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DIY LED indicator relay

One of the problems of fitting low wattage or LED indicator bulbs, is the wrong flash rate using standard relays.
People sometimes cure this by putting resistors in the circuit, this does work, but the resistors do tend to get hot, and in a glassfibre car this can be a fire risk.
You can buy, at a considerable cost, a special LED relay, however it is simple to modify a existing relay to do the same job

The old mechanical indicator relay is powered by nothing more than a bi-metal strip.
The current flows through a small coil, heating the bi-metal strip, which bends away from the contact, breaking the circuit and letting the strip cool, where it straightens and starts the process again.

mechanical flasher
mechanical flasher
mechanical flasher opened
open mechanical flasher

I had a look on the web, and discovered that most modern relays, such as the Lucas 19FL, used the same chip, with a few external components to provide the same function.

A look around a scrapyard, gave me a couple of relays, one was a Lucas unit, the second was from a Vaxhall and seemed to be standard Lucas type, I was wanting.

19fl relay

Once home I tried them out, connected to a test bulb, to find they just buzzed and the test bulb stayed on. No problem as this was what I was expecting. I quickly opened both relays up and checked the small IC fitted on the boards.

relay open
relays opened

These both turned out to be U243B chips as I guessed. A look at the datasheet again confirmed that pin 7 on the chip was the one that controlled the "bulb out" feature, needed under some E.U. regulation, to alert people to the fact one bulb had gone.
I then disconnected this pin, by cutting through the PCB track at the rear.
This means that the chip now oscilates at a fixed rate, just what I was needing.

pin 7 disconnected
Track cut through

I then hooked it back up to my test bulb and it worked.
A nice regular flash.

Wiring the relay is just a case of swapping the relays and adding an addition earth wire,
I then preformed a quick test of the hazard lights, all seems to be ok.
They do go off, just my camera seems to lag a little.