Pause Mod LED

[ChildOfCv]

This is my latest. I've added the existing +5V to BUSRQ via a 3.3K resistor for context, as well as showing the full DPDT switch with audio. The switch is shown with the system in normal operation (i.e. not paused).

 

Thoughts?

 

Pause Mod.png

 

The schematic here ought to work.

 

The main reason that the LED would look "on" when connected to WAIT is that internal components are always pulling WAIT down and therefore activating anything with a +5V supply (such as the proposed LED). If it weren't for the wait state generator or the sound chip, you'd be able to use your original circuit with no fuss.

 

Using BUSRQ should allow this to work as is though, since nobody else is always pulling it down.

 

Since BUSRQ is a high-impedance input and does not supply voltage of its own, I don't think you'll need the diode though.

[Ikrananka]

 

The schematic here ought to work.

 

The main reason that the LED would look "on" when connected to WAIT is that internal components are always pulling WAIT down and therefore activating anything with a +5V supply (such as the proposed LED). If it weren't for the wait state generator or the sound chip, you'd be able to use your original circuit with no fuss.

 

Using BUSRQ should allow this to work as is though, since nobody else is always pulling it down.

 

Since BUSRQ is a high-impedance input and does not supply voltage of its own, I don't think you'll need the diode though.

 

Thank you. I'm going to order some parts and should be able to test this in a week or two. I'll report back then.

[Ikrananka]

I just did a test with pausing my CV via a standard 1N4004 diode and it paused fine (both via WAIT and BUSRQ). The 1N4004 has a forward voltage of 1.1V while a Shottkey is around half of that. Regardless, seeing as the system paused via a 1N4004 diode doesn't that mean that the diode doesn't really need to be a Shottky? My interpretation is that the Shottky would be used to ensure that the WAIT or BUSRQ were drawn down low enough to be activated and that if the forward voltage across the diode were too great then this may not happen. But seeing as a forward voltage of 1.1V doesn't seem to be too high then a Shottky is not necessarily warranted.

 

Or am I missing something?

[SiLic0ne t0aD]

Sorry I couldn't help more but I thought the diagram and pics Parkfun showed had an LED added to it as well but guess I was wrong.

 

I'm not sure exactly what switch you're using but with the NKK branded switch I used (like Doubledown and Grips had), it was super easy, since it already had a built-in Led and resistor. I simply tapped into 5v and a ground somewhere up front on the board and the Led works as it should.. It lights up when engaged and is off when not pressed. I wish I still had pics but I did all this a while ago and they're long gone, lost on an old phone I had. I'm sure you'll figure it out though. good luck!

[ChildOfCv]

Okay, I'm dense. I now realize the rationale for the diode: If someone other than the switch pulls the line low, you don't want it turning on the LED. The diode in the circuit for just your pause mod (but nothing else) will do just that, even with other components on the other side of that diode tugging on the line.

 

On the other hand, it might be entertaining to watch the LED pulse with the music as you play Frogger

[Ikrananka]

Okay, I've now taken a rather different approach to what I was trying to do before. I have always intended adding a power LED mod to my CV as well as the pause mod LED so this would have meant two LEDs being added. One LED would stay continuously illuminated when the system is turned on, while the second LED would blink on/off when the system was paused via my added DPDT switch. So, I got to thinking that it would be far more elegant to only have a single LED to do both jobs. The single LED would stay continuously illuminated when the system is turned on and NOT paused. When the DPDT switch is thrown, and the system is paused, the same LED would start to blink.

 

It's taken me some time to try and figure this out but I believe I'm more or less there. I'm using a 555 timer in one of the standard astable circuits that are readily available by Googling around. The standard astable 555 circuit makes the LED blink, but my challenge was how to make the LED stay on when the DPDT switch is in one position and to blink in the other. I believe this can be accomplished by feeding +5V to one pole of the switch and GND to the other pole. In normal operation the +5V goes to the GND pin of the 555. This forces the output from the 555 to stay high and for the LED to stay lit. A diode in the line to the Z80 BUSRQ prevents this +5V from interfering with it. When the DPDT switch is set to pause, then both the BUSRQ and pin 1 of the 555 are routed to GND. Therefore the CV system pauses AND the 555 goes into it's normal operating state causing the LED to blink.

 

I've only tested this using an online breadboard simulator and have not optimised the values of the resistors and the capacitor to get exactly the right blinking frequency etc. However, I wanted to throw this up here to see what those who actually understand electronics think of my suggestion.

