Wiring panel

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.

Circuit diagram:

 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

Mechanical contacts have the disadvantage that they wear out. That is why it is practical to use an electronic ‘touch switch’ in some situations. With such a touch switch the resistance of the human skin is used for the switching action. The schematic shows the design of a circuit that senses the resistance of the skin and converts it into a useful switching signal. The touch switch contacts can be made from two small metal plates, rivets, nails, etcetera, which are placed close together on a non-conducting surface. In this circuit a comparator of the type LM393 has been used. In the idle state there is, via R1, a voltage equal to the power supply voltage on the non-inverting input of IC1a. Because the inverting input of IC1a is set with R2 and D3 to D5 at the supply voltage minus 1.8 V, the open-collector output of IC1.a is, via R3, equal to the power supply voltage. This voltage is inverted by IC1.b. The voltage at the non-inverting input of IC1.b amounts to half the power supply voltage (through voltage divider R4 and R5) and is lower than the voltage on the inverting input.

Circuit diagram:

Electