Protect a Vehicle-Reverse Camera


The circuit in this
Design Idea uses a simple comparator circuit to make a power-on time
delay for an automotive rear view camera. Auto manufacturers typically
power reverse-view cameras from the reverse-light circuit. In
automatic-transmission vehicles, a short power pulse is applied to the
camera when you shift through reverse as you go from park to drive, or
vice versa. This sudden voltage pulse is bad for the sensitive circuits
in the camera and may reduce its lifetime.

This Design Idea suggests a simple and cheap method for avoiding
this situation.Simple circuit helps to protect a vehicle-reverse camera
figure 1The input to this circuit connects to the positive and negative
terminals of the reverse light (Figure 1). The circuit powers the camera
using a MOSFET. R1 and C1 form a time-delay
element (Reference 1). When the reverse light turns on, it slowly
charges the capacitor through resistor R1. R3 and R4 form a voltage
divider, which you use to set 6V on the inverting pin of the comparator.

At the instant of power application to the circuit, the comparator output is low, and the MOSFET is off. Once the voltage of C1 rises above 6V, the comparator’s output becomes high, and the MOSFET
turns on. The values of R1 and C1 set the time delay to 2.2 sec. You
can calculate this time based on the exponential charging of a capacitor
using the following equations:

You can set a different time delay by changing the value of R1 or
C1. When you shift the gear lever from the reverse position to any other
position, capacitor C1 discharges within 60 msec through D1, R3, and
R4. As you pass through reverse, shifting between park and drive, the
camera does not turn on due to the 2-sec delay.


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