Electrical help re: common earth

[EightBySix]

Hi,

I wonder if someone who is better at electronics than me explain something.

If you have (say) an iPac (gets a voltage and ground from the ps2 cable) and a coin mech that is also powered.
The coin mech needs to be connected to the iPac to send a coin signal.

Do they need to share a common ground?
Do I not then need to get the power from the same source as the iPac? Wont be possible if they are a different voltage - and there maybe wouldnt be enough current to drive both.

Probably missing something here.

[mcseforsale]

1.) Yes, you can chain the *SWITCHES* on the coin mech to a common ground on the iPac and their associated connectors for coin1 coin2, etc.  The iPac relies on the ground to see if the switch has been "clicked", or changed state, to tell the PC through the USB port that that associated switch was pushed.
2.) The *LIGHTS* on most older coin doors are usually around 14V.  The iPac can't power that...it's a different type of device and iPac doesn't supply power.  Indeed the iPac gets its power from the USB or PS/2 port of the computer which is usually only 5 or 5.5V and about 500mA. 
3.) Go to Autozone (or your neighborhood auto parts store of choice) and get some 12V bulbs that fit the sockets that you are using on the coin door (my particular door took blade type bulbs, so I got some LED blades off Amazon).  Then, wire *ONLY* the wires from the bulbs to a power header from your PC power supply on yellow/black.  Incidentally, on a PC power supply, where you have a connector for a HD, or floppy (usually labeled P1, P2, etc), yellow is 12v+, black is ground and red is 5.5V+.

Here's an explanation of the pinouts on most modern ATX power supplies:
http://www.scary-terry.com/atxps/atxps.htm


Hope this helps.

AJ

Hi,

I wonder if someone who is better at electronics than me explain something.

If you have (say) an iPac (gets a voltage and ground from the ps2 cable) and a coin mech that is also powered.
The coin mech needs to be connected to the iPac to send a coin signal.

Do they need to share a common ground?
Do I not then need to get the power from the same source as the iPac? Wont be possible if they are a different voltage - and there maybe wouldnt be enough current to drive both.

Probably missing something here.

[EightBySix]

Thanks for the detailed answer!

I was trying to generalise though - looks like I got caught out!

Specifically, I've made a coin mech with its own micro controller, that has its own power source and ground. It currently lights an LED for testing, and I want that signal to go to the iPac instead.

I'm thinking they would both need to have reference to a common ground. 

[mcseforsale]

Ahh.  I understand.  I'd check the references over at ultimarc.com.  I'm pretty sure there's a way to power a *FEW* LEDs with it, but I've never done it.  As long as your LEDs are 5V and you wire them to the right polarization, I don't see a problem.  I'm guessing the switch and LED are on different circuits, no?  Or, how about this, an optical coin switch where the LED is on and you have a photoreceptor watching it and when a coin passes by, it switches.  But, then you'd have to do it optically with something like an optiwiz...

hmmm...

AJ

Thanks for the detailed answer!

I was trying to generalise though - looks like I got caught out!

Specifically, I've made a coin mech with its own micro controller, that has its own power source and ground. It currently lights an LED for testing, and I want that signal to go to the iPac instead.

I'm thinking they would both need to have reference to a common ground. 

[mcseforsale]

Here's the answer:

LED HARNESS: (Optional). The LED harness plugs onto the 10-way header on the board. The LEDs are connected to the harness via a removeable plug. This can be disconnected to allow the LEDs to be mounted through a panel. Ensure to note which way round the plug is fitted to the LED. The LEDs can be tested by pressing caps lock, scroll lock and num lock on a pass-thru keyboard. If you have connected the I-PAC to the PC using USB, in mame.ini change "led_mode" from "ps/2" to "usb"

From here:
http://www.ultimarc.com/ipac2.html

AJ

[rCadeGaming]

+1 with everything above.  Just be sure you understand the ATX pinout and what you're connecting.

Also, it won't hurt to add extra ground connections between components just be safe.  In audio/video connections this is usually helpful to reduce hum from potential differences.  Just be sure you're connecting to a common ground point on each component.

EDIT:  Just saw you latest post while typing this.  Whatever button connection you're using on the I-PAC, your coin mech needs to connect it to ground to indicate a coin, NOT voltage.  No connection = button not pressed, connection to ground = button pressed.

EDIT 2:  Can't keep up.  As long as the computer rail is providing enough current (amps) you can probably power the LED's through the IPAC or directly from the PC, whichever you prefer.  You might want to place a resistor in series with the LED to restrict the current flowing through to only what's needed to light it up.  Otherwise, it's nearly a straight connection to ground, and other things like the I-PAC may lose power.  I don't know if the I-PAC's LED connection is regulated for this, but it shouldn't hurt anything to add a resistor and be safe.  When calculating resistor size, keep in mind the forward voltage drop through the resistor.

[EightBySix]

Hmm, still a bit of confusion - the LEDs are a bit of a red herring here.

My 'coin-o-matic' in the link above (there is a vid, a few images down) sets a high signal when a coin is detected (just happens to light an LED so I could test it). Sounds like i need it to pull the line low instead to connect to the iPac.

So as long as the grounds are connected, they can each have their own voltage?

[mcseforsale]

OK.  Sorry, I didn't see the link.  Can you program the PIC to send the correct number of pulses out to a lead on the iPac (to the coin1)?

AJ

[rCadeGaming]

My 'coin-o-matic' in the link above (there is a vid, a few images down) sets a high signal when a coin is detected (just happens to light an LED so I could test it). Sounds like i need it to pull the line low instead to connect to the iPac.

Exactly, and there will be voltage coming out on the iPac's button wire, so make sure that's not feeding into your circuit; it should just go straight to ground.

So as long as the grounds are connected, they can each have their own voltage?

Yes.  The voltage in the two should be completely isolated from each other.

[EightBySix]

Can you program the PIC to send the correct number of pulses out to a lead on the iPac (to the coin1)?

Yep - thats how I wanted it to work. Put a pound in and you get 10 goes, or 5p + 2p +2p + 1p = 1 credit. You get the idea 

Thought it would be a good place for all the spare change in my pocket, and teaches the kids maths too ;-) When they get older, I'll set it to - say - 1 credit = 8p to help them with their times tables. Muahahaha.

Yes.  The voltage in the two should be completely isolated from each other.

Cool. thanks everyone.