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Pause Mod LED


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#51 Osgeld OFFLINE  

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Posted Sun Feb 17, 2019 8:27 AM

https://www.digikey....k=1&pageSize=25



#52 Ikrananka OFFLINE  

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Posted Sun Feb 17, 2019 8:44 AM

 

Yep - that's exactly what I found before.  The two switches I posted pictures of are both in that list.  I've used similar searches on DigiKey, Mouser, RS Components, Element14, switch manufacturer websites and local suppliers with nothing better coming up.  

 

Snap-in panel mount switches are problematic for two reasons, (i) the ColecoVision is 4mm thick which is too much for all of the snap-ins I looked at, and (ii) this then throws me back into looking for another way to mount my PCB.

 

I much prefer, in this case, a switch that has a circular threaded mounting using a nut to tighten.  Easy to drill a hole, easy to install and will hold my PCB in place.  The two switches I posted meet those criterion but I hate the 4mm thick bezel.  In my opinion, these two switches will look ridiculous on a ColecoVision.


Edited by Ikrananka, Sun Feb 17, 2019 9:09 AM.


#53 Ikrananka OFFLINE  

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Posted Sun Feb 17, 2019 9:08 AM

Well, a friend of mine would say "Use a knife switch."  He loves the mad scientist look.

 

On a more serious note, perhaps an analog switch IC could be used as a relay, so that the switch itself doesn't have to do so much.  For instance, an Intersil DG413 has 4 independent switches.  2 are normally closed, and 2 are normally open.  So by hooking the same input to all the switch "actuator" pins, you have a functional DPDT relay.  This would open up your switch choices to anything from SPST up.  This particular chip claims to come in a PDIP package too.

 

Knife switches are cool, in a bringing Frankenstein to life kind of way  :-D

 

Anyway, looks like we had similar thoughts (although I'm a lot slower than you).  I woke up early this morning and couldn't get back to sleep thinking about switches (how sad is that) and then the quad SPST HEF4066 that is in the ColecoVision sprung to mind.  So, in a round about way I came to the same conclusion as you.  Use a CMOS analog switch.

 

After some research this morning, I'm moving towards continuing to use my original Radio Shack (DPST) switch.  One pole of this switch will handle the audio on/off while the other pole will be the +5V on/off signal to a SPDT (or dual SPST) CMOS switch.  I'm currently favouring the MAX4544CPA+ which is an 8-pin DIP, seems to have good specs, and is cheap at ~$2 each.  This switch will be the one that activates the system pause and LED blinking.  I'll knock up a revised circuit diagram later today.

 

Am I right in thinking that CMOS analog switches will not work with momentary switching signals, i.e. the internal switches are not latching but will always revert to their "normal" position when the signal line drops low?


Edited by Ikrananka, Sun Feb 17, 2019 9:12 AM.


#54 ChildOfCv OFFLINE  

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Posted Sun Feb 17, 2019 12:14 PM

 

Am I right in thinking that CMOS analog switches will not work with momentary switching signals, i.e. the internal switches are not latching but will always revert to their "normal" position when the signal line drops low?

 

Well, there might be a latching switch or two out there, but the one you're looking at does not latch.  You'll need to use a pull-down resistor for when the switch disconnects though.



#55 Ikrananka OFFLINE  

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Posted Sun Feb 17, 2019 6:45 PM

Okay, here's an updated draft schematic with the MAX4544CPA analog switch included with an input signal pulldown resistor (R4).  I do have some questions about this which I'll put at the end of this post.

 

CV Pause Mod Schematic Draft 2.png

 

Updated draft PCB images:

 

CV Pause Mod Front Draft 2.png  CV Pause Mod Back Draft 2.png

 

Questions

  1. The MAX4544 has an max 60 ohm ON resistance.  Will this be a problem pulling the Z80 BUSRQ low?
  2. How do I calculate the value for the pull-down resistor?  I understand that I need to know the MAX4544 IN leakage current but it doesn't seem to be specified in the datasheet.
  3. With the current design I have the MAX4544 IN pulled high (+5V) during normal operation of the CV.  IN is only pulled low when the system is paused.  Is this okay or would it be better to have IN low during normal operation and only pulled high when the Z80 is paused.  Or does it not really matter?  I'm thinking about leakage past the pulldown resistor drawing unnecessary current.  Is it best to minimise the duration of this?  I am able to swap the IN high/low around by a manual rejig of the Radio Shack switch internals (which can be easily done if necessary).

Thanks



#56 ChildOfCv OFFLINE  

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Posted Sun Feb 17, 2019 7:26 PM

1)  60 ohms dividing the voltage with the 3.3K pull-up resistor will give you a 0.01V pull-down.  That ought to be enough.  Nobody else should be dumping amps into this part of the circuit--everybody on this line will only pull to ground.  If you connect it to WAIT instead, that's still only 0.02V.  Anything below 0.8 is a low.

 

2) 10K will work.  So would 1M probably, and that will keep your leakage low.  All you need for the resistor is for it to offer a little gravity.

 

3) Well, you don't really have a choice about how to wire the mechanical switch since "paused" has to be the same as "audio disconnected."  For the analog switch, though, the IN is such a high resistance that you don't need to worry about whether it sees a high or a low most of the time.  The IN itself measures leakage in nano-amps.  And either way the chip will be connecting the common to one of the switching pins most of the time.



#57 Ikrananka OFFLINE  

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Posted Mon Feb 18, 2019 9:48 AM

Fantastic - thanks ChildOfCv for all your help with this - I'm nearly ready to order parts to prototype and test.

 

I did a little more reading this morning and realised that I could switch things around and change from a pull-down resistor configuration to a pull-up resistor configuration.  So, in the latter case I take the +5V supply, put it through a pull-up resistor and tie that to IN on the analog switch.  IN is also then fed to my Radio Shack Switch which now simply connects that line to GND when closed (which it would be in normal operation).  The NO and NC lines are also swapped.

 

Is there any advantage to using one case over the other in this circuit? 

 

Pull-Down

 

Pull-Down.png

 

Pull-Up

 

Pull-Up.png



#58 Ikrananka OFFLINE  

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Posted Mon Feb 18, 2019 9:49 AM

3) Well, you don't really have a choice about how to wire the mechanical switch since "paused" has to be the same as "audio disconnected."

 

The nice thing about the Radio Shack switch is that it can be easily changed internally so that instead of operating with both poles being ON or both poles OFF, it can be made to work with one pole ON and the other OFF and vice versa.



#59 fdr4prez OFFLINE  

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Posted Mon Feb 18, 2019 5:23 PM

Instead of blinking, would be any easier to use a multi-color LED, so one color for ON and different color for pause?

#60 Osgeld OFFLINE  

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Posted Mon Feb 18, 2019 11:13 PM

or just use a blinking led hehe



#61 Ikrananka OFFLINE  

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Posted Tue Feb 19, 2019 8:12 AM

Instead of blinking, would be any easier to use a multi-color LED, so one color for ON and different color for pause?

 

 

or just use a blinking led hehe

 

I've already considered both of these options.  I thought about using a multi-colour LED but decided that personally I much preferred the idea of the power LED slowly blinking while paused.

 

As I want to only have a single LED installed, an off the shelf blinking LED won't work as it always blinks and can't be made to stay lit continuously.  I suspect that it was an off the shelf blinking LED that Yurkie used for his mod - but he then had a separate LED for power which isn't what I want.






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