Alternative to Optocouplers for switching a higher load resistance - Page 1
Forum section: electronics / projects

author: foetusmachine
author_rank: Contributor
timestamp: February 04, 2021, 07:26:33 am
content: Hi,I'm a beginner, and this is my first post. I think this is a bit more of a technical question.I'm working on a camera trigger for my Sony A7. The circuit I have is working fine with the optocoupler able to switch the Focus and Trigger pins to camera ground. That's how the Sony cameras work using the external multi port cable. The Focus and Trigger pins sit at 3.15v and if they drop below a 0.5v threshold (by grounding), the camera shutter operates.All good.However, i'm measuring in the order of 40-50us Tf (time to fall) across the optocoupler output, which seems about right - According to the datasheet. I say about right, becasue I've estimated the camera circuit as having an internal resistance of 15k-20k ohms. I used a simple battery internal resistance calculation to estimate this. This is a quite high resistance for an optocoupler, and the response time correspondingly suffers as per just about every datasheet i've seen.Does anyone have any ideas on a specialized opto that may not be affected so much by the load resistance or perhaps has anyone used a digital isolator such as the TI ISO121x series and know whether these are affected by load resistance? Ideally the unit would be a 4 or 6 pin DIP package.I'd like to get the Tf (time to fall) back down below 10us or even less if possible.

author: fcb
author_rank: Super Contributor
timestamp: February 04, 2021, 08:25:55 am
content: Use a faster optocoupler, or a bit more drive current (or both).Digital isolators normally require extra power on the isolated side.

author: gnuarm
author_rank: Super Contributor
timestamp: February 04, 2021, 09:05:28 am
content: Why exactly are you using an opto? Is there some need for high voltage isolation? If not just use a 2N7002 to make the ground connection. You can use a bipolar like a 2N2222 as well.On the other hand, why does the rise and fall time matter? You said the remote is working well. What is the goal of this "improvement" exactly?

author: magic
author_rank: Super Contributor
timestamp: February 04, 2021, 09:12:16 am
content: If you pull down to ground, high internal pullup resistance only helps you. OTOH, the time for the signal to rise back may be higher.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 04, 2021, 09:25:44 am
content: With a rather high pull up resistance part of the delay is from the cable capacitance - the optocoupler can not do anything about this.Not sure if it still helps at the higher resistor range, but with higher current it helps to no drive the LED of the OK with too much more curren than needed to switch. Driving the detector hard into saturation adds some delay. This more like a reduced delay - the rising slope still looks the same.There are a few OKs with base terminals. Here one can add a resistor base to emitter to increase the speed a little, at the cost of more LED current needed. However this are mainly older types, so not sure if this is faster than a modern type without the base pin.

author: foetusmachine
author_rank: Contributor
timestamp: February 04, 2021, 09:45:39 am
content: Thanks @gnuarm,I was considering a 2N7000 just now. I'll pick one up tomorrow and see how it makes a difference.I was wanting to improve the times because the shorter the delay, the better the potential image i'll be capturing. I'm not just doing it for the sake of it.@Kleinstein,I was driving the Opto LED moderately for 10 milliseconds. I tried reducing the current but it made little difference. I increased it to 80% of max and it made a noticeable improvement.I've attached the RL vs Tr graph from the datasheet.I've also shown a screen grab from my basic USB scope to help explain how the optocoupler is operating. It's quite telling. Yellow is input signal from the sensor and green is the opto grounding the camera trigger pins.

author: Zero999
author_rank: Super Contributor
timestamp: February 04, 2021, 09:47:11 am
content: Put the opto-coupler, as close to the camera as possible, preferably inside the connector, if there's room. Connect the LED side to the long cable.

author: fcb
author_rank: Super Contributor
timestamp: February 04, 2021, 10:26:19 am
content: Assume you are using an opto-coupler as you prefer isolation of the trigger circuit that you are driving from some other process/equipment. Using a transistor won't give you ioslation.The A7 has a small Li-Ion pack, and chunks through it, so the the user maybe powering externally (ground loops?).Tr (off-time) for the circuit is probably less important as the shutter triggers on closing contact, unlikely that the user is determining exposure from the length of the pulse (not sure that's possible on the A7).

