Computing stuff tied to the physical world

Almost, but not quite

In Hardware on Nov 9, 2010 at 00:01

There’s a new Infrared Plug in the works – it’s almost ready. The IR transmitter uses the venerable 555 chip:

Screen Shot 2010 11 08 at 20.36.42

I used the formulas from the datasheet to calculate the different resistor and capacitor values to arrive at 38 KHz, which is the most common IR frequency (and which most 36 and 40 KHz systems will pick up as well).

To check things out, I hooked the plug up for constant oscillation (DIO to +3V):

Dsc 2235

Despite using 1% parts for all the timing critical components, the output isn’t quite what it should be:

Screen Shot 2010 11 08 at 20.33.12

That’s 35.5 KHz … i.e. almost 10% off target!

(note that I measured the signal across the transistor – the IR current is actually the opposite, i.e. fast rise time)

I’ll have to tweak one of the resistors empirically (16.0 kΩ i.s.o 17.4 kΩ seems to fix it) and order some new SMD values. And then I need to check that the frequency across multiple plug builds will remain within bounds.

Not quite back to the drawing board (the PCB seems to be ok), but still: back to the soldering iron!

  1. You’re not falling fowl to the tolerances in the resistor and capacitors are you?

    • Oh hang on… 1%… Didn’t see that line hidden between the pictures. Move along, nothing to see here!

  2. Yey, nice with a IR plug! With out IR-in How will I know what to send?

    Keep up the great work! // Jesper, Sweden

  3. Why not use a trimmpot instead? Analog circuits usually needs individual calibration unless there is a feed back loop or some other kind of “simple” auto calibration.

    This is the reason many (faint hearted (-; ) people use “something digital” (yes it is analog to at the transistor level but it is “invisible” (-; ). I guess you have looked at NCO:s and rolled them out due to cost?

  4. May I suggest you replace the ‘555 and a bunch of the discrete components with a small avr chip, perhaps the ATtiny4 in SOT-23?

    • It’d be great, but so far those chips aren’t available AFAIK. In this circuit I’m also a little worried that the internal RC clock is not accurate enough – but that’s just guessing for now.

  5. I know the internal RC on the PICs are good to about 1%, due to factory calibration. Maybe the Atmel parts do that also? I recently replaced some logic and a one-shot and discrete R,C parts with PIC 12C508 in a design, and the result was better all around. Smaller, cheaper, and much more accurate and repeatable.

  6. accuracy isn’t much of an issue with these IR transmitters. I’ve had good luck with internal osc for picmicros. As for smallish AVR, I just took a wild guess. Seems like you’ve got one you are using on a different plug, maybe an analog input?

Comments are closed.