 

Edited February 12, 2019 by Ikrananka

[ChildOfCv]

Okay, I've now taken a rather different approach to what I was trying to do before. I have always intended adding a power LED mod to my CV as well as the pause mod LED so this would have meant two LEDs being added. One LED would stay continuously illuminated when the system is turned on, while the second LED would blink on/off when the system was paused via my added DPDT switch. So, I got to thinking that it would be far more elegant to only have a single LED to do both jobs. The single LED would stay continuously illuminated when the system is turned on and NOT paused. When the DPDT switch is thrown, and the system is paused, the same LED would start to blink.

 

It's taken me some time to try and figure this out but I believe I'm more or less there. I'm using a 555 timer in one of the standard astable circuits that are readily available by Googling around. The standard astable 555 circuit makes the LED blink, but my challenge was how to make the LED stay on when the DPDT switch is in one position and to blink in the other. I believe this can be accomplished by feeding +5V to one pole of the switch and GND to the other pole. In normal operation the +5V goes to the GND pin of the 555. This forces the output from the 555 to stay high and for the LED to stay lit. A diode in the line to the Z80 BUSRQ prevents this +5V from interfering with it. When the DPDT switch is set to pause, then both the BUSRQ and pin 1 of the 555 are routed to GND. Therefore the CV system pauses AND the 555 goes into it's normal operating state causing the LED to blink.

 

I've only tested this using an online breadboard simulator and have not optimised the values of the resistors and the capacitor to get exactly the right blinking frequency etc. However, I wanted to throw this up here to see what those who actually understand electronics think of my suggestion.

 

Pause Mod 555 v1.png

It's kind of weird that this works. When the 555 is not grounded, I would not expect any output at all, and at least by a "legal" design of the 555 type, it's possible that some brand won't work this way.

 

I would keep the GND always grounded. Instead, hook the base of the capacitor to the switch. When the switch is off, the trigger and threshold will stay high, and it will be in a "monostable" configuration with the LED on. When the switch is on, it goes into astable configuration.

 

So I *think* it will work to replace the connection at the DPDT with C1 lower pin, and GND to straight ground. The +5 to the upper part of the switch shouldn't be necessary, but of course it still needs to be connected to the resistor series and the VCC.

 

Try that setup out.

[Osgeld]

disconnecting grounds is a sure fire way to get head on desk syndrome (at this point me personally would have said screw it and used a microcontroller)

[ChildOfCv]

Well, the feather in my cap is ruffled and deformed....

 

My last suggestion probably falls flat. Instead, try this:

 

 

555 101:

 

The timer is basically a flip-flop, two op amps, a resistor network, an inverter buffer, and a discharge transistor. The resistor network provides two threshold points: 1/3vcc and 2/3vcc. The "Threshold" input is connected to the op-amp that references the 2/3 mark. When the voltage climbs higher than that, it flips the flip-flop and activates the discharge transistor, which sinks that DIS pin to ground. At the same time it causes the output to go to VCC. When the voltage on the "Trigger" input drops below 1/3vcc, it flips the flip-flop which turns off the discharge and pulls output to GND.

 

So for astable operation we set up an external resistor and capacitor network around those principals. As the capacitor charges, it goes through R1 and R2 to charge C1. When the voltage at C1 reaches 2/3vcc, DIS pulls it towards 0 and it discharges through R2 only, until it reaches 1/3vcc and then restarts the cycle. So total charge time once established is 1/3(R₁+R₂)C seconds, while discharge time is 1/3R₂C.

 

So this circuit's intent is to pull the trigger and threshold to 0 while not paused. This keeps the output pulled towards ground, which pulls power through the LED from +5. So, always on. Then when you switch to pause mode, it allows the 555 to work in astable mode since it's basically disconnected from the pull-down.

 

So as Bullwinkle says, THIS time for sure!

 

PS: This setup also removes the need for a diode.

 

PPS: It's actually not that odd that the 555 sorta works without a ground. I remember a youtube video from Dave of EEBlog a while back where he demonstrated that a microcontroller can still do a blinky LED thing without all of its proper power and grounds. It was a bit weaker, and most likely it won't be reliable, but it was at least functional. Note that he was *not* recommending such a setup, but was pointing out that it could lure you into a false sense of security just because it "works".

