AVR-Chat

I've seen this before - the 2 pin approach is to use the transmitter
in a "push-pull" manner.  set pin A to high and pin B to low, after
12.5 uS, pin A to low and pin B to high.  wait 12.5 uS, repeat for the
number of cycles you want in your pulse.  You could also just connect
one pin and the other to ground though you will get more power with 2
pins. 

Use a transistor if you want to drive the transmitter with a higher
voltage (more power, greater range). A mosfet would be a good choice.
However, the transmitter has a fair amount of capacitance - I just
checked one of mine and it has .002 uF.  You will need to provide a
way for the transmitter's cap to discharge (not necessary in the
directly connected case).  A reasonably small resistor across the
xmitter's leads will do it.  You'll have to do the calculations for
your specific device.  iirc, you should pick a value of R so that 5RC
is much smaller than 12.5 uS (one half the period of your xmitter's
frequency) yet large enough to allow the transmitter's cap to actually
charge.  For the device I have, 5RC = 12.5 uS yields 1.25K.  I think I
used 470 ohms because it was laying around.  There is a circuit cellar
article about ultrasonic homing from about 2 years ago that discussed
these issues. check their site.

Phil

--- In AVR-Chat@yahoogroups.com, "Daniel Boyer" <dpboyer@d...> wrote:
> 
> I'm working off of a schematic that I found on the web which is an
> ultrasonic rangefinder utilizing a PIC... The transmitter section has
> the PIC driving an Ultrasonic Transmiter at 40khz... The schematic shows
> one side of the transmiter connected to one pin on the PIC and the other
> side connected to another pin on the PIC.... I am doing this with a
> Tiny15; shouldn't one side of the transmitter be connected to ground and
> the other to the avr?  Do I need to use a resister or will the
> transmitter draw a low enough current?  Should I use a transistor to
> drive the transmitter?
> Thanks,
> Daniel