The circuit explained here is basically a warning signal generator designed largely for automobile application. It triggers a warning signal in a situation when the turn indicator switch is not restored back to its neutral status. The circuit is especially useful for all those cars where the turn indicator system is not supported with an audible clicking sound.
In addition to this, the circuit creates a warning signal as soon as the ignition switch is turned off but car headlights continue to be in the 'on' position. The circuit is specifically designed for 12 volt negative earth cars.
Turn Indicator Warning
When the turn indicator is in the pulsating mode, the circuit sends out a nonstop four step intermittent tones in a random sequence. The functioning can be learned as I have explained below:
Point C obtains its supply voltage through the ignition switch. Point A is fed with several intermittent pulses from the turn signal relay contact which flashes the side indicator lights.
Each one of the astables produces a frequency determined by the linked T3/T4 RC stage, with a fairly reduced frequency. The second astable T1/T2 oscillates at a varying frequency with respect to the pilot voltage derived via D5 or D4. T5 and T6 produce the timer interval necessary for bypassing the pulsating periods at times when the warning light is not operational.
The alarm frequency is adequately boosted by transistors T7 … T11 for loading the loudspeaker.
Headlight Warning
This car warning circuit could similarly be applied like a 'switch off your headlamps' warning message indicator.
The involved operations may be learned as described below:
Switching the ignition switch "off" takes away supply from point C. Point B, on the other hand, remains energized through the light switch.
This results in both astables and the amplifier stages to continue being functional, apart from T5. This turns OFF the control signal from T6 resulting in its collector to increase to 12 volts and evoking the AND gate D6/D7 to feed the signal frequency over to the transistor amplifier stage.
A two-tone warning signal then begins operating. The terminals A, B and C, should be wired up with the flasher unit, the headlamp switch and the ignition key switch respectively, and you should not modify any of the car electrical substantially except the specified three connections as stated, along with the chassis ground connection.