Edited February 12, 2019 by ChildOfCv

[Ikrananka]

So as Bullwinkle says, THIS time for sure!

Thank you for taking such a hard long look at this, I can't thank you enough.

 

I tried your proposed circuit in two different simulators with the same result. When the switch is set to "pause" the LED does indeed blink as expected. However, when I set the switch to the other position the LED stays off. Here's a schematic from EveryCircuit.com showing that most of the current passes through the 555 to ground with some passing via the resistors. While threshold and trigger are indeed pulled low, this doesn't seem to pull the output to ground as you expected.

 

Edited February 12, 2019 by Ikrananka

[ChildOfCv]

Perhaps I got my output sense backwards. Check the voltage at "out". If it's high, then just return to the original LED to ground configuration.

 

Edit: After taking another look at the 555 internal schematic, output should be high while TRG is pulled low, and your simulation agrees. So yeah, output to LED and resistor to ground should work.

Edited February 12, 2019 by ChildOfCv

[Ikrananka]

Perhaps I got my output sense backwards. Check the voltage at "out". If it's high, then just return to the original LED to ground configuration.

 

Edit: After taking another look at the 555 internal schematic, output should be high while TRG is pulled low, and your simulation agrees. So yeah, output to LED and resistor to ground should work.

 

Yes, switching to the original LED to ground configuration works - THANK YOU. Now to decide upon what flashing frequency I want.......

 

BTW - is adding the capacitor to pin 5 (Control Voltage) necessary in this application? On one of the 555 datasheets it shows astable operation with pin 5 not connected, but does state that "Decoupling CONT voltage to ground with a capacitor can improve operation. This should be evaluated for individual applications". How does it "improve operation"?

[ChildOfCv]

Pin 5 is for applications that require more stability in the oscillator. It's like a decoupling capacitor but only for the internal voltage divider. If you use one at all, it doesn't need to be a big one.. maybe 10nF like most of the decoupling capacitors on the motherboard. I suppose you could also use it as the trigger voltage reference if you wanted to control that directly, but for this application it's not an issue.

 

The datasheet does beg and plead for power decoupling "to protect associated circuitry" though. One 0.1uF capacitor and a 1uF capacitor in parallel between Vcc and GND (both low ESR for best results). Place as close to the 555 as possible for best results. This is not a functional requirement either, but could potentially help with ruggedness.

 

For RC combinations, the TI LM555 datasheet has a nice graph (figure 16) of resistor vs capacitor vs frequency values. Using a relatively low value for the upper resistor R1 will give you a near 50% duty cycle if that matters to you. For example, R1=1K, R2=1M, C=1uF would give you a frequency just a little faster than a blink per second with about 50.02% duty cycle.

Edited February 12, 2019 by ChildOfCv

[SiLic0ne t0aD]

Thanks for this info.. I might have to do this to mine as well.

I've always wanted the Led to blink like Yurkies mod but was never exactly sure how to do it.

[Ikrananka]

Thanks for this info.. I might have to do this to mine as well.

I've always wanted the Led to blink like Yurkies mod but was never exactly sure how to do it.

 

I'm now working on putting together a very small PCB design for this which I'll make available here when it's ready. Probably sometime over the coming weekend.

[SiLic0ne t0aD]

Outstanding! Thank you. I'd definitely buy a couple if you end up selling them.

[Ikrananka]

Here is the schematic I'm now using. I've switched to a red LED (5mm) as I think it's more stereotypical for the ColecoVision era (e.g. Commodore 64) and so have adjusted the LED resistor value accordingly. I have also selected the values of R1 (150) and R2 (1.3M) to blink the LED, when the CV is paused, at a rate of around 0.5Hz.

 

 

There is a reason I have two ground connections, one is an input for the circuit and the other is the ground connection to the switch.

 

As for the circuit board, I agonised about where to put it and how to fix it inside the CV and eventually landed on a location which I'll talk about in a moment. I'm basing my design on using the same Radio Shack toggle switch (#275-0015) that Yurkie used in his pause mods. The first picture below shows one I did myself a few years ago with my preferred location for a 5mm power LED. With these switch and LED locations I thought it would be a good idea to try and tuck the circuit board directly underneath the switch and LED in the location shown below outlined in yellow. The circuit board will rest on the raised plastic that borders this area on three sides and a plastic spacer will be located under the board. The idea being that the nut that clamps the switch in place will clamp against the PCB holding the switch, PCB and LED firmly in place. The spacer underneath will provide support to the PCB allowing the nut to be nice and tight without stressing the PCB. When installing all of this the LED will need to be mounted to the console first and only soldered in place once the PCB and switch have been put in place.

