Touch probe woe's

[JohnHaine]

That's the sort of thing, but you need to know more about the Gecko inputs. Their manual is silent on the input config except to say that they expect to be grounded by a switch which implies there's a pullup resistor. Two things are needed, the internal voltage to which they are pulled up (which determines the max Vcc of the transistor needed) and the value of the resistor (which sets the transistor on current), It may well be that the voltage is 48v as you mention which would be OK for the 2n3904 m_c suggests. You could measure the on current by just connecting your multimeter on the current range between the input and Gnd. The maximum collector current for the 2n3904 is 200mA.

Then really the transistor needs a series resistor between the probe output and its base to limit the base current (as we don't know what the internal pullup in the probe is). In practice probably a 4k7 (4.7 k, 4700 ohms) would probably be fine to make sure the transistor saturates to pull down the input to ground.

Alternatively you could use a logic level MOSFET such as the IRLZ34N. This is reasonably beefy and would easily handle the voltage and any reasonable current. You just need to connect the probe output to the gate, the drain to the Gecko input, and the source to ground. Good practice would be a 100k resistor from gate to Gnd just to make sure the device is switched off when disconnected.

[Smokie555]

This is just the info I need guys to get this thing working. Thanks to both of you for the time. I've just gone on a weeks holiday so won't be able to try this stuff until I get back but it will give me lots to think about until then. Cheers Ross

[Smokie555]

Just be looking at IRLZ34N online John, looks like a great solution. Just not sure how I power the probe, does it need to be from the Gecko's 48v supply and if so what is the best way to achieve this? If its ok to run it off a 5v USB supply or a 12v wall wart and do I need to connect the earths of the two supplies?

Cheers
Ross