Dimming LEDs in a dashboard using factory controls

[CasualTinkerer]

I recently installed leds in the dashboard of my car and while they dim, they don't dim much before they simply drop out. The factory dimmer works by varying the output voltage to the bulbs from 9-12vdc, which explains why the bulbs drop out without significantly dimming. So, I'd like to build a circuit to go inline with the factory dimmer to provide significant dimming to the leds.

It seems the most common method of dimming a led is to use pwm, so I googled for pwm circuits. There's certainly a bunch of circuit designs out there, but they seem to be controlled by varying resistance from a pot, and not from a variable voltage source. I need to control the pwm using my 9-12vdc voltage source. I do have access to 14vdc power from the fuse panel that I can use if needed.

I'd like to be able to vary the duty cycle as much as possible, but an old scratchy pot that only has a 3 volt range isn't going to give a lot of fine-grain control. And no, changing the pot or how it's wired up isn't an option. My only available spot to tie in without affecting other circuits is between the pot and the dash.

Any suggestions on how to build the circuit?

[alimirjamali]

... There's certainly a bunch of circuit designs out there, but they seem to be controlled by varying resistance from a pot, and not from a variable voltage source...

The POT in those designs simply acts as a voltage divider, providing a variable voltage source....

I'd like to be able to vary the duty cycle as much as possible, but an old scratchy pot that only has a 3 volt range isn't going to give a lot of fine-grain control.

3 volt range is more than enough to provide a proper range for LED PWM brightness control.

Any suggestions on how to build the circuit?

A 555 timer, an Op-Amp based PWM generator or even a small Micro-controller could be used to build the circuit. I am a digital guy and have the tendency to do everything the hard way! (Micro-controller). If non of the Analog guys out there post a schematic for you by tomorrow, I could show you how to do it with a Micro-controller . You would better use digital for everything and avoid analog as much as possible .

[shroomduke]

I was thinking a Voltage Controlled Oscillator might work here. Just a thought!

[CasualTinkerer]

I could really use some help with a schematic. While I can identify various parts and generally describe what they do, I don't know enough theory to actually design a circuit beyond basic 12v stuff for cars.

While the micro route sounds cool, it also sounds like severe overkill for what I need for this project. The 555 timer or voltage controlled oscillator sounds more reasonable.

[CasualTinkerer]

This is the LEDs I'm using:

https://www.superbrightleds.com/moreinfo/miniature-wedge-base/194-led-bulb-15-smd-led-wedge-base-tower/910/

I thought about the interference issue. If it helps, the instrument cluster is a late-80s Chevrolet cluster, so it's not super-modern. Will PWM affect the gauges, I have no clue.

[Rufus]

A typical hardware solution to generate PWM from a control voltage would be to have a ramp generator running at the PWM frequency and a comparator comparing the ramp voltage and control voltage. The output of the comparator is the PWM signal.

A big problem you have is the light output of incandescent lamps is about proportional to the applied voltage to the power 3.6 so to match brightness the relationship between the lamp voltage and PWM duty cycle needs to be very non-linear.

[CasualTinkerer]

Just finished watching today's Mailbag, and I saw the tiny led pwm controller. Maybe that would be a good basis to go off of and just figure out the details of getting the variable voltage from the car's dimmer into the brightness pot. Here's a link to the project:

https://www.tindie.com/products/madworm/led-strip-pwm/