The O.E.M. turn signal flashing units installed in automobiles have two basic functions i.e. Flasher and Lamp outage detection.
These flashers are usually built with an 8-pin IC like U2044B, U6432B etc. which are specifically made for automotive flashers.
Designed and Written By Abu-Hafss
Circuit Operation
NORMAL OPERATION:
-input = Square wave between 11.89V – 12.0V
+input = 11.9V (reference voltage)
The comparator U1 compares the two voltages and the output is a square wave between 0-12V. This output is rectified thru diode D1 and filtered thru capacitor C1.
Now, we have a triangular wave form which is fed into another comparator U2.
+input = Triangular wave between 7V – 8V -input = 1V (reference voltage)
The comparator U2 compares them; the output is constant 12V which goes to the base of the NPN transistor.
This switches on the NPN and hence C3 is connected to ground. In result, the 555 timer oscillates at about 1.4Hz.
The output of 555 is connected to relay RLY1 which relays 12V direct from the battery (thru shunt) to the lamps.
OPERATION WITH A DEFECTIVE LAMP:
When a bulb is defective, there is an increase in the resistance of the bulb network hence the voltage drop across the shunt is changed. So, in this case we would have:
-input = Sq. wave between 11.95V – 12V
+input = 11.90V (reference voltage)
The comparator U1 compares them and the output is almost zero volts. After the diode and the filter network, we finally have a few millivolts at the +input of U2 which is compared with the reference voltage, 1V.
This results in low output of U2 which ultimately switches off the NPN and hence C3 is disconnected from the ground.
Now, the timing network of 555 has only C4 to work with, therefore the frequency of the oscillation is doubled. This causes the remaining bulbs to flash at doubled rate.
Circuit Diagram
Comments
I put LED tail lights on my truck. Doing so required me to swap out the wiring harness for each light. After the install, I noticed the bulb outage detection wired were not connected to anything, as in the factory harness coming from the BCM has 5 wires and the one I purchased to make my new tail light wires work has 4.
The bulb outage detection wire has a reading of 11.9V and I’ m getting hyperflash on both sides. How can I wire the bulb outage detection wire (or what can I connect it to) to trick it into thinking there isn’t a bulb out?
Thanks!!
The outage detection wires probably requires a load to satisfy the detection circuit. You might have to connect an actual light bulb between the detection wire and ground to satisfy the load conditions. A resistor might not work since it might get too hot.