PROJECT                          
RECYCLED  E-THERMOMETER
CONVERTED  INTO   A-V-Ω  - MULTIMETER


... It starts with this dumped ( analog ) ELECTRICAL THERMOMETER with lost sensor.
Because the high-sensitive SIEMENS 20 μA INSTRUMENT is OK, an  I D E A  came up ...

Make  a   P R O J E C T   WITH KIDS  -  Build a  » A-V-Ω  MULTI-METER «  out of it!
===================================================================================

(A)  P L A N N I N G :
**********************
Use the existing switch for 3 Ranges ...

   ( _ )  OFF
   (1st)  12 Volt
   (2nd)  600 Milli-Ampere
   (3rd)  Ohm ( with 1.5 V BATTERY in box on back side )

On the empty top a  T E S T  board with  PLUGS  &  POTI  will be installed ...

          

How the 20 μA instrument looks INSIDE ...

          

The current in the little spool generates a magnetic field, what interact with the
magnetic field of the permanent magnet to a torque of the axle. The 2 spirals are
both contacts of the spool and provide the counter-moment. The torque is readable
via arrow on the scale and proportional to the value of the current in the spool.

R E M A R K S :
===============
In the  F O R M S  the  LETTERS  U, I, R  refer to  Voltage, Current & Resistance.
Their  U N I T S  are  VOLT (= V ), AMPERE (= A )  &  OHM (= Ω ). 

(1st) The VOLT Range:
---------------------
Common used batteries are 1.5 V types, as »AAA«, »AA«  &  »MONO« Cells,
4.5 V  &  9 V - Blocks  and  "combined" 12 V - Blocks  or  CAR BATTERIES ...

          

With "OHM's LAW" we calculate the (Serial-)RESISTOR (= R ) for a  12 V  MAX-Range ...

   U = R * I   ==>   R = U / I  =  12 V / 20 μA  =  600 kΩ

Refer to Fig. 1 in the following sketch ...

          

(2nd) The AMPERE Range:
-----------------------
Common ranges for charging  NiCd  or  NiMH  - Cells  are 50 mA .. 600 mA. To calculate
is the (Parallel-)Resistor (= "SHUNT") for a 600 mA Range of our 20 μA instrument. We
have measured:  20 mV = Ri * 20 μA   ==>   Ri = 1kΩ ...

   U  =  Ri * Ii = Rs * Is  ==>  Rs = Ri * Ii / Is = 1 kΩ * 20 μA / 600 mA = 0.33 Ω

This value is too small! It is not realistic to have a smaller shunt than 1.5 Ω (= R1 ).
Therefore:  We have to make a  "Special Arrangement"  of resistors ( Ref. to Fig 2 ):

Nearly 600 mA go through the shunt R1,  20 μA  have to go through the instrument!
A (Serial-)Resistor  (= R2 )  is neccessary, much bigger than the shunt ...

   U  =  R1 * I1 = R2 * I2  ==>  R2 = R1 * I1 / I2 = 1.5 Ω * 600 mA / 20 μA = 45 kΩ

(3rd) The OHM Range:
--------------------
We use the same configuration, add the 1.5 V BATTERY, the TEST-OBJECT-Resistor, and
for  0 Ω  (= "ShortCut" )  a  "ZERO-ADJUST"-Potentiometer  of  100 Ω.  To limit the
"Short-Cut-Current" in  0 Ω  Adjustment, an additional 40 Ω resistor is placed serial
to the 100 Ω poti.

We calculate the value of the (Serial-)Resistor (= R3 ) to place  "100 Ω"  in the
middle of the scale! ( Ref. to Fig 3 ):

   I = U / R  =  1.5 V / ( 100 + 100 ) Ω  =  ~ 8 mA

   R3  =  1.5 Ω * ( ~ 8 mA / 10 μA )  =   ~ 1.2 kΩ

(B)  R E A L I S A T I O N :
****************************
These theoretical results have to be CONFIRMED and MODIFIED by EXPERIMENTS:

(1st) The VOLT Range:
---------------------
To reduce swing into its final state, a 20 kΩ resistor ( small compared to R !)
is placed parallel to the instrument.  R = 570 kΩ was found as the final value.
                                      ============
(2nd) The AMPERE Range:
-----------------------
As the final value was found  R2 = 46 kΩ.
                             ============
(3rd) The OHM Range:
--------------------
  R3 = 0 Ω  was found to be optimal.
 ==========

For safety usage, the  V, A & Ω - RANGES  get separate COLORED PLUGS ...

T H E   W I R I N G   D I A G R A M :
-------------------------------------
Because READABILITY the SCALE-Sequence is  A-V-Ω  - So the Switch-Sequence too - But
the PLUG-Sequence is choosen  0-Ω-A-V  because measuring VOLT will be most common ...

          

... The "OFF" Position is used to "Short-Cut" the instrument for transportation.
Moving the instrument generates "Counter-Inductivity" what has a damping effect! 

R E C Y C L I N G   O F   T H E   E X I S T I N G   P R I N T B O A R D   ...
-----------------------------------------------------------------------------
               TOP SIDE                                  BACK SIDE  

  

... This is another challenge !!!

DRAWING THE  S C A L E S :
--------------------------
Now the SCALES have to be drawn on white cardboard.  The  V & A  Ranges are linear
to divide,  BUT the  Ω-Scale  has to be found by experiment ...

        
This is the final wiring. Bottom (right) shows the battery box with contacts ... ... for the 1.5 Volt Battery, a 1/2 of a 3 Volt, eg. VARTA DUPLEX 3010 See in "FAQs" a picture of this BATTERY TYPE !!! impressum: *********************************************************************************** © C.HAMANN http://public.beuth-hochschule.de/~hamann 07/19/12