frequency as I turn the pot.  It just dawned on me to use my digital camera to make sure the IR led is working.  Back to the basement...

I'm desperate for a new desktop, but can't justify the expense yet. :'(

 

[Glass-Giant]

And I'm an idiot.  I knew that 72Hz - 41KHz range sounded wrong.  I calculated again and it's actually 41KHz to 72KHz.  Duh.  So I added another resistor and a value of 9.18 Kohm should give me 38.1Khz.  I think that should be plenty close enough to the ideal frequency.  But I tried it and the receiver still isn't receiving.  I'll likely still try the scope/decade counter thing to make sure the transmitter is okay.  The digital camera indicated the led is firing.

Thanks for the input everyone,
Shawn

 

[Codyhtml]

about your desktop if you build one your self it is cheaper i'm sure with a little tutorial and some help from this forum you could build one (i built mine)

 

[Codyhtml]

o don't use ceramic caps they won't give you good timing and use somthing like a myler they give goood timing

 

[Glass-Giant]

If I do find some money for a desktop, I've got a friend who builds them and gets his parts at wholesaler prices.  So I'm good there.

I'll have to see what I can get then for caps.  I've been using ceramic.  :-[ :-[

 

[audioguru2]

Hi Glass-Giant,
I used a smiley because I'm glad to have an accurate frequency counter in my portable Digital Multimeter. I thought everyone knows to use a frequency counter for measuring frequencies. ;D ;D

Hi Cody,
Your TSOP1738 is designed for data, not for continuously modulated IR. It has Automatic-Gain-Control which reduces its gain when it receives modulated pulses longer than 70 pulses to reduce or eliminate interference from new compact florescent bulbs that are also modulated at about 38kHz.
You can avoid the AGC reducing the gain by modulating the IR in bursts of about 20 cycles on then 20 cycles off of the 38kHz. Then use a simple peak detector at the output of the receiver to blend the bursts into a continuous signal. A kit uses a peak detector. 

View attachment 37293

View attachment 37294

 

[Codyhtml]

o ok thanks audioguru it looks like i will be updating the schematic i gave glass giant with a 555 that pulses the other 555 right and then a peak detector right  ok i'll do that soon

 

[Glass-Giant]

I just realized I forgot to respond to the last 2 posts.  Thanks, audioguru, for the clarification.  That helps with my confidence in the transmitter (I wasn't sure if the transmitter or the receiver was the problem).

And thanks for all your help codyhtml.  If you don't get around to it, I think I can figure it out with your schematic and audioguru's picture.