Choosing an optocoupler for a digital interface with old -ve polarity -5V electronics

Question

This is an amateur application - 'adding auto control to a manual motor control circuit' using an optocoupler to bridge opposite polarity circuits.

The existing 20 yo motors x 2 (-12V) are controlled manually, forward and reverse, by push buttons. The push buttons switch -5V to motor control hardware (TTL presumably).

I am adding auto control of the motors from a +5V (ST-4 guide camera) source.

With help from Stackexchange experts, I have come up with a basic layout, below.

Switch off auto to use manual. The push buttons will be physically guarded and wont get much use. This might need some modification to lockout inadvertent activation of the PBs.

My questions...

I need to choose an optocoupler (there are 4 channels). The optocoupler must be capable of a similar output as the push buttons (which I presume is logic level) and fast enough to be responsive to the ST-4 pulse guide commands.

The circuit should not be overly complex, have few components and therefore be reliable - it's mission critical.

I have perused a number of likely datasheets and to my inexperienced eye the FOD0721 look promising. It's fast and has an appropriate voltage rating.

1. Is this a sane choice, overkill or just inappropriate for the application?

2. What should I be looking for in an optocoupler for this type of application?

^{simulate this circuit – Schematic created using CircuitLab}

This is the edited version based on discussion below.

^{simulate this circuit}

Answer 1

I'd suggest that since you are unsure of the voltage and current levels on your switches (and you recognize that you don't have a lot of experience with electronics) you should use relays to activate your pushbutton circuits.

There are plenty of small relay boards designed for microprocessors such as Arduino that would allow yo to simply solder in voltage free contacts across your existing pushbuttons.
For example for about $5 you can get a 4 relay board with opto-isolated inputs that will run from 5 V.

You might not need opto-isolation, and those variants are available too.

Certainly this would seem to imply the absolute minimum of modification to your Telescope circuitry.

Answer 2

The optocoupler must be capable of a similar output as the push buttons (which I presume is logic level) and fast enough to be responsive to the ST-4 pulse guide commands.

The FOD0721 is not a good option for emulating a push-button switch. Just use a standard opto-coupler which has an NPN bipolar transistor or MOSFET output (not Darlington or logic gates) and a good Current Transfer Ratio (eg. Sharp PC812). The opto-coupler is simply wired directly across the switch (in series with SW1, which I presume is for disabling it).

Don't worry about speed. Any opto-coupler will be much faster than a pushbutton. Mechanical switches 'bounce' for several milliseconds during operation, whereas the opto-coupler should switch cleanly in a few microseconds.

A standard opto-coupler should work just as well as a switch, so long as it can pass enough current with low enough voltage drop when turned on.

Answer 3

First your VDD2 is max 6 vdc, If you are going to send -12Vdc into the MOSFET I think it's not going to like it,
Second, are you sure of the motor current consumption. What king of motor runs on less then 10mA. 10ma is the max current that the FOD0721 can handle, potential problem here.
Is V2 the actual motor, or just the input control of a motor.
Why the 750ohms ?
If SW2 does the job directly to the motor, why using a 750ohms for SW1-FOD721 series ?
ThePhoton is right, Your FOD721 should be connected between SW1 and the R750, not from GND to SW1, this way you are shorting the TTL signal to ground. Or it could be connected from -5TTL to the SW1 switch then to the R750 then to the motor...

Please clarify some of those enigmas so we can better help you. Cheers...

Answer 4

I tested the circuit below on a breadboard (one side -ve polarity, the other +ve polarity and an ILCT6 bridging the gap).

A second look at the pushbutton wiring on the actual device confirmed ground side (low side?) switching.

Input was a basic Arduino digital pin HIGH LOW sketch. Emitter to ground (+ve) carried more current - brighter LED

Irrespective of emitter to GND or collector to GND, the output voltage did not vary - on my multimeter.

Having also run this circuit in a spice program, I added a small cap across the emitter collector to supress voltage spikes in the order of 3 - 4 volts.

Great learning experience.

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