A small circuit to enable the adjustment of LED panel lamp’s brightness

Technology has moved on since I modified tungsten screw-in panel lamp bulb cases for my TA (see TTT2 issue 11, page 13). Bright LED panel lamps in various colours are now available in positive and negative earth. They do not dim easily; a reducing voltage will cause the LED to extinguish.

To overcome this, variable duration but regular voltage pulses, will illuminate the LED for a longer or shorter period of time. LEDs switch very quickly, and as observed, the effect of this pulsing is bright or not so bright. To the eye if the switching is fast enough this brightness is even. The technical term for the type of switching, is pulse width modulation. I have seen articles showing ways to achieve this using programmable logic integrated circuits. This is a simpler way.

Here is a circuit which uses a small number of components, adjusted with just a knob. It is built around a 555 timer integrated circuit. All components are easily obtainable.

The components are mounted on a small piece of Vero board and enclosed in a plastic box. The driver transistor is a metal oxide silicon field effect transistor and is mounted in a heatsink on the outside of the box. It is of the plastic encapsulated type and therefore it and its heatsink will be fully insulated from any wiring. Heat transfer compound is used between the transistor and its heatsink.

The works.

Where on the car to mount this will be a personal choice. The feed to the panel and dash lamps can be inserted into the wiring after the panel lamps ‘on’ switch on the dashboard (wire coloured blue on TA). An earth connection to the box will be needed.

I used a knob from an old panel lamp ‘on’ switch, drilled to push fit the potentiometer spindle and a small strip of Velcro to attach to the back of the dash. (See photo).

Many suppliers now stock LEDs all colours including www.classiccarleds.co.uk – Tel 0800 2465678 – or from their eBay store.. The driver transistors and integrated circuit are cheapest via eBay.

Other components from Maplin or Rapid-online etc:

-Small piece of Vero board (unfortunately the minimum size is about £5 from Maplin)

-Maplin Plastic box

-8 pin IC holder.

-Resistor 10K ohms,1/8w .

-Capacitors. 0.01 micro-farad (1)

-0.10 micro-farad (2)

-Push fit heatsink for the transistor

-100K ohms linear potentiometer.

Misc:

-3mm Bolt and nut for heatsink, wire, tag for earth connection, connector from box to lamps feed. Heat transfer paste.

I regret that I cannot supply kits or built units. Any questions via email to email(at)robbut.plus.com – please substitute @ for (at).

One thought on “A small circuit to enable the adjustment of LED panel lamp’s brightness

  1. BRIAN YATES says:

    Hello,

    If you are not an electronics enthusiast an easier solution might be to buy a dimmer for 12 volt LED lights by looking on Google. These are available for about £4. Dimmable LEDs will of course be needed. I haven’t made this conversion, but hope to do so over winter.

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