transistor mosfet touch switch

[jeremygaughan]

My project is to build a grinder/sander for the edge of glass to make stained glass windows. I put a darlington triplet on a speed 400 motor with a 4.5v battery pack, and I am loosing a lot of voltage. So my idea is to use a mosfet at the end of the darlington triplet to improve the efficiency. So, do you think it will work? Here is my schematic...
**broken link removed**

 

[audioguru]

The link to your schematic doesn't work.
Attach your schematic here to your reply, instad of posting it over at Kodak.

 

[jeremygaughan]

I think that fixed it.

 

[audioguru]

You had the Mosfet connected to short the motor not to power the motor.

A Mosfet doesn't need transistors to drive its no-current gate, the switch by itself can apply voltage to the gate.

Ordinary Mosfets need 10V from gate to source to fully turn on. I used a logic-level Mosfet that turns on fully with a gate to source voltage of 4.5V, and I connected it to apply power to the motor. The 10k resistor turns off the Mosfet when the switch is released.

 

[jeremygaughan]

Thanks for the clarification on the mosfets. I will get one that turns on fully with less voltage. The reason I used the three transistors is that it is a switch that you just touch with your finger and it turns on. Is it possible to turn on a mosfet with the current that would pass through a person?

 

[audioguru]

A touch switch (only one wire) works because a person picks up radiation from the power lines in a home or business. Then the 60Hz (or 50hz in some countries) needs to be rectified and filtered so it doesn't cause the load to be powered by 60Hz 9or 50Hz) on and off pulses.

A two-wires touch switch works from the conduction of a person's finger on two wires. It doesn't work when a person's hands are dry or if they wear gloves.

A touch switch usually also has a flip-flop latch so you touch it and the latch keeps it on until it is touched again which turns it off.

Your two-wires touch switch would cause the motor to run only while you held the inputs then it would stop when you let go.

The gate of a mosfet is destroyed by static electricity so it is not suitable for a touch switch unless you add protection diodes. It would also need a flip-flop latch.

 

[jeremygaughan]

so I´ve modified it a little. How about this?
"EDIT" I saw your post after I wrote this. I´ll research flip-flop and figure out where to put the protection diodes, I assume shocky because of the fast switching rate.

 

[audioguru]

You need to use a low loss common-emitter transistor to drive the motor.
You are using emitter-followers that have a voltage loss.

Your new circuit will apply only 2.5V to the motor when your finger is a little damp and salty. Then the motor will stop when tou move your finger away or if it dries.

 

[Hero999]

You also need to connect the gate inputs to the + and 0V with diodes connected in reverse to protect the gates from ESD.

 

[jeremygaughan]

Do you mean diodes in this orientation and possition?

 

[audioguru]

The Mosfet won't turn on when it is driven from 3 transistors that reduce the gate voltage to only 2.5V.
Just use a logic-level Mosfet without the transistors, a high value resistor to turn it off and two protection diodes connected properly.

The pins layout looks like you are using a little 2N7000 Mosfet that might be able to drive a tiny low current motor. Its gate needs 10V.