John Schimmel, 12/01/2013
When you open the case of the remote you will see a circuit. Depending on the toy, it can be very simple or a bit more complicated.
In the toy train we're adapting now, the two black push buttons are the circuit's switches. The remote control's plastic switches (the ones with arrows top shell of controller) sit directly on top of these internal switches. When the cover switch is depressed, these internal switches are also pressed and trigger the toy's functions.
We will be soldering one switch jack to the backside of each of the push button switches.
Detach the circuit from the plastic shell
If you can not access the push button's solder pads, you will need to remove the circuit from the plastic shell to get underneath the circuit board. Look for some screws on the circuit board and unscrew them.
Place screws in an area where they will not roll away. Finding replacement screws that fit is a pain and it is important that the circuit is replaced correctly once the switch jack is installed.
Next we'll need to locate the 2 pins that "connect" when the push button switch on the circuit board is pressed. It's those two points were we'll be soldering the leads of the switch jack.
A multimeter can be used to determine connectivity. Put your multimeter into "connectivity" mode - it's usually denoted by something that looks like the "increase volume" symbol. Your multimeter should beep and change it's numeric value when you touch the red and black leads together. The beep and number change indicates that electricity is making it out of the black lead into the red lead and back into the device.
We will only need to determine how the push button switch's pins are connected, which pins become connected when the push button switch is pressed and which solder pads are connected to which pins.
Flip the circuit over and you'll see the traces and connections from all the components. Locate the backside of the push button switches. On this particular toy, there are 4 solder pads per switch.
Once you've located the switch, with some finger gymnastics touch one lead of the multimeter to a solder pad and the second lead to another solder pad of the switch. Now, press the switch with your thumb. if your multimeter beeps that means you successfully found the two points that connect to make your toy operate this switch's function.
If your multimeter does not beep or change it's numeric value on the digital display, Check to see if your leads are touching the solder pads correctly and test that your multimeter is operating as expected.
Your toy may have another style of internal switch that is different from the 4 pin push buttons we had here, but you can use the same process to determine the pins where you'll need to attach your switch jack.
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