How to use the same switch to control LED and vary resistance?

Question

I am struck with a simple board design problem.

The requirements are : I have a 4 position slide switch according to the position of the slide switch, I should turn on different colored LEDs and also vary the output frequency of 555 timer in astable mode and drive a piezo buzzer.

I plan to vary the output frequency by varying the resistor values connected to 555 timer.

Problem : I cant use the same switch to control LEDs and vary the resistor values. How to achieve this that is changing the switch position should select different LEDs and also vary the resistance, thus varying the frequency of 555 timer.

Answer 1

You can simply use your 4 position switch to generate a control signal that then switches control elements (like MOSFETs). You need to change your LED drives to this (note I only show one LED and One switch)

^{simulate this circuit – Schematic created using CircuitLab}

Something similar can be done with the 555 as well,

^{simulate this circuit}

Of course signal ctrl2, 3 etc. is not shown.

the main concern with this approach is two fold.

1)you must ensure that the gate swing is large enough that the transistor is remains fully conductive over the whole operational range.

2) if you put too large of a MOSFET in there the stray capacitance on the resistor chain will change the operating conditions.

Answer 2

An alternative to doing it with logic is to use a Double Pole, 4 throw (DP4T) switch. A double pole switch is actually two switches that are coupled to the lever, or knob and they move simultaneously. Each pole can switch a different circuit. This one made by Grayhill. It's a rotary switch, not a slider but it has the same function.

OK, searching a bit further found a much cheaper DP4T slider here. Cost about $3 in single pieces.