Re: Ultrasonic serial communication transmitter.
From: Andrew Holme (andrew_at_nospam.com)
Date: 01/30/05
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Date: Sun, 30 Jan 2005 10:38:33 -0000
Rubicon wrote:
> Hello,
>
> I'm trying to make a 24kHz ultrasonic transmitter capable of sending
> AM serial data to a receiver over a min 30ft range. I have no scope or
> frequency counter (looking for a counter locally) and have managed to
> get it just-going on occasion over a couple of feet by using high
> tolerance components and using an ultrasonic tone controlled switch
> kitset as a tuning referance. I'd foolishly thought when I bought the
> kit "What are the chances of there being 24kHz lurking about?" Pretty
> good actually. Drop a tool on the garage floor, tap the workbench and
> away she'd go. Lucky I didn't connect it to the main garage door!
Ultrasonic transducers are usually resonant at 40 KHz and their efficiency
and sensitivity are greatly reduced at other frequencies.
> Transmitter.
> At present I have a 16F84A PIC sending serial out at T1200 (True
> logic) to the reset pin of a 555 timer set for 24kHz. Its output goes
> to a 4069 inverter that drives a transducer. The inverter is wired in
> parallel, 180 degrees shifted for maximum output. My PIC programmer is
> old and doesn't go beyond the 16F84A which is why I'm trying to do it
> this way.
>
> TX PIC----555 24kHz----INVERTER----TRANSDUCER
>
> If I connect the TX PIC serial output pin straight to the RX PIC
> serial input pin then the receiving LED lights up during the program
> cycle just fine. However if I connect the 555 output pin to the RX PIC
> then it lights up only occasionally. A sync problem I'm guessing
> caused by the 555 duty cycle? It was set to 55% and I changed it to
> 50% which made things worse. I'm unsure what to do here as I read that
> the duty cycle for a transducer should be less than 60% for it to
> operate properly and also that connecting a transducer straight to a
> 555 isn't a good idea. The 555 isn't stable at the resonant frequency
> as a reverse voltage causes the frequency to jump about.
>
> I've just seen a circuit where a modulated input goes to the Control
> Voltage pin of a 555 (astable mode), the output having a pullup
> resistor. Another circuit provides control over the duty cycle without
>
> changing the output pulse frequency.
> I can combine them and try it but is it the best way to go about this?
> Is there a better way considering the limitations of my PIC?
Trying to modulating the duty cycle is a bad idea - this will not be
transmitted faithfully over a narrow band link because the "information"
will be in harmonics. Modulating the frequency (FSK = Frequency Shift
Keying) sounds like a better idea. Keep the frequency deviation fairly
small though.
>
> Perhaps a tone controller or are they not fast enough?
> PIC----567----INVERTER----TRANSDUCER
The NE567, if that's what you're referring to, is a tone decoder. That
would be a good choice for demodulating FSK at the receiving end. The NE567
can detect a signal buried in noise, but you still might need a little
pre-amplification.
An oscilloscope is essential to develop something like this.
>
> Maybe going up to 40kHz with a crystal locked TX inverter as in the
> link below but how does one connect it to the PIC for serial AM? I
> have a 40Khz crystal I pulled from some old junk which I could use and
> the crystal eliminates tuning. I can't find any 24kHz crystals
> locally.
> http://info.hobbyengineering.com/specs/DIY-k49.pdf.
> PIC----CRYSTAL LOCKED INVERTER----TRANSDUCER
You would need a varicap diode to generate FSK from a crystal oscillator.
You can generate AM by making a crystal oscillator from logic gates and then
using a spare NAND or NOR to gate the carrier on and off with the
transmitted data.
- Next message: Anthony Fremont: "Re: Ultrasonic serial communication transmitter."
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