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PWM circuit control

Practical Motorsport Wiring - Club Level

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Hi All,

I'm looking for a simple circuit to dim a LED dash. It's for a project I'm helping a friend with. He is building a factory 5 cobra.

The new style dash they offer is a full LED backlight and clearly won't dim on a standard rheostat. The limitation I'm working to is that he wants to keep the dash looking original, so he will use the Lucas-style bakelite knobs.

The options are as I see it:

1, Use a potentiometer (modify it to take the Lucas knob); this clearly is not the most efficient circuit. But given its light use, it should be OK.

2, Use a 555 timer to build a PWM circuit.

The question I have is, can I control a 555 timer with a rheostat rather than a potentiometer? Using the original Lucas rheostat will mean I don't have to modify the dash and the overly brittle bakelite knob.

thanks, Stu

At first, I thought you could, but now I'm not sure it would work. I've browsed a couple of schematics and all of them used a pot.

A rheostat is just a variable resistor - the circuit will be exactly the same as say a coolant temp sensor, connect a pull-up to one side, ground to the other.

Thanks, Adam, I tried installing a pull-up, but I hit the same issues, so I decided to install a POT in the rheostat housing.

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