Fuel Tank Switch

Introduction

The Simkits fuel tank switch is an expensive option, when you consider it is no more than a three position switch and a knob, the estimated cost being £200 (including tax and shipping).

 

 

The actual switch has three positions, left, both and right, while the angle between each position is 90.

The switch provides two output signals the logic is 01, 10, 00 (left, both, right).

 

 

A simple single pole rotary switch is not capable of generating this output sequence. It would appear that Simkits have used something like the T9000 series switch from Arcoelectric, the logic for which can be derived from this diagram:

 

GND connected to (3)

Sig1 connected to (2)

Sig2 connected to (1)

 

Switch Position 1 Sig1 GND, Sig2 N/C (01)

Switch Position 2 Sig1 N/C, Sig2 GND (10)

Switch Position 3 Sig1 GND, Sig2 GND (00)

Switch Position 4 Not used

 

No UK distributor for this switch could be located.

Solution

The solution was to use a single pole, 3 position, 90 indexing switch and a MicroChip 12F508 microcontroller. The total cost of this implementation was £2, excluding the knob.

The switch was connected to inputs GP0 and GP1 such that in the left position both GP0 and GP1 were open-circuit and because these inputs have pull-up resistors, they would be both at logic 1. In the both position GP0 was pulled low and in the right position, GP1 was pulled low. Hence the logic output from the switch was 11, 01, 10 (left, both, right).

The software for the 12F508 was trivial, a simple lookup table to convert the physical switch inputs to the response expected by the Simkits interface and this was transferred to GP4 and GP5, configured as outputs.