10/15 Update: 88 cents. That's just crazy. http://www.ebay.com/itm/New-Arduino-Ultrasonic-Module-HC-SR04-Distance-Sensor-Measuring-Transducer-/181849383810
08/14 UPDATE: Renaming the thread One-Buck Ultrasonic Sensor due to Price Drop: http://www.ebay.com/itm/New-Arduino-Ultrasonic-Module-HC-SR04-Distance-Sensor-Measuring-Transducer-HE-/400756229697
11/26/12 UPDATE: After a year, I finally changed the thread title from Four-Buck to Two-Buck: http://www.ebay.com/itm/New-Arduino-Ultrasonic-Module-HC-SR04-Distance-Sensor-Measuring-Transducer-Hot-/271040857767?pt=LH_DefaultDomain_0&hash=item3f1b4b46a7
I lied. $3.95 w/free shipping. Prices on these sensors have been plummeting recently and this is a new low.
I received several of these today, which I paid $4.50 for on October 29. They are pictured and described as SRF-04, which I have used before. 4 pin units. Not PING quality, but very nice. Main bad thing is that they take 2 I/O pins. One trigger, one echo. PING uses just one.
What I received today are newer 4-pin SRF-05 units. Never used these before, and the only info I have found in 10 minutes of searching is at
which implies that these can be used with just one pin, like a PING.
The pinouts on mine are labeled differently than either of these. Mine are labeled:
Vcc---Trig---Echo---OUT---GND so these are some variant or knockoff. I hooked one up to a BS2 tonight using 2-pin mode and it worked flawlessly. I'll request info from the seller about the OUT pin, might take a day to get a reply.
With luck, I'll find out that I can tie the OUT pin to ground and get to one pin mode.
Without any reply or luck, I may tie the OUT pin to ground and release the magic smoke.
So right now, I can't say if they function on one pin, or that the seller will continue to ship 05 units instead of 04's, but they are quite a bargain either way. I just ordered some more. I'm a gamblin' man.