 

 

My first draft PCB design is shown below.

 

 

The large hole is for the switch to pass through. The white circle around the hole is the extend of the nut that will hold everything in place. I plan on using a 4-pin locking Molex connector for the inputs.

 

As usual comments and/or suggestions are welcome (this is more or less the first PCB I have ever designed and I really don't know if I'm making mistakes).

Edited February 15, 2019 by Ikrananka

[ChildOfCv]

Neat. You should also include a paper template for people who have not yet done either the LED or pause mods, so they can accurately drill the holes.

[Osgeld]

if your doing a pcb why not ditch the wires all together

[Ikrananka]

Neat. You should also include a paper template for people who have not yet done either the LED or pause mods, so they can accurately drill the holes.

 

While this is mainly a personal project, I will get some extras made for those who want them. I already have a template from my prior power led mod (the one in the photo) and I'll include that for anyone that wants to buy a board.

 

I plan on getting the first test boards (3) from Osh Park. Any reason not to use Osh Park for any subsequent boards? Alternatives?

[Ikrananka]

if your doing a pcb why not ditch the wires all together

 

There will always be two wires needed as input to the mod from the CV mainboard, i.e. +5V and GND. Then there's the two audio wires from the mainboard to cut off sound when paused. So there will always be four wires.

 

The other two wires, between the switch and the PCB, will be short and I guess could be eliminated if I could source a suitable through hole soldered switch. However, as a hole needs to be cut in the ColecoVision case for the switch, my preference is for any switch to have a bezel to hide any defects in the hole that has been cut. I've searched both the Digi-key, Mouser and NKK catalogs for alternatives to the Radio Shack switch without success. My criterion just don't seem to be met, i.e. bezel, DPDT, not-illuminated, black, ON-ON, easily available, appropriate termination style and is aesthetically complimentary to the CV.

 

Of the PCB mountable switches that were close, none included a bezel and none were panel mountable. So, I'd have to find some other way to physically mount the PCB and this mounting would also have to provide the support and mechanical strength behind the switch, even if the switches were suitable.

 

Now, I am a newbie at this so if I'm missing something I'd love to hear suggestions on alternative switches.

Edited February 16, 2019 by Ikrananka

[Osgeld]

put slots in the board and solder to switch after its mounted

 

edit I guess there that plastic separator on the switch which would not allow this

Edited February 16, 2019 by Osgeld

[Ikrananka]

put slots in the board and solder to switch after its mounted

 

edit I guess there that plastic separator on the switch which would not allow this

 

I could cut the plastic separator off.

 

But, mounting the PCB to the switch this way will move the PCB at least half an inch away from the top of the console meaning that it is essentially floating in mid air. The only mechanical support/mounting the PCB would get is from the solder joints to the switch (5 off) and to the LED (2 off). I really don't think that relying on solder joints to "mount" the board is a good idea. Although I do like your idea if it could be made to work.

[Ikrananka]

Agh - I'm now searching for a new switch. The Radio Shack one I have and wanted to use cannot be modified to work as an on-on DPDT switch. It's sold as a DPST switch and it could be modified so that one side could be off with the other on and vice versa (still technically single throw though). I thought I'd be able to add contacts from spare ones to make it a double throw switch but unfortunately the metal toggle in the switch is designed for single throw use only. So, for the switch I'm back to the drawing board.

 

I want something that is plain black, discrete (low profile) and mounts in circular holes (for ease of DIY installation). I've struck out on pushbuttons with only a few industrial ones being remotely suitable and at a high price. Lots of momentary DPDT to choose from but latching on-on seems to be a problem to find with pushbuttons. So, I'm back to rocker switches and have the following two options so far. What do you think? I really would like something that is more discrete - the Radio Shack switch that I now can't use has a bezel that is only 2mm thick.

 

Round (~$4 each)

 

Personally, I'd go for the unmarked plain black and not the one with markings on it. Downside is that the bezel is 4mm thick - so not very discrete.

 

 

Rectangular (~$6 each)

 

I'm not that keen on this one as I can't find it without markings and the bezel is thick at 4mm - so not really very discrete.

 

 

 

 

 

[ChildOfCv]

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.