Using an optocoupler to connect insulated rail to input pin of shift register

For whatever reason I can't read your crude docx but in any event some comments:

- You only need to drive an optocoupler with, say, a few mA.  A 10V AC source thru a 330 ohm resistor into a ~1V optocoupler is overdriving the optocoupler if the output is driving a low-current digital input to a shift register.  That's a peak current of ((10V x 1.4) - 1V) / 330 ohms = 40 mA into the optocoupler.   A 1k resistor is more than enough.

- If you're driving any "modern" digital input, you don't need to pull it up with 10k.  You can lower current by 10x with a 100k pullup resistor.  10k pullups are a holdover from the last century with bi-polar (TTL) inputs.  Anything digital today is CMOS which has negligible input currents.

- By using 100k (instead of 10k) you can use, say, a 0.1uF capacitor across the input to smooth out the AC ripple.  Yes you can "debounce" or smooth the ripple in software but that's just silly if a 1 cent capacitor can do the same.

- Then, if you put that filtering cap on the input you can use a single-ended optocoupler rather than an AC input optocoupler.  AC input optocoupler can run you 2-3 times the cost and are fewer choices.  Of course if you already have a bag of AC optocouplers then never mind.  By using a capacitor as described above it will still filter the half-wave output from a single-ended optocoupler so your digital input still sees a steady DC voltage (no software debouncing).  Then just put in a 5 cent diode on your 10V AC source to feed all your single-ended optocouplers with half-wave DC (so as not the apply reverse voltage to the optocoupler).

Here is a circuit I use to generate a clean logic signal to a shift register from an insulated rail:

Note that I use an RC debounce and feed the signal into a Schmitt trigger buffer to generate a clean transition from a noisy signal. This takes care of the AC ripple as well as intermittent contact between the wheels and the third rail.

Debouncing and filtering could be performed in software, but I originally built the circuit for a microcontroller that had very little memory or processing power, so software debouncing wasn't an option.

Stan,

Thanks for the advice. I wasn't sure how much current the optocoupler needed. I will change the resistor on the input to 1k and the pull up to 100k. I already have some AC optocouplers, so I think I will stick with them. 

Professor Chaos,

Your circuit diagram was very helpful. Do you have any recommendations for a specific part that I could use for the Schmitt trigger. I would prefer a chip with 6 or 8 channels because I have several switches to control. I am a novice, and I'm having difficulty deciphering all the specifications and deciding which part to buy.

John

SN74HC14N is a suitable hex Schmitt trigger.

GUNRUNNERJOHN,

Thanks for the parts information.

I used the 74HCT7541, which has 8 lines, but I'm not sure it's still available as a through-hole part - only surface mount.

My sensor board integrates 32 of those sensor circuits and feeds them into daisy-chained shift registers, so I can read 32 sensors with the SPI interface of the Arduino:

The DIP package of the 74HCT7541 is no longer produced.

I would rather have the through-hole part, so I think I will just order the chip GRJ suggested.

Professor, that's an impressive board that you put together. I'm guessing that there are companies that will make custom PCBs. I may have to look into that so I won't end up with a rat's nest of wires.

GRJ found a good place for getting circuit boards made very inexpensively, but I can't recall the name just now. It was 2 words I think. Maybe he will be by, I'm sure he remembers. Or I will hunt for it tomorrow, I have it here somewhere.

The place is OSH Park PCB Fab WebSite, but note they don't assemble, just make blank boards.  If you want assembly, you need to build enough to make it cost effective, the tooling almost any board starts at $200 before you get any boards, then there's a per/board assembly charge.

Finally have a moment to get back here, sorry for the delay.

Using the optocoupler from track power as described above should work out just fine, but I'm a little concerned if it would work properly under varying track voltages.  Under command operation it wouldn't be a problem, but in any case I prefer a different method.  

I would use a separate DC power source with its ground connected to the outside rail bus of the layout.  When the insulated rail is bridged by passing wheels it would complete the circuit to allow that DC source to power the optocoupler.  This would insure that the LED in the opto is fully turned on and not over driven.  

Something else to note on the output stage is that you will need to use transistors or some other driver between the shift registers and relay boards or LEDs.  The 74HC595 can supply 25mA on one output, but can only supply 70mA total, so if more than a couple outputs are turned on it will over drive the chip if outputs are directly driving relays/LEDs.  

I looked over the sketch posted in the other thread, and if it does the job that's needed it's great.  It is massively less complex than the sketch I have going.  I have some concerns about what is going on there, but will need to test it out with hardware to see  exactly how it is working and if my concerns have any merit.  

The sketch I have going reads all inputs, stores their values, then decides what to do with them before sending out all data to the outputs.  All this makes for a pretty inefficient program, so I'm hoping that what you have will actually get the job done.  

It isn't really a problem yet, but also just wanted to make sure you're aware that the EEProm in the Arduino has a limited life.  Under normal conditions as used in your sketch it should last many, many, years, but under testing conditions it's life can be quickly expended, so just be aware of that and make sure your code doesn't write to the EEProm any more than it needs to.  

Anyway, that's about all for right now.  

JGL

JGL,

I hope you don't mind that I am working on this at the same time as you. I initially wanted to learn enough so that I would understand what you are doing, but I got interested in the Arduino and decided to go ahead and take a stab at the program. I'm also becoming comfortable with the components so you won't have to hold my hand through the wiring process.

I have an unused fixed 10V post on my transformer that I plan to connect to the optocoupler.  Do you think that would be okay? It would be easy to switch to another source at this point. 

