Re: [mowbot] A Lurker Says, "Hello".

Dave Everett (Deverett nospam at
Tue, 24 Sep 1996 21:27:08 +1000

robin nospam at wrote:
> Thomas A Maier <tmaier+ nospam at> said:
> > ... The only problem is that
> > most 40 KHz ultrasonic transducers have such a long "ringtime" (~1msec)
> > that they are blind to anything in the first ~.5 ft (.5msec out and
> > .5msec back). I expect that we're going to be closer to the ground
> > than that.
> Ah, but it doesn't actually matter here where the edge of the pulse
> lies, just how strong the echo is. So this shouldn't be an issue.
> The Polaroids are no better at short-distance rangefinding, and would
> be terrible in this application because they emit a complicated chirp.

I just realised that this will be very easy. We aren't trying to measure
distance, just signal strength.

On the system I built I schmitted the return signal to feed an
I can just leave the 40khz signal running, integrate the pulses into a
voltage and read it through one of the AD pins. The minimum distance is
not relevant
if the sensors are fixed to a permanent height portion of the mowbot
inline with the axle) as we are only reading the strength of a
train of pulses.

Dave Everett.