DEFINITION of the S90 SHUTTER CONTROL SIGNALS and an EXTERNAL SHUTTER CONTROLLEROVERVIEW
One of the great advantages of a PFP-configured CCD optical instrument especially for low light photography such as astronomy I haven't discussed yet, is the removal of the lens increases flux by the speed of the lens you remove. For example, the S90 has an f/2 lens. By taking all the glass out from an S90, you go from f/2 to f/1, giving you a +2EV immediate flux advantage. Also, in a telescope you typically remove the eyepiece optics and position the CCD on the telescope's focal plane, further improving signal strength.
In a camera lens, the exit pupil diameter defines the minimum diameter a shutter can be. In the S90, the exit pupil is 5 mm diameter, hence the shutter clear aperture in this camera is 5.5 mm diameter. This is about half the size of the CCD itself and cannot be used in the PFP architecture. Most often, the lens in your instrument will be larger than a camera lens. Therefore the complete solution is an external shutter that has a clear aperture larger than the diagonal of the sensor area. For a 1/1.7" CCD that is 9.6 mm, hence to be safe and have some installation tolerance, a minimum 10 mm shutter is required.
CAMERA SHUTTER SOLENOID DRIVE SIGNALS
The S90 employs a bistable shutter that is used for CCD data readout and dark frame acquisition when enabled, as discussed earlier. This means that the shutter changes state with a brief application of current polarity reversed pulse generated by an H-bridge driver on the main board, and stays in that state mechanically until excited again.
Figure 1 shows the two states. The left panel shows the OPEN->CLOSE transition pulse, and the right panel is the CLOSED->OPEN pulse. The drooping tail is inductive back-EMF. The pulses are 6.1 and 1.3 ms with a voltage of 4.9V. Thus the native H-bridge is capable of delivering +/- 550 mA into 9 ohms.
DEFINING an external SHUTTER
In terms of timing your own shutter, the OPEN->CLOSE reaction is the more important of the two. That's because after closure either data readout begins or dark frame acquisition begins. Hence to guarantee imaging quality is not subject to deterioration by stray light, your shutter has to close at least as fast as the Canon shutter. According to the measurement in Fig 1, we can comfortably make the assumption that firmware operations (readout or dark) will not start any earlier than 6.1 ms after the rising edge of this pulse. Because of hardware latency, this time could be longer than 6.1 ms. The right way to get the firmware start-operation time mark is to actually measure the Canon shutter with a photodiode, which I have not done as of this writing. Staying with the first assumption for now, after you detect the rising edge, you have 6 ms to make sure your shutter is closed, all told ... signal detection, level translation, drive, solenoid energization, and mechanical settling time. Therefore the shutter you select or build, has to meet this specification.
SPECIFYING an external SHUTTER CONTROLLER
If your shutter can be driven by 5V at less than 550 mA, then you can wire it directly to the Canon native H-bridge. However, because more mass has to be moved over a larger distance for a 10 mm aperture, it is likely the external shutter will need more power to change state.
Figure 2 shows a very flexible general-purpose shutter controller that you can use to drive your bistable shutter. It can deliver up to +/- 1.5A from a DC supply of +4V to +13V, and you can set the open and close pulse widths from 1 ms to 23 ms independently. All you need to do is connect the input to the 2-wire points on the Canon shutter, and run the circuit at the rated voltage of your shutter. The circuit has an auxiliary output that stays latched in the open or closed states. For example, you use this signal to enable an illumination lamp during the open state and disable the lamp during the closed state, simulating an external shutter if your camera is in a completely dark environment. The lamp cannot be an incandescent type such as a halogen because of thermal lag.
CIRCUIT FUNCTIONAL DESCRIPTION
When interfacing your own powered equipment with camera electronics, it is prudent to use optoisolators, as I've done here. There are two independent symmetrical paths in this circuit that split up at the TLP2160 input and recombine at the Si9986 H-bridge output. A Canon open or close pulse turns ON one of the two LEDs in the TLP2160. The -ve going output of the TLP2160 gets inverted by the 74HC00 and clocks the D flipflop (74HC74) to generate a -ve going TRIGGER pulse to the LMC555. The LMC555 output goes HIGH to begin its timeout. Its output is connected to 3 nodes:
1) D-flipflip PRE pin inverted via 74HC00. The LMC555 LO->HI transition immediately resets the D flipflop. This retriggerable feature allows the shutter excitation signal (to the TLP2160 LED) to persist longer or shorter than the set delay in the LMC555, guaranteeing that the external shutter will always get its prescribed pulse width regardless of input excitation pulse width;
2) SR flipflop. The LCM555 output changes the state of NOR-gate (74HC02) SR flipflip which drives the state LED indicators and is connected to the lamp enable/disable output;
3) Si9986 H-bridge. Drives the H-bridge for the timeout period in the corresponding current polarity.
The RC network on the 74HC00 is PUP reset.
CALIBRATION and USAGE
When you first try your circuit, it is possible that you may experience swapped behavior, such as the GREEN LED turning ON after a close signal. Simply swap the input terminals. Likewise, if your shutter is operating in reverse, swap its terminals. You don't need a scope to set the delays. If you use a 10 or 11 turn trimpot, delay will be ~2 ms/turn, starting at 1 ms fully CCW. You can test your circuit directly with the native shutter on the camera by setting the operating voltage to +5VDC:
a) disconnect the ribbon cable from the solenoid, attach the 10 ohm resistor as shown,
b) connect the ribbon pair to INPUT, and
c) connect OUTPUT back to the solenoid.
Set OPEN to 1.3 ms and CLOSE to 6 ms. The device should work identically to the native Canon connection. Always PUP the controller first, then the camera. Then take one dummy shot to allow it to set itself into its correct standby initial state.
BOM - DigiKey
1X 296-1187-1-ND SN74HC00DR IC QUAD 2-INPUT NAND GATE 14SOIC
1X 296-1188-1-ND SN74HC02DR IC QUAD 2-INPUT NOR GATE 14-SOIC
1X SI9986DY-T1-E3CT-ND SI9986DY-T1-E3 IC BUFFERED H-BRIDGE 8-SOIC
1X TLP2160(F)-ND TLP2160(F) ISOLATOR 2.5KVRMS 2CH TOTEM SO8
1X 296-17360-1-ND LP2981-33DBVR IC REG LDO 3.3V .1A SOT23-5 (can be soldered too)
2X SM-42TW204CT-ND SM-42TW204 TRIMMER 200K OHM 0.25W SMD (11 turn SMD)
2X LMC555CMXTR-ND LMC555CMX/NOPB IC OSC MONO TIMING 3MHZ 8-SOIC
2X 478-1239-1-ND 0603YC104KAT2A CAP CER 0.1UF 16V 10% X7R 0603
BOM - Mouser
2X Schmartboard 204-0004-01
Resistors LEDs and other capacitors are your choice. It is advisable to disconnect the Canon solenoid and replace it with the 10 ohm resistor to prevent false triggering due to the inductive kickback.
See also:
http://chdk.setepontos.com/index.php?topic=8801.msg92704#msg92704
