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DPRG: Radio Shak Tape Measure Hacking

Subject: DPRG: Radio Shak Tape Measure Hacking
From: Jim Brown jgbrown at spd.dsccc.com
Date: Mon Jan 19 15:44:55 CST 1998

I got this off the comp.robotics.misc newgroup.  I figured that maybe someone
out there that might be interested in this:

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	 Newsgroups: comp.robotics
         From: seeker at indirect.com (Stan Eker)
         Subject: RSTM (ultrasonic ranger) min range modification
         Date: Fri, 26 May 1995 07:52:27 GMT

          Here's a small improvement in the Radio Shack Electronic Tape Measure
         (RSTM) minimum distance.  Stock, it'll read down to ~1.7 feet as long as
         you're using the TEST mode (continuous bursts, one per second).  In it's
         normal (useless) mode, it'll only *ever* read down to 2 feet.  With the
         following two capacitor changes, you can get down to 1.0 or 1.1 feet.

          Simple modification (no brainer): change C8 to 6.8nF (6800pF) Mylar cap,
         and change C11 to a 2nF (2000pF) 5% Mylar cap.  This minimizes the time
         that the input amplifier stays in saturation (C8) and reduces the time
         that the Schmitt trigger (C11) holds off switching.  There's still a fairly
         major (about 400uS) time it could be reduced further, for a gain of another
         5 inches or so, but that'll have to wait for further study (fooling around).

                     shown from                sensor
                bottom side of board            wires
                                             |        |                
                | S     555     ------- CA3160                 |
                | W   -------   |     |  amp     tank          |
                | I   |     |   |  U2 |         circuit        |
                | T   |  U3 |   -------       -----            |
                | C   -------     ||          |   | XFMR       |
                | H     =====     || C8       -----            |
                |        C11                                   |
                |                                              |
                |  ------------------------------------------  |
                |  |       LCD DISPLAY  (other side)        |  |

          The changes above just shorten the time the input stage is idle after the
         initial burst.  As an added benefit, it seems to have also killed the odd
         results I'd had with returns at less than 1 foot.  Before, it'd often show
         a 30 foot (or more) value, now it doesn't.  I'd recommend using the same
         type of Mylar caps as the originals to keep the temperature coefficient
         the same, but you can experiment with other types if it's for indoor use
         only (limited temperature range).  The changes are easy to get parts for
         and duplicate, and don't appear to screw up the calibration.

          The remaining 400uS or so of wasted time (and distance) is mostly due to
         ringing in the tank circuit that drives the transducer.  From the research
         I've done it appears to be purely a function of the high Q of the L-C tank,
         and not because of any ringing in the electrostatic transducer.  However,
         every passive method I've tried to reduce the Q (and ringing) has failed
         miserably.  When I get my hands on some high voltage (450 to 500V) MOSFETs
         I may be able to damp it out actively, but I haven't found any in the usual
         hobby channels, yet.  Attempts to reduce the initial ping from it's nominal
         500uS value cause an ERROR display, so the micro is obviously watching it.
         I'll post the final results when I get the time and MOSFETs...

          If active damping works, I expect that C8 will end up at around 3nF, and
         C11 will need to be reduced to around 1.2nF, in addition to the circuitry
         required to maximally damp the tank circuit right after the ping.  It won't
         help to drop them to this value WITHOUT the damping, and will only cause
         problems until the final hardware is in place.  If the total requirements
         for the added circuitry cost over $5-8, it's useless, as you can get the
         Polaroid unit (that works down to the 6-8 inch range) for only $35 or so.

          In case you missed the earlier post, the TEST mode is entered by shorting
         a split circular pad on the top (display UP) side of the board, and it's
         in an area roughly centered between the legs for R33 and R34.  A legend on
         the bottom side shows "R31-34", and R33/R34 are the bottom two resistors.
         This will cause the RSTM to ping every second (without added controls), and
         you only need a single pin to monitor the distance.  When U3 pin 3 goes high,
         a measurement cycle has started.  When it goes low again, the echo has been
         received.  Since sound travels around 1100 feet per second you don't have
         to be overly accurate in sampling the pin, although doing it with an
         interrupt service would induce less error.  20-30uS would be unnoticeable
         for most hobby robotics uses, as it's less than a half inch error.  Heck,
         the circuitry in the RSTM can cause *that* much error range.

          Warning to newbie hardware hackers:  make sure to remove the batteries
         before attempting any modifications to the RSTM.  It uses around a 300-400V
         pulse burst to ping the transducer, and will sting if you get hit by it.
         It probably won't kill you unless you're wearing a pacemaker, but....

          And, no guarantees, YMMV, LTBB, other legal mumbo-jumbo.  All warranties
         are hereafter null and void.  Don't take any wooden nickels.
          |  Stan Eker            |                                                  |
          |  seeker at indirect.com  |      (this mind intentionally left blank)        |
          +-----------------------+                                                  |

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My employer won't claim these opinions so I'm giving them away for free.


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