Low-cost comparator circuit prevents sudden voltage pulse

Automotive manufacturers typically use the reverse-light circuit to power reverse-view cameras. Specifically in automatic-transmission vehicles, a short power pulse is applied to the camera when it shifts through reverse as it goes from park to drive or vice versa. This sudden voltage pulse can affect the sensitive circuits in the camera and may reduce its lifetime. This Design Idea features a simple, low-cost comparator circuit that can prevent this situation.

The 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. 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 seconds. 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 60msec through D1, R3 and R4. As you pass through reverse, shifting between park and drive, the camera does not turn on due to the two-second delay.