132 Chapter 4
of terminals the output can be connected to. In the SPST switch there is just one. What if
we had two terminals to which the output could be connected? Because there are now two
throws, this kind of switch is called a single pole, double throw switch. In this switch there
are actually three terminals arranged in a row. The input attaches to the center terminal,
and the other two terminals go to the two outputs. The SPST switch, as we’ve seen, is the
type used to switch an appliance on and off. The SPDT can be used to switch either one of
two lights on. This kind of switch is not too useful in real life, as there is a chance you may
want both lights off. But it illustrates the point. Incidentally, there is a less common type
of enhanced version of the SPDT switch with a center off position. The toggle is biased
mechanically so it can be positioned in between the two extreme positions. That switch
will turn off either light (in our example). In the aforementioned example we have had the
switch connected just in the positive supply line (where it is usually connected). The other
terminal, that is, the negative terminal, if we were considering, say, a battery being hooked
up to a light, would be permanently connected into the circuit. In situations where both
sides of the battery need to be switched, we use a switch that is essentially a dual version
of the SPST switch. This switch has two sets of terminals, each set identical to the other in
function. As you might have guessed, this is a double pole, single pole , or DPST switch,
where a pair of inputs can be switched to a pair of outputs. This switch type is useful
because it makes possible more than just the basic on/off function. An even more versatile
switch is thedouble pole, double throw , or DPDT switch, where two separate inputs can
be switched to two separate pairs of outputs.
Table 4.1 illustrates the use of the different switch types. Figures 4.9 and 4.10 depict the
switch types very clearly.
The dotted line for the DPST and DPDT switches indicates that these switches have
ganged contacts, that is, they are switched together with each mechanical toggle. For a
seemingly simple mechanical device, there’s certainly more to the humble switch than you
Table 4.1 : Uses of Different Switch Types
Switch type Purpose
SPST Used to switch a single monoamplifi er speaker on or off
SPDT Used to switch a monoamplifi er between two speakers
DPST Used to switch a single pair of stereo amplifi er speakers on or off
DPDT Used to switch a stereo amplifi er between two pairs of stereo speakers