dedicated meter to the project. If you do this, remember
to include a switch to disconnect the meter (until you
want to read it). This will eliminate voltage from ruining
the meter (when it is set to the resistance scale) and also
improve accuracy by not having the meter input resistance
affect the box value. Both circuits are shown in this article.
Construction of the box is straightforward and wiring
is not critical (see the schematic). As you can see, I used
three types of switches to construct the project. That’s
because I couldn’t secure all single-pole single-throw
switches at the time I wanted to build the unit. So, be
flexible, and either use what you have on hand or order
Theory of Operation
Each pot is wired to a corresponding switch. The
switch either “shorts” the pot or allows its resistance to
add with the other pots (which are connected in series).
The resulting total resistance appears across the binding
posts. The precision is achieved by the measurement
process using the digital meter. One
should note that since the pots are
connected as rheostats (i.e., one side tied
to the wiper electrically), if they are
adjusted for minimum resistance it should
be zero. Therefore, the switches would
not be needed. However, as pots age, the
contact resistance can increase from zero
ohms. The switches further decrease this
resistance so it becomes negligible.
Using the Box
Using clip leads, connect to the
binding posts to place the box resistance
across your circuit.
; Inside wiring.
; Close-up of pot and switch wiring.
Description Part Number Supplier
1 meg Potentiometer 023-640 Parts Express
100K Pot 023-634 Parts Express
10K Pot 023-628 Parts Express
1K Pot 023-624 Parts Express
100 ohm Pot 023-604 Parts Express
SPST Switch 275-634 RadioShack
*DPDT Switch 275-626 RadioShack
Dual Binding Posts 274-718 RadioShack
7x5x3 Case 270-1807 RadioShack
Five Control Knobs 274-415 RadioShack
Misc. Hook-up Wire 278-1222 RadioShack
*Optional For dedicated meter (refer to the schematic)
Parts-Express.com • RadioShack.com
36 April 2014