Design of automobile turn signal circuit

Brief Introduction

Today we are going to design a car taillight (turn signal) circuit effect as shown below:

That is, when the car turns, the driver turns on the turn signal, and the tail lights will be lit up in turn according to the direction, and after a certain interval, all will be turned off. Finally, it will be repeated until the driver turns off the turn signal.

This effect can be achieved by the following circuit:

The complete circuit diagram is as follows:

There are two main points in circuit design:

When the turn switch is pressed, the LEDs need to light up one by one;

The process of LEDs being lit up one by one is repetitive.

Effect display When the turn switch is pressed, the LEDs need to light up one by one.

The process of LEDs being lit up one by one is repetitive.

How it works

The PNP transistor (Q3/Q4/Q12/Q13) controls the LED power supply in the circuit, and the condition for its conduction is that the corresponding NPN transistor needs to be turned on.

When the driver does not press the turn signal, the NPN has no base current Ib, so the NPN is not turned on, and the corresponding PNP is also not turned on, so the light will not light up unless the turn signal switch is pressed.

When the steering switch is pressed, the NPN will flow through the Ib current, so the NPN will be turned on, and at this time the PNP will also flow through the Ib current, so the PNP will also be turned on. Therefore, the LED will receive voltage and be lit.

When the first Q3 is turned on, Q14 will flow through Ib, so Q14 will also be turned on. However, since the voltage of C7 = Vcc, Q4 must wait until the voltage of C7 is released through Q14 before it can be turned on. This achieves a certain conduction time interval between LEDs.

The control principle of the remaining two LEDs is the same as that of the second LED. The interval conduction time of the LEDs can be adjusted by setting the capacitor size.

Finally, the repeat function is implemented. Since the control signals of the four LEDs are cascaded, to turn off all of them, you only need to turn off the power of the first LED. So we can add a triode circuit at the last end to control the control signal of the first stage.

The length of the repetition time is proportional to the size of capacitor C13.

Summarize

The working logic of the circuit is relatively simple, and the entire circuit can be divided into three parts:

Power supply control circuit

Time control circuit

Repetitive control circuit

The above circuits all utilize the working characteristics of basic components. If you find something you cannot understand, you need to improve your circuit basics.