The Gentle Touch

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Consumer appliances
these days hardly ever have a proper mains switch. Instead, appliances
are turned on and off at the touch of a button on the remote control,
just like any other function. This circuit shows how a device (as long
as it does not draw too high a current) can be switched on and off using
a pushbutton. The approach requires that a microcontroller is already
available in the circuit, and a spare input port pin and a spare output
port pin are required, along with a little software. When power is
applied T1 initially remains turned off. When the button is pressed the
gate of T1 is taken to ground and the p-channel power MOSFET
conducts. The microcontroller circuit is now supplied with power.
Within a short period the microcontroller must take output PB1 high.
This turns on n-channel MOSFET T1 which in turn keeps T1 turned on after the push-button is released.

Now the microcontroller must poll the state of the push-button on
its input port (PB0) at regular intervals. Immediately after switch-on
it will detect that the button is pressed (a low level on the input port
pin), and it must wait for the button to be released. When the button
is next pressed the device must switch itself of f: to do this the
firmware running in the microcontroller must set the output port pin to a
low level. When the button is subsequently released T1 will now turn
off and the supply voltage will be removed from the circuit.

The circuit itself draws no current in the off state, and for
(rechargeable) battery-powered appliances it is therefore best to put
the switch before the voltage regulator. For mains-powered devices the
switch can also be fitted before the voltage regulator (after the
rectifier and smoothing capacitor). Since there is no mains switch there
will still be a small standby current draw in this case due to the
transformer. Be careful not to exceed the maximum gate-source voltage
specification for T1: the IRFD9024 device suggested can withstand up to
20 V. At lower voltages R2 can be replaced by a wire link; otherwise
suitable values for the voltage divider formed by R1 and R2 must be
selected.

The Gentle Touch Circuit

The Gentle Touch Circuit Diagram

The author has set up a small website for this project at http://reweb.fh-weingarten.de/elektor, which gives source code examples (which include dealing with pushbutton contact bounce) for AVR microcontrollers suitable for use with AVR Studio and GNU C. Downloads are also available at http://www.elektor.com.

Rainer Reusch
Elektor Electronics 2008