Automotive Turn Signal Flasher
with Lamp Outage Detection
Circuit :  Abu-Hafss, Pakistan
Email  :   Email

Here is a circuit of a flasher which I designed for automotive use. It has a built-in function of lamp-outage detection. In normal operation, it will flash at about 1.4Hz and when a lamp goes bad, the flash-rate is doubled. The flashers faster clicking sound and the dashboard indicators faster flashing attracts the drivers attention that one of the bulbs has gone out.

The circuit consists of two parts - the flashing unit and the lamp outage detection module. The flashing unit is built around 555 timer configured as an astable multivibrator. The resistors R12/R13 and capacitors C3/C4 sets the required frequency. Note that C3 is connected in parallel to C4 through BC547 an NPN transistor, acting as a switch. When there is positive voltage at the base of the transistor, it conducts and connects C3 to ground. C3 & C4 in parallel doubles the capacitance value i.e. 220nF + 220nF = 440nF. This capacitance value together with R12 and R13 result in frequency of about 1.4Hz.

In the lamp outage detection module, a shunt resistor (a thick wire) with a calculated minor resistance (30mΩ) is the key to detect the lamp outage. The voltage to lamps is fed thru this shunt. Hence, the shunt is connected in series to the network of the bulbs which are connected in parallel. The inverting input of the comparator U1 is also connected to the shunt. The non-inverting input is connected to a potential divider providing a reference voltage of 11.90V

Square wave between 11.89V - 12.0V at (-)ve input of U1
11.90V (reference voltage) at (+)ve input of U1

The comparator U1 compares these voltages and the output is a square wave between 0-12V. This output is rectified through diode D1 and filtered through capacitor C1. Now, we have a triangular wave form which is fed into another comparator U2.

1V (reference voltage) at the (-)ve input of U2 and
a triangular wave between 7V - 8V at (+)ve input of U2

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:

Square wave between 11.95V - 12.0V at (-)ve input of U1
11.90V (reference voltage) at (+)ve input of U1

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.

A single quad op-amp IC LM324 could also be used for building this circuit. Two of the op-amps to be used as comparators in place of U1 & U2. Another op-amp can be configured as astable oscillator in place of 555 to flash the bulbs. This astable oscillator will have two timing capacitors as used above with 555.

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