A Clean Output

[xenobie]

My dear friends,

I am currently designing a circuit whereby using 16 pins of a micro controller, I would like to trigger 64 outputs using the attached schematic. I plan to do this by using a matrix style connection (8 x . The schematic only shows this using 6 pins (3 x 3) otherwise it would have been too large to draw. The problem follows:

1. Apart from the resistor needed on the input of the opto to limit the current, what else do I need on the input side to protect the uC as some pins will be sinking current while others will be sourcing it.

2. I will drive everything using a 12V battery but the uC has a 7805v regulator. How do I continue the circuit from the output side of the opto in order to drive a mosfet using 12v? Can you provide a small schematic of how to wire the rest of the circuit from a single opto?

3. The switching from the uC must be very fast. Can you kindly suggest a reliable and robust opto coupler and mosfet? The current that the mosfet will be driving will not be more than 500mA but I do not wish to load the opto coupler.

4. Finally the most important thing, is that the output (of the mosfet) is very clean and that NO VOLTAGE whatsoever is present on the output unless the uC wants to. How do I ensure this life or death situation ?

Thanks for your help
Regards,
X

[xenobie]

I have a small update. A friend of mine just introduced Shift Registers to remove the grid thing.... any other ideas?

[Hero999]

1) Nothing.
2) What do you mean? It's an opto-TRIAC not a MOSFET coupler.
3) How about using a BJT? It's much easier. What frequency and duty cycleare you talking about?
4) Semiconductor outputs alone shouldn't be used in a safety critical situation. If the circuit is safety critical you need to build in some redundancy and monitor the outputs to verify they're in the intended state.

[xenobie]

Hey Hero,

Thanks for your reply.  Ok so basically I chose the wrong symbol when making the diagram. The current design is now slightly different.

I basically have the uC -> Shift Register -> Opto Coupler -> Transistor

The loads are all 12V 50mA so no need for large switching stuff but it is a safety critical application and I do need monitoring and the like. How do I do this please? Any ideas where I need to look at?

I will look for BJT in that case. When I said fast, I really meant fast response time. In fact in every application each output will only be used once. The system will then need to be reset.

Much Obliged.

[Hero999]

YOu haven't provided enough inofrmation.

What do you men by fast? <1ms, 1us, 1ns 1ps?

What level of safety you need? If it goes wrong, what's the worst that could happen? What's the SIL (Safety Integration Level)?

Safety engineering is a complex subject, here are a few links to get you started.
http://www.hse.gov.uk/research/hsl_pdf/2000/hsl00-01.pdf
http://www.gavazzionline.com/safety/pdf/generalsafety.pdf
http://4-sightconsulting.co.uk/Current_Papers/Determining_SILs/Methods_of_Determining_Safety_Integrity_Level.pdf

[xenobie]

Hi Hero999,

Thanks for your constant cooperation. There has been some development since the last post. So I intend to use an Arduino to connect it to a 74HC595 Shift Register. I will then use a PC817 Opto Coupler as this is fairly new, small and fast. I haven't thought about the output of the opto yet but most probably I will drive a mosfet to trigger the output. So for speed, that's settled.

Regarding the Safety of this project well basically it will be triggering explosives such that if an explosive is trigger abruptly, it might kill someone. I have given a lot of thought about safety and what I intend to do is to actually have a relay which supplies voltage to the output (the mosfets which I have yet to think about mentioned above) will only be switched when the system has the green light. Furthermore once the relay has been switched, I still cannot afford to have the outputs go off because even if all humans are clear of the area, I still need to have perfect timings.

To summarise the safety issue, I wouldn't worry about killing someone as the Arduino controller Relay will not supply voltage at all HOWEVER a clean output is a must. Not only this, I need to monitor the outputs to know if everything is going smoothly.

Thanks once again, any design ideas are greatly appreciated.

X

[Hero999]

It sounds like you should make the safety system manual, i.e. the power needs to be disconnected when there are people in the area.