Simple circuit helps to protect a vehicle-reverse camera
Add a delay to prevent short power pulses as you shift through reverse.
Aruna Prabath Rubasinghe, University of Moratuwa, Moratuwa, Sri Lanka; Edited by Paul Rako and Fran Granville -- EDN, October 6, 2011
The circuit in this Design Idea uses a simple comparator circuit to make a power-on time delay for an automotive rearview 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.
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
(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.|
Reference |
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Talkback
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momentary oscillation at trip point will be compensated by gate resistor and gate capacitance of FET
Aruna Prabath - 2011-25-10 20:11:14 PDT -
In case of a rear view camera combined with a Japanese car navigation system, such circuit is not helpful. The "rear" contact switch will be connected to the navigation system, which provides the delay, and the power to the camera is also provided by the car navigation.
Alexander Schnase - 2011-10-10 18:50:31 PDT -
Attempting to protect the camera by interrupting the negative lead will probably not work as planned, since the negative connection is probably also on the shield side of the video output cable. How about, instead, just a larger capacitor shunting the power at the camera power terminals, and a series resistor to limit the inrush current? Consider that the cold filament inrush to the backup lamp filaments is probably a few amps, is it that probable that there would be a problem? Of course another even simpler solution, if it is needed, would be to have the camera controller processor just delay the startup of the circuitry for a second or so.
William Ketel - 2011-10-10 18:16:24 PDT -
As C1 slowly charges up past the trip point set by R3/R4, the comparator will begin to sporadically trip, more than likely spitting out a sputter of pulses until the threshold is well-crossed. Of course, the purpose of this circuit was to eliminate chattering. Adding a small amount of positive feedback to provide hysterysis will help to eliminate this pitfall. A large resistance from the output pin 1 to the positive input pin makes all the difference. It would be an improvement to also add a low resistance between C1 and the opamp pin/hyterysis resistor to provide more corrective hysteretic effect.
Norm Novotney - 2011-10-10 14:33:23 PDT
