Making touch switch independent of current?

Thread Starter

Steven Middleton

Joined Feb 7, 2017
22
Hey guys,
I was wondering if you could help me out with this project. I am trying to make a touch switch that can provide higher current. I tried a darlington pair, but that doesn't provide enough current. I need about 300 ma. Is there a way to make a touch switch that can provide that kind of current, without being dangerous? Thanks.
SM
 

AlbertHall

Joined Jun 4, 2014
12,344
The 2N7000 will only handle 200mA.
We really need to know the voltage available from the sensor and the voltage to be switched.
 

Thread Starter

Steven Middleton

Joined Feb 7, 2017
22
Oh, yes the voltage supplied is pretty low, 3.7 volts. I didn't think about using a mosfet. Is there a circuit diagram to show how to wire it? I want to make it so the switch will turn on when it is in contact with the skin, and turn off when it is removed from the skin.
 

Thread Starter

Steven Middleton

Joined Feb 7, 2017
22
I checked those out. Thanks. The problem is that the circuit stays on when contact is broken. I need the circuit to turn off when contact with skin is broken.
 

ian field

Joined Oct 27, 2012
6,536
Hey guys,
I was wondering if you could help me out with this project. I am trying to make a touch switch that can provide higher current. I tried a darlington pair, but that doesn't provide enough current. I need about 300 ma. Is there a way to make a touch switch that can provide that kind of current, without being dangerous? Thanks.
SM
300ma isn't a big ask - a TIP121 Darlington should do it with ease if you remember the 2x Vbe requirement. A pretty foolproof MOSFET would be something like an IRFL530.
 

ericgibbs

Joined Jan 29, 2010
18,766
Hi Steven,
Ref the two Vbe.
As you know a conducting transistor is assumed to have a Base to Emitter voltage of approx 0.7V.
A Darlington has two Base/Emitter junctions in series, so thats approx 1.4V.
I would use MOSFETs.
E
 

Thread Starter

Steven Middleton

Joined Feb 7, 2017
22
in order to make the mosfets work, should I hook them up the same as a darlington pair? So they turn on when there is contact, and turn off when there is no contact?
 

ericgibbs

Joined Jan 29, 2010
18,766
hi S,
Do you have a Darlington transistor that you could post, so we can see what you are trying to do.?
What is the 300mA load.?
E
 

BobTPH

Joined Jun 5, 2013
8,804
You have not shown us how your darlington is hooked up, so how can we comment on whether a MOSFET could work with the same circuit?

Bob
 

Thread Starter

Steven Middleton

Joined Feb 7, 2017
22
I used two transistors and hooked them up in a darlington pair configuration. I'm using it to power a DC motor. It works, but isn't supplying enough current for the motor to run at full RPM's.
 
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