Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!apple!vsi1!wyse!mips!mark
From: mark@mips.COM (Mark G. Johnson)
Newsgroups: sci.electronics
Subject: A whole lot of fun
Message-ID: <23631@obiwan.mips.COM>
Date: 19 Jul 89 16:52:28 GMT
Lines: 59

Here's a design problem that I had some with; hopefully you will too.
I enjoyed the tightwad-tradeoff aspects.

    0.  The overriding goal is to have the cheapest possible parts cost.
        Saving $0.01 is Ultimate Victory; cost table is below.
    1.  Given a transducer and a load to drive.  Design a controller
        circuit that varies the voltage applied to the load (approximately)
        linearly with the transducer "reading".
    2.  The transducer is a pressure sensitive resistor.  Its resistance is
        <= 5Kohms when there is too little pressure, and >10K if too much.
    3.  The circuit and the load operate from 12VDC (+/- 10%).
    4.  When there is too little pressure, apply the full 12V (99% of VCC)
        to the load.  Not (VCC-2*Vbe), VCC.
    5.  When there is too much pressure, apply only 6V to the load.
    6.  When the transducer reading is between 5K and 10K ohms, linearly
        control the voltage applied to the load as shown in the curve below.
    7.  The load is basically a 50 ohm resistor (240 mA at 12V).

   Voltage applied to load (% of 12V supply)
                 |
                 |
            >99% |----------+
                 |           \       (approximately linear curve but
                 |          . \       not required to be perfectly precise)
                 |          .  \
                 |          .   \
  !! NOTE:  50%  |          .    +--------------
                 |          .    .                   resistance of pressure-
                 o----------|----|---------------->  sensitive transducer
                           5K    10K ohms


Here are the prices of components; minimize the total parts cost:
   Resistors                      1 cent
   Capacitors <= 0.5uF            3 cents
   Capacitors > 0.5uF            15 cents
   Diodes                         2 cents
   Zener Diodes                   5 cents
   Transistors rated < 0.5 Watt   8 cents
   Transistors rated > 0.5 Watt  20 cents
   Power MOSFET,     > 0.5 Watt  45 cents
   Dual Opamp, 0.5 Watt          20 cents
   Quad Opamp, 0.5 Watt          20 cents
   Transducer (REQUIRED)        100 cents

(My "solution" costs 152 cents; I suspect it can be done for about 141
 cents by a sufficiently clever design).


Transistor and diode VBE is between 0.45 and 0.85 volts (mfg variability
plus temperature sensitivity).  Beta (Hfe) is 40-200 for the small
transistor, 10-80 for the big one.  Vcesat is 0.05 to 0.10 volts.
MOSFET VT is 1.5 to 3.0 volts, and RON is 0.05 to 0.15 ohms.  Resistors,
capacitors, and Zeners are 5% tolerance.  The circuit should not
oscillate or exhibit other instabilities.
-- 
 -- Mark Johnson	
 	MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086
	...!decwrl!mips!mark	(408) 991-0208