Single Bulb, Two Filament Setups
This turn signal system was designed to use separate turn signal and
brake light bulbs.
We are aware that some of the more simple lighting systems used a
single bulb for the rear lamps with a dual filament bulb. The larger
filament is used for both turn signal and brake function.
Here is why that is not a good idea: the filament can be either on or
off, but it can't be both at the same time. Let's say you are at a
traffic light and signaling to go left. If your foot is off the brake,
the rear will signal left; but, if your brakes are on, there is no signal
as the brakes are commanding that the filament remain on.
A much better approach to the problem is to add another socket to the
existing rear lamps. You could then add one of the superbright LED's
into the rear shell, and use that for the turn signal.
A suggestion: use Mouser part # 697-D29B9sic-12V. This is a 10mm Dome
BA9S Red Sunbrite LED w/ a built in dropping resistor to connect directly
to your 121V source. It uses a miniature bayonet socket and will fit almost anywhere.
If you really must stick with a single bulb dual filament setup, then
you are going to need two additional power diodes (to keep the brake
signal from feeding back into the turn signal module) and a relay to
tie the LH and RH lamps together when the brake lamps are supposed to
Any questions? Please call, we will be happy to talk to you.