Nuts and Volts - April 2014

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

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 them.

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)