🚀 go-pugleaf

Recetly I posted a design problem which was claimed to be "a whole
lot of fun" to solve, mostly because it required a minimum-parts-cost
design.  (And thus rewarded "clever" circuits).

The problem was to vary the voltage applied to a 50 ohm load depending
on the value of a variable-resistance sensor, with this xfer function:

   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


The two most interesting replies used very different approaches.
One circuit employed two discrete transistors:

	X1   VCC 1       /* (Transducer: 5K - 10K)	$1.00  */
	R1   1   GND     11K			/*	$0.01  */
	R2   1   2       30K			/*	$0.01  */
	Q1   VCC 2   3   NPN-small		/*	$0.08  */
	DZ1  4   3       4.7V   /* Zener diode		$0.05  */
	R3   4   GND     4.7K			/*	$0.01  */
	Q2   5   4   GND NPN-power		/*	$0.20  */
	R4   2   5       150K			/*	$0.01  */
	D2   5   3       diode  		/*	$0.02  */
	RX   VCC 5       50    /* load */
----------------------------------------------------------------------------
        TOTAL COST					$1.39


The second approach used an opamp.  Although one way to account for
this is to charge 1/4 the cost of a quad opamp pkg, I decided to count
it as 1x the full cost of a dual opamp pkg.  Computing parts cost the
other way, the price would be $1.39 (same as the previous design).

	R1   VCC 1       100K			/*	$0.01  */
	R2   GND 1       5K			/*	$0.01  */
	R3   1   2       33K			/*	$0.01  */
	R4   2   3       1.5M			/*	$0.01  */
	A1   3   0   5   2
	   /* OPAMP with   out = 3, in+ = 5, in- = 2	$0.20  */
	R5   3   4       250			/*	$0.01  */
	Q1   6   4   VCC PNP-power		/*	$0.20  */
	R6   6   5       100K			/*	$0.01  */
	X1   5   GND     /* (Transducer: 5K - 10K)	$1.00  */
	DZ1  6   4       4.7V   /* Zener diode		$0.05  */
	C1   6   4       0.1U			/*	$0.03  */
----------------------------------------------------------------------------
        TOTAL COST					$1.54


The discrete circuit is rather non-precision, as it does not compensate
for the temperature-sensitive VBE's or the finite base currents.  However
it certainly *is* inexpensive, mostly because it doesn't need any
capacitors for stability.

The opamp circuit *does* need frequency-compensation elements (R3,R4,C1)
which adds $0.05 to the cost.  However, this topology is much more
likely to be linear than the previous circuit.  Note that the opamp
must handle input signals very near GND, and put out an output signal
near VCC.

Interestingly, both circuits used the same "trick" to get the breakpoint
at (10K, 50%-of-VCC).  A zener diode is placed across the collector-base
junction of the power transistor, to limit the collector-emitter voltage
drop at 6 volts or less.  (DZ1 and D2 in the first ckt, DZ1 in the second).

Special thanks to Charles Flaig of Apple and TomB of HP for the two
most interesting (and lowest cost) solutions.
-- 
 -- Mark Johnson	
 	MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086
	...!decwrl!mips!mark	(408) 991-0208