I am not an electrical/electronic engineer, so please bear with me.

I have a monochromatic camera (www.qimaging.com/resources/pdfs/QICAM-12bitManual.pdf) which can be triggered (i.e. capture image) by a 5v TTL signal provided by an external triggering system (a function generator). Also, the camera comes with an electronic triggering board related to this application (www.qimaging.com/resources/pdfs/ExtTrigApNotes.pdf) (www.youtube.com/watch?v=LuC_-cUO97c). So, the function generator output is not directly connected to the camera. The BNC cable from the function generator would be first connected to the triggering board; then the triggering signal will reach the camera through a circular DIN port. One thing this triggering board does is it allows you to monitor the exposure time and readout time of the camera (so I think this is one way to see if the camera is triggered or not, right?).

So, basically I want to feed the camera a 5v TTL square signal (DC) and make the camera capture an image (i.e. trigger) on edge high mode. I also, have an external light source, supplying the required light for the image, which also need to be synced with the triggering TTL signal on edge high mode (www.hsps.com/2006/pages/nanodren.htm).

I have come across a few issues for which I need help from you:

1) I am using a function generator with an in-built TTL output (BNC connection). So, when I monitor the function generator output, the voltage amplitude is not 5v, but around 4.2v. That persists even if I set the function generator's impedance to High Z. I tried two different function generators and I get the same 4.2-4.4v. What's the reason behind this? How can I get an exact 5v TTL signal?

2) You may ask, your camera surely works (will trigger) even with a 4.3v signal. You are right. But it gets problematic when I also connect the external light source (required for higher quality image) to the function generator to be triggered at the same time as the camera. So, when the BNC cables from the function generator output is connected to the oscilloscope, my voltage is around 4.3v, but the moment I connect another BNC cable to the same output in the function generator and plug its other side to the external light source, the voltage amplitude drops to around 3.6v (probably due to internal impedance of the external light source), which may not enough to trigger the camera.

So, although the function generator might be giving out a 5v signal when there is nothing attached to it, but this surely drops to a value of 3.6v with all the BNC cables connected to it. My question is that how I can make sure a 5v signal (DC square) will reach both my camera and my external light source.

Please let me know if you have other suggestions for my case. Hope my post won't get deleted due to not following the forum guidelines. Thanks in advance.

Note: internal impedance of the function generator is 50 ohm.

  • \$\begingroup\$ A legal TTL High level is anything above 2.4 volts, if I recall correctly. \$\endgroup\$ – Peter Bennett Jan 31 '17 at 1:00
  • \$\begingroup\$ So, do you mean the camera should work even with a 2.4 volts signal? The camera description says: "An external trigger source can be supplied through the BNC connector labeled External Input, using 5V TTL signals." \$\endgroup\$ – Muhamad Mohaqeq Jan 31 '17 at 1:06
  • \$\begingroup\$ interfacebus.com/voltage_threshold.html \$\endgroup\$ – Bruce Abbott Jan 31 '17 at 5:35

(big edit)

"Triggering TTL for on/off control of selected channels using a BNC connection Switching time < 30 uS"

"Pulse Mode allows the user to control the integration period where the time is dictated by the duration of the pulse. If “Pulse High” mode is used, then the time the pulse is high is the time of the integration period. Vice versa for “Pulse Low” mode."

SO you have the choice of positive edge or negative edge, or positive duration or negative duration somewhere in software.

You only want the camera to start capturing before the flash goes.

But we do not know the flash delay time. It is probably a few to 10 microseconds, based on my experience. You can determine this by asking them or testing it with a fast light sensor (e.g. Photo diode biased and 50 Ohm to scope.)

The xenon flash time of 1ns is not critical about the capture duration AS LONG AS IT IS TRIGGERED BEFORE the flash actually occurs.

However the trigger delay time may be critical as we do not know the setup and delay times for each device to be triggered and I did not find an explicit spec for this. We call this a "Race condition" when two events are triggered and you want the camera to capture the output of the flash.

IN logic design it usually works the other way around. We use a Flip Flop or memory cell to capture the data before it changes on a clock edge. ... and not after some delay when a 1ns flash occurs. THe Xenon flash works by a secondary emission arc of a capacitor discharge, so once ionization starts which can take a few us, it completes in a hyper abrupt "tunneling mode" in 1ns arcing across the inner gas. It also causes really huge EMI pulses everywhere if done at a fast repetitive rate and can interfere with cell phone reception. ( like Safety Xenon Bulbs flashes at 2 pulse/sec. are notorious for wireless interference)

( I finally discovered all the options on your camera to indicate either polarity or edge can be used on your camera)

By using a slow ramp signal generator 0 to 5V or power supply you can carefully measure the threshold for edge trigger on each device to understand where it triggers. I am not 100% sure they are TTL compatible precisely as TTL is strictly defined with a threshold of 1.5V or more likely it is CMOS with 2.5V threshold which can be TTL compatible but does not work when you load both devices from your generator.

This means your generator is poorly matched to your 2 loads Normally TTL can drive 10 loads or more and this does not seem to be the case.

You can make a quick fix by adding 330 to 470 Ohm pullup to the TTL signal generator nearby to 5V and connect grounds together so that you get 0.5 to 5V output swing or adjust as necessary.

Once you have verified trigger to both devices successfully, then it may work. Otherwise Sync delay on flash may be needed if it triggers to fast.

Again sorry for not reading your manual thoroughly as you...( ought to have)

"Simple" possible solution


simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – Dave Tweed Feb 1 '17 at 12:47

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