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I have a homemade arcade controller which uses a USB gamepad board. I would like to add some EL wire to it such that when a specific gamepad button is pushed it lights up the EL wire.

The Gamepad circuit has one wire per button. When the wire is floating the button state is up (as in NOT pressed), when the wire is connected to ground the button is down (pressed). I don't remember what that is called but basically the button is a simple pushbutton switch that connects the two sides together (3.3 V and GND). When the button is down, very little current is actually flowing through the wire.

The EL wire inverter is operated via a more simple on/off switch. When the switch is closed, whatever current the EL wire is using will be flowing through the switch.

So all I know is I can't just connect the two grounds together and the other sides of the two switches together and control both inputs with a single pushbutton.

I think I need a solid state relay or transistor or transistor network or something where a single switch closure causes the other two switches to close and they can operate at different voltages and currents. But I don't remember much about transistors and solid state relays seem really expensive and are targeted at high voltage / high current applications.

Help?

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  • \$\begingroup\$ Nope. It's just a black box to me, when I connect a battery it draws 107 mA and the wire lights up. \$\endgroup\$ – EEdiot Mar 14 '13 at 3:38
  • \$\begingroup\$ What voltage is your EL driver input? \$\endgroup\$ – Oli Glaser Mar 14 '13 at 3:42
  • \$\begingroup\$ At least 3V. I may go with a 4.5 V driver for a brighter glow. \$\endgroup\$ – EEdiot Mar 14 '13 at 14:35
  • \$\begingroup\$ If you change to a higher voltage for the EL driver then you need to use one of the last 2 circuits. \$\endgroup\$ – Oli Glaser Mar 15 '13 at 7:07
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Depending on your EL driver box input voltage, you have a few options. All are based on usign a simple P-channel MOSFET as a switch. Note that all values and part numbers are just guidelines, any general purpose NPNs will do, similarly any general P-ch MOSFET will do (as long as it has a low enough threshold voltage, e.g. < 3V or so, and can handle the maximum EL box current easily - check the rating)
Also resistor values are not set in stone, you have considerable leeway with the values here also (the 2.2kΩ R_PULLUP is a guess at the gamepad pullup)
For example, R1 could be anywhere between, say, 10kΩ and 1MΩ (the higher the impedance the more prone to noise, the lower, the higher power consumption and the harder for the transistor in the last circuit to pull down)

EDIT - Okay, now we know the voltages, then we can settle on this circuit. I will leave the rest of the circuits below in case they are of any use to future readers.

This assumes that the EL wire driver supply is always a bit lower than the gamepad supply:

EL Wire 3V circuit

Make sure you choose a MOSFET with a low Vth (turn on threshold voltage) and use a schottky diode (make sure it is the right way round) The EL Driver supply and Gamepad supply ground wires need to be connected. The Switch represents the button, R_Pullup the assumed internal gamepad resistor, and the +3.3V represents the gamepad supply.

Simulation:

EL Wire 3V Simulation


Circuits below left for interest to future readers

If the input voltage is +3.3V, then things are very easy, this circuit will work (note - this assumes two 3.3V regulated supplies that are not going to vary by more than a couple of percent):

EL Wire switch 3.3V

EL_BOX is a simple representation of the load from the EL driver box. When the button is pushed it pulls the gate of the P-ch MOSFET low and turns it on, supplying power to the EL driver.

Simulation:

EL Wire Switch Simulation

If the voltage of the EL wire box is different, let's say 5V, then if you can hack a schottky diode in between the button and it's +3.3V connection, then this circuit would be an option:

EL Wire Switch 5V

The diode prevents the 3.3V rail interfering with the 5V rail, but still allows both to be pulled to (near) ground. A schottky is used as it has a lower drop than a normal silicon diode, to make sure the game controller IC pin sees a logic low.

Simulation:

EL Wire Switch 5V Simulation

If it's not possible to hack the diode in (i.e. all you have access to is the +3.3V line and ground line) then you will need a couple of transistors to invert the signal something like this:

EL Wire Switch 5V Inverting

You could use N-ch MOSFETs for Q1 and Q2, but bipolars are cheaper so unless you need the lowest power consumption NPNs will do fine.

Simulation:

EL Wire Switch 5V Inverting Simulation

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  • \$\begingroup\$ Thanks so much for your detailed answer. I think circuit 1 will work for my situation, though I admit I do not fully understand it. On the controller board, if I measure the voltage between the button input and ground I get 3.3 V. If the button input is connected to ground aka logic low then it is "pressed". So it seems like as long as the two circuits share a common ground, as in your diagram, the button input will get the logic low it needs when the switch is closed and R_PULLUP does not need to be there. I am probably failing to understand the point of pullup resistors. \$\endgroup\$ – EEdiot Mar 14 '13 at 19:05
  • \$\begingroup\$ R_PULLUP is a guess at the internal gamepad resistor, and needs to be there in order to "Pull" the line high when the button is not pressed (otherwise the line is left floating and this is not a good idea) You are right that you don't actually need both resistors if the voltage is the same, just one of them, so R1 (or R_PULLUP, but this is already present) is not strictly necessary, but I left it in there accidentally (the circuit was adapted from another example) Note that circuit 1 will only work as long as the EL driver voltage is also +3.3V otherwise you need one of the other options. \$\endgroup\$ – Oli Glaser Mar 14 '13 at 19:44
  • \$\begingroup\$ Oh, so circuit 1 assumes that there is only one power source? Because no 2 power sources are going to have the exact same voltage in the real world, especially when they are different types of batteries with different discharge curves. In this case I have ~3V and ~3.3V. \$\endgroup\$ – EEdiot Mar 15 '13 at 2:20
  • \$\begingroup\$ As far as leaving the to_gamepad_ic connecting hanging, that is actually how it normally would be on the gamepad I'm using, so I guess that means there is a pullup resistor on the board already. \$\endgroup\$ – EEdiot Mar 15 '13 at 2:21
  • \$\begingroup\$ For Circuit 1, there can be two supplies - it doesn't matter if the voltages are slightly different, but if the gamepad is battery powered with no regulator IC then it's not usable (this was an "if you're lucky option" really). Also, as I mentioned above, R_PULLUP is meant to represent the internal gamepad resistor that is assumed to be already present. So are you saying you have 3.3V for the gamepad (how many batteries of what type?) and 3V supply (what type? batteries or regulated/unregulated DC adaptor?) for the EL wire? \$\endgroup\$ – Oli Glaser Mar 15 '13 at 5:33

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