Amp-Off test - Creative Uses of CHDK - CHDK Forum

Amp-Off test

  • 7 Replies
Amp-Off test
« on: 16 / November / 2012, 19:45:19 »
I've been working on a circuit to reduce the nasty bright spots that show up on long exposures on CCD cameras.
In the attachments pictures of the so called amp glow (single 322s iso80 dark frame on a sd1300/ixus 105) and the circuit i used.
« Last Edit: 16 / November / 2012, 20:44:45 by casrap »

Re: Amp-Off test
« Reply #1 on: 16 / November / 2012, 19:45:28 »
What the circuit does is lower the supply voltage of the ccd when a signal >1,5V is entered on 'IN'.
By lowering the supply voltage most of the electronics on the sensor shut down, stopping the creation of the glow. With most of the electronics on the sensor inactive you wont be able to read data from it, so you have to restore the supply voltage at the time the shutter closes and the sensor readout begins.
The circuit: Q1 is the input stage, it is a high impedance inverter that pulls the cathode of a adjustable zener diode to ground. The anode of the zener is tied to the basis of the output transistor, and to the original supply voltage of the sensor via a resistor. The output transistor is a simple emitter follower; the emitter follows the voltage at the basis - 0.6V. So the output voltage of around 14V is reduced to 13.4V when the circuit is inactive and to a preset voltage (here ~7.5V) when the circuit is active.
Besides the circuit 2 more things are needed;
- a signal to enable/disable lowering the supply voltage
The simplest usable signal i found was the voltage to the green led.
- a way to to steer the signal in the software.
Find a place in capt_seq.c and insert code to turn the led on and off. I inserted the code to turn the led (and the circuit) on after a call that waits for the release of an external remote ("BL wait_until_remote_button_is_released\n" )
The camera turns the led off when the shutter closes (no idea why, but thank you).

Attached: the same exposure with amp off and the circuit (1cmx1.3cm)
« Last Edit: 16 / November / 2012, 20:21:32 by casrap »

Re: Amp-Off test
« Reply #2 on: 16 / November / 2012, 20:06:26 »
I've also tested the sensitivity for possible side effects the amp-off circuit might have. Up to now i have not found any. The only thing i do see is that the lcd flashes the live view image after every picture (thats when the picture is saved and the led lights up).

nb: the amp off picture seems to have more hot pixels than the amp on picture. This is caused by the jpeg; during creation of a jpeg a palette is chosen so most of the information in the picture can be displayed. In the picture there is the amp glow that 'has to be' in it, taking up most of the available colors. In the amp off picture there is only some noise an some hotpixels so the entire palette is used for them making them better visible. (I should have used raws but i can't display them in the forum).
« Last Edit: 16 / November / 2012, 20:22:32 by casrap »

Re: Amp-Off test
« Reply #3 on: 16 / November / 2012, 20:12:16 »
The circuit can be modified to different voltages (if you want to experiment or you have other components and want to use those)
- The buffer/inverter: any mosfet with low gate charge and Vds max ~20V and VgsTreshhold<3V should work
- The adjustable zener; you can set a different voltage by changing the values for r2&r3, or replace this with a fixed voltage zener (~8V) or you can even leave it out and use the resistors to set your output voltage (but this will create more noise in the image).
- The output transistor is a bog standard NPN, any general purpase NPN will do.
« Last Edit: 16 / November / 2012, 20:24:05 by casrap »

Re: Amp-Off test
« Reply #4 on: 16 / November / 2012, 20:23:01 »
Nice hack !
Ported :   A1200    SD940   G10    Powershot N    G16

Re: Amp-Off test
« Reply #5 on: 16 / November / 2012, 20:37:04 »
'Inserting'the circuit in the camera. I have no picture of it yet, but the next time i open it up i'll post one (or more).
The circuit needs the 14V going to the sensor, and the trace of that signal  to the sensor connector must be cut, a connection must be made to both sides of the cut (to the +14V and the 'Out' in the schematic). Connect the 'Out' to the trace to the connector.
Use a multimeter to trace the signal from the led to a resistor on the main pcb, connect the 'IN' from the circuit to the other connection of this resistor.
That only leaves ground to connect. Most capacitors carry ground on one connection, pick one.

A little warning; Everything is fiddly in these cameras and making the cut to the trace and even more connecting wires to this trace can be extremely hard to do..
« Last Edit: 16 / November / 2012, 20:48:18 by casrap »


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Re: Amp-Off test
« Reply #6 on: 17 / November / 2012, 02:56:27 »
Interesting solution, good job!  ;)

Re: Amp-Off test
« Reply #7 on: 01 / February / 2013, 05:30:59 »
I have the same problem ... and your solution is great!
It is possible to have some more info? some more pics?



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