Table of Contents

Single Pole Single Throw (SPST)

SPST rocker switches are commonly found in the automobile industry as a standard switch for various dashboard functionality such as windshield wipers. Usually, SPST switches are accompanied by an LED light to indicate when the switch is on.

There are many YouTube videos that demonstrate the usage of SPST rocker switches, however, there are very few indications on how these switches are wired internally. For the hobbyist, three terminals instead of a typical passthrough (2 terminals) can be rather confusing.

Circuit

The circuit consists in a switch between the positive terminal (+) and the LED terminal (-) - although the later is counter-intuitively indicated with a LED symbol rather than using the (-) symbol. 

                                Ved=5V
                                    
            +               //      
             \     R=?     .-.      
        +     +---+\\\+--+(->|)+    
        |     |            `-' |    
        |     |                |    
        |     |                |    
        +    ---              ---   
Vin=12V       -                -

When the switch is toggled, the current flows from the $V_{in}$ terminal (marked with the + symbol) to the terminal marked with the LED symbol. This is sufficient to use the SPST just as a common switch. However, to make the LED glow, the additional ground terminal (marked with the ground symbol on the switch) must be grounded as well. Only then will the switch emit light as well as working as an interruptor.

LEDs are commonly rated at $V_{ed}=3.7V$ and draw about $I_{ed}=0.02A$ ($20mA$) but this may vary between the LED type. Since automobile SPST switches usually draw power from a car battery, the input voltage is rated at $V_{in}=12V$ such that the voltage must be lowered to $V_{ed}=3.7V$ using the resistor marked in the schematic with R.

Using Ohms law, it follows that:

\begin{eqnarray*} U&=&R*I \implies \\ R&=&\frac{U}{I} \end{eqnarray*}

Since the circuit is a serial circuit, then the sum of individual voltage drops must be equal to the voltage applied to the circuit, such that:

\begin{eqnarray*} R&=&\frac{U}{I} \implies \\ R&=&\frac{V_{in} - V_{ed}}{I_{ed}} \end{eqnarray*}

It is known that:

such that all values can be substituted to find out the rating of resistor R. For the ratings enumerated above, the calculations would reduce to:

\begin{eqnarray*} R&=&\frac{V_{in} - V_{ed}}{I_{ed}} \implies \\ R&=&\frac{12V - 3.4V}{0.02A} \implies \\ R&=&430\Omega \end{eqnarray*}

Practical Applications

The following is a modification of an ultrasound emitter by adding an SPST switch. This was performed by drilling through the plastic and then mounting the switch on the device and then wiring the device according to the schematic presented above.

hardware_switches_single_pole_single_throw_assembly_switch.png
hardware_switches_single_pole_single_throw_assembly.png