author: Zero999
author_rank: Super Contributor
timestamp: February 04, 2021, 10:31:48 am
content: How long does the trigger pulse need to be? I wonder if a small transformer could be used instead. You'll need to add a DC blocking capacitor, so it doesn't short the biasing current from the pull-up and a diode, to protect against inductive spikes.

author: foetusmachine
author_rank: Contributor
timestamp: February 04, 2021, 10:43:50 am
content: As it turns out, I think there is something wrong with the internal circuitry of the a7iii - i've probably damaged it in the process of prototyping the circuit with jumper leads running everywhere.I looked through some previous load testing info I had on my cameras and I should be seeing a much lower internal resistance on the camera's trigger circuit.I've connected the trigger circuit to my older a6000 and run the test again. See attached plot - a much better outcome with <15us total circuit response time.Live and learn, but i'll probably make the same mistakes again and blow something else up in the future.Thanks for the input and suggestions from everyone.

author: gnuarm
author_rank: Super Contributor
timestamp: February 05, 2021, 12:00:19 am
content: Quote from: fcb on February 04, 2021, 10:26:19 amYes, but it is that isolation requirement I am questioning. The camera is battery powered equipment. It's likely the trigger is battery powered. Why would the two need optical isolation?QuoteWith external power they are typically also isolated, in particular if AC powered.QuoteOptos are inherently slow device, but certainly better than us delays can be obtained. I designed a TV video interface using an opto back in the days of analog CRTs. The opto was rated for MHz operation, maybe as high as 10 MHz.I don't quite understand the OP's info on his circuit, so I don't understand the graphs he has provided. The way to turn on the opto rapidly is to drive with a higher current. I would use a resistor to limit the current into the LED of the opto, then bypass that resistor with a small cap to provide a "bolus" of current, causing the output to turn on more rapidly then cutting the current back to prevent damage.i would simulate this, but it still depends on using an adequately fast opto and getting simulation models for LTspice can be a big PITA. Actually, a part in the LT library is the 4N25 which seems to have the sort of slow response with a large value pullup resistor on the output. What is counter intuitive is the fall time also slows with a larger load resistor. This spec is with a 2 mA LED drive current, so a 20 mA LED current should improve matters.I'm beginning to think this will require a different circuit for significant improvements. To do that will require more information on the requirements.

author: fcb
author_rank: Super Contributor
timestamp: February 05, 2021, 07:43:04 pm
content: Quote from: gnuarm on February 05, 2021, 12:00:19 amThe A7 (I have one) has a modified micro-USB on it (15 pin, called a 'MULTIPORT'). Power, LANC, trigger etc.. are all on this. All share the same ground(s). We use ours plugged into a laptop running an app and a seperate HDMI monitor for help with setup/focus. The camera effectively star earths the HDMI monitor and the laptop.If you have an external power adaptor it is likely to be floating - so for basic triggering (i.e. remote release, intervalometer) you wouldn't need isolation. But if you are are interfacing to something else (and the requirement for a few microsecond accuracy indicates this) then not a bad shout to have isolation. Of course only the OP knows why/what they are doing.There's power available (probably off the LANC) so you could build all manor of buffer for an opto or digital isolator. Just a matter of understanding the effort/result ratio.

author: gnuarm
author_rank: Super Contributor
timestamp: February 06, 2021, 12:18:48 am
content: Quote from: fcb on February 05, 2021, 07:43:04 pmThe OP has indicated he will try working with a transistor instead of an opto, so obviously the isolation is not the reason for initially trying the opto. He has said he is looking for the fastest trigger he can get. With the transistor he can get low going edges very fast or he can use a simple gate like a 74HC04 and get sub µs delays.This is one of the reasons why I hate designing a circuit without full information.QuoteI wish I had my drawings, but it is likely the circuit I used was active on the photo diode side of the opto. Still, until there is a clear need for an opto, no reason to consider the circuit further.