Here is a link to the relay board that I purchased for testing. It uses a separate 12v power supply to run the relays. The relays are activated by a LOW signal from the shift register, and I read somewhere that this only uses 3 mA of current.  I can also power the indicator lights through a relay to reduce current flow through the shift register. Let me know if you think I need the transistors with this setup. I am going to need some help on the transistor circuit if you think it is necessary.

I was aware of the memory issue with the Arduino. I'm trying to be careful while testing, but I will probably shift the storage to a different memory location once I've finished all the testing. The code is written so that it only writes to memory when a switch is thrown and only reads once in the startup.

Let me know if you need an explanation of anything in my sketch. Here is a basic outline of the flow of the sketch:

1. Read all inputs from shift registers (assumes 4 switches to each register, each switch on 2 consecutive pins)

2. Check to see if a timer is running for each input (a timer running indicates that power has already been turned on to the switch coil and led)

3. If the timer has been running less than 1 second then maintain power to the switch coil

4. If the timer is between 1 and 3 seconds then remove power from switch coil (this keeps down chatter from the insulated rail)

5 After 3 seconds reset the timer to 0 unless the input indicates that the insulated rail section is still occupied or a button is being held down. (This way the sketch will keep checking every loop and reset the timer to 0 once it no longer senses a signal from the insulated rail.)

6. If no timer is running then if a signal is present at an input, turn on power to switch coil and led. Also turn off power to opposite led and start timer.

7. Send all outputs to shift registers. ( 2 output registers for each input register. 1 for switch coils and 1 for led's)

I just want to update everyone on my progress. Today I had a successful test on my layout controlling a total of 8 inputs and 16 outputs (8 switch coils and 8 indicator lights). Here is a picture of what I have now:

It's a jumble of wires now, but I found some solderable breadboards that have the same layout as the larger solderless ones I used. I will be able to fit enough components to control 8 switches (16 inputs/32 outputs) on 3 boards. I should also be able to clean up some of the wiring and add screw terminals for connections to the layout.

Thanks to everyone for your help. You gave me a lot of great ideas and kept me from making several mistakes.

Looks like you have made great progress! This is an interesting project. Any chance you could post a wiring diagram and final details when you are finished? The final parts you used would be good too!

RTR12,

For the first part of my project I used the circuit proposed by Professor Chaos. 

I only made a few small changes. I am using 10V AC Hot as an input so I changed the resistor to 1k from 1.5k. The arrow going to AC common leads to an insulated rail section and a momentary push button on the control panel. The AC is connected to this optoisolator , and this is the Schmitt trigger that I used. It is slightly different from the one drawn above because it inverts the output signal. (When the button is pressed or the insulated rail connected, the signal going into the Schmitt trigger is LOW and the signal coming out is HIGH). I had to use this part because I could not find a non-inverting one with through hole mounting. The Schmitt trigger is connected directly to an input pin of a 74HC165 parallel in shift register.  My Arduino sketch requires that the 2 inputs (straight and diverging route) on each switch are connected to adjacent pins on the shift register. I plan to chain 6 shift registers together on one board so I can handle a total of 24 switches. The basic purpose of the Arduino sketch is to make sure that power is only applied to switch coils for a short period of time even if a train is parked on an insulated rail. The sketch also maintains power to the appropriate indicator light and will remember the switch positions even after power is turned off.

The shift registers are connected to the Arduino by 4 wires. The connections are described in the comments at the top of my Arduino sketch. The sketch outputs data to twice as many 74HC595 parallel out shift registers. These are connected to the Arduino by 3 wires. The data is output in the following order: input from the first register (closest to Arduino in chain) is sent to the last (farthest in chain) output shift register. This register controls the relays for the switch coils. The next to last output shift register controls the relays for the led's. The data output continues in this alternating pattern of coil / led control.

I am using this relay module to receive the output from the shift registers. It does not come with instructions, but the only required connections are the 12v dc power source (2.1mm plug), a 5v connection from the Arduino, and one wire from each shift register pin to a numbered pin on the relay board. John Galt Line suggested using transistors to power the indicator lights instead of the relays. I ordered a few transistors, and I am going to experiment with them. At first glance it seems like they would be less expensive and take up less room.

I am planning to build permanent boards to control 8 switches and try them out on my layout for a few weeks before continuing. Let me know if you have any questions about the details.

John

Wow, that was a great description of EVERYTHING, Thank You! I need to do a little studying now and look at the data sheets for the devices. I am getting a little better at figuring those out. Also, I think I may have a couple of the parts already and I might try to breadboard this myself. You certainly picked up the Arduino programming very quickly, as well as ALL of the other stuff too. I would agree that the transistors that JGL suggested would make for a smaller package when finished.

The only thing that didn't come through was the picture, it says 'Image Not Found'. I am guessing that it was the same picture that Prof Chaos posted above? If so I will just use his schematic.

Thanks again for taking the time to explain all this and with good parts links too. I am guessing there will be others interested in this as well. Great project and you got is all figured out pretty quickly too. 

Please keep posting your progress and how your testing goes.

Thanks again for the update on the project with all the details!

RTR12,

Yes. The missing image is the same as the professor's diagram. One thing that helped me was to put each component on the breadboard individually to make sure that I knew how it worked. I then started combining components and testing the results. The optocouplers and Schmitt triggers both have pretty simple wiring schemes, so they were pretty easy. The shift registers were more complicated, and I was glad I spent some time experimenting with them.

Ok, thanks. I will follow your lead and do it the same way. I usually have to stop and think or re-read things a few times for stuff like this anyway. I have used a decade counter chip, but have not done anything with shift register chips before. Thanks for all your help.