A voltage pulse resistance design for a car reversing camera

The circuit in this example uses a simple comparator circuit to delay the power-on time for a car's backup camera. Car manufacturers often use the backup light circuit to power the backup camera. In an automatic car, when the gear changes from park to drive (or vice versa), the camera is given a short power pulse when the gear changes through reverse gear. This sudden voltage pulse is not good for the sensitive circuits in the camera and may shorten its life. This example presents a simple and inexpensive way to avoid this situation.

This circuit creates a 2s power supply delay for the car's reversing camera.

The inputs of this circuit are connected to the positive and negative terminals of the backup light. The circuit powers the camera through a MOSFET. R1 and C1 form a time delay element (Reference 1). When the backup light is on, it slowly charges the capacitor through resistor R1. R3 and R4 form a voltage divider that sets the inverting terminal of the comparator to 6V. The moment power is applied to the circuit, the comparator output is low and the MOSFET is turned off. When the voltage of C1 rises above 6V, the comparator output is high and the MOSFET is turned on. The values ​​of R1 and C1 set the delay time to 2.2 seconds. This time value can be calculated based on the charging time of the capacitor using the following formula:

VC(t)=VIN×[1-e-(t/R1C1)],

6 = 12 × [1-e-(t/33kΩ×100μF)], and t = 2.287s

You can set different delay times by changing the value of R1 or C1. When the lever is switched from reverse to any other position, capacitor C1 is discharged within 60ms through D1, R3 and R4. When you switch between park and drive and the lever passes reverse, the camera will not be powered due to the 2s time delay.