CCTV Camera Long Exposure Control Circuit

The circuit was designed to control long exposures of the CCTV camera using a simple AT89C2051 microcontroller. It can be used with modified B/W cameras based on Sony chips CXD2463R ( CCD timing control ) and CXA1310AQ ( CCD signal processing ). These ICs may be found with Sony HAD CCD sensors in small board cameras with "1004" or "1043" designations.

The circuit contains slightly more transistors and resistors than necessary. It is useful to have the camera in the "normal" autoexposure mode when the MCU is started or even if the MCU socket is empty. Software functions are controlled and monitored by five push buttons and three LEDs. MCU ports assignment may be different - it depends on software. The schematic described below allowed me to design a simple single sided prototype PCB. There are four unused port pins for future developement : UART RX/TX for the remote control ( computer serial port connection ) and analog comparator inputs which may be used e.g. for supply voltage checking.

Exact values of resistors are not critical as well as types of transistors and diodes. I used BC547 ( NPN ) and BC557 ( PNP ) transistors, 1N4148 diodes and SMD resistors and capacitors with values listed in the schematic. I can't provide a PCB layout of the daughterboard because the original prototype design was slightly different ( more complicated ) and I modified it using the original PCB which is now not optimal for this final version. I'll prepare some "production model" in the future.

The CXA1310AQ amplifier gain control circuit ( at the top - one switch, potentiometer and two resistors ) is optional. It is not related directly to long exposures but it helps to obtain a desired image brightness when the AGC output is not optimal. Sometimes a minimum noise is more important than the maximum gain.

I/O description

Connections to the camera board are denoted by the IC pin numbers. "A" means the CXA1310AQ chip, "D" means CXD2463R and "C" is for CCD sensor pins. Following camera connections are used :

Gain control input ( CXA1310AQ pin 16 ) may be switched from the signal provided by the chip to the manually selected voltage. Control voltage range is approx. +1.8V .. +4.1V ( lower voltage causes higher gain ). The pin 16 is hardwired to the pin 17 on the PCB and must be lifted to enable the manual control. Op. amplifier output ( CXA1310AQ pin 17 ) provides a control signal for the automatic gain control Shutter control signals ( CXD2463R pins 20 and 21 ) have on-chip pull-down resistors so the auto iris mode is selected by default. The shutter is disabled when +5V signal is putted on the pins. Vertical driver +15V ( VH ) supply ( CXD2463R pin 5 ) must be disconnected during the long exposure to inhibit the CCD readout. CCD +15V power supply ( CCD pin 9 ) should be set to lower voltage ( through a 10k resistor ) during the long exposure to eliminate the CCD amplifier emission and to lower the influence of switching pulses on the AGC circuits in the CXA1310AQ and/or in the capture device. +15V supply for the vertical driver and CCD is connected to both Vdriver and CCD supply pins on the PCB. It must be disconnected and wired independently to the daughterboard. Vertical driver V3 signal ( CCD pin 2, CXD2463R pin 4 ) provides both negative pulses for CCD vertical registers control and positive pulses for CCD readout control. The positive pulse must be blocked before the first field readout to keep the second field accumulated charge in the CCD. Synchronization signal ( CXA1310AQ pin 4, CXD2463R pin 25 ) is used by the MCU to count fields and lines in the frame. Blanking signal ( CXA1310AQ pin 29, CXD2463R pin 26 ) is used by the MCU to discriminate between odd and even fields.

SYNC and BLK signals were originally brought to drive an embedded digitizer and capture circuit which is not completed yet ( I have a problem digging up some special parts ).

It is necessary to decouple +5V, +12V and +15V power supplies by capacitors because the MCU frequencies interference may cause some strips or noise in the CCD signal. If there are problems with a correct decoupling I recommend following measures :

Remove LEDs from the circuit. They are switched fast during the push buttons check causing some interference. LEDs are used mostly for the software debugging and they are not necessary for the basic function. Don't place the daughterboard too close to the camera board and use some type of shielding between both boards.

It is possible to test the circuit without any MCU in the socket. Control ports P1.4 .. P1.7 ( pins 16 .. 19 ) are pulled high and the camera should work in the "normal" mode without the MCU. Shorting the pins to the ground ( pin 10 ) should switch corresponding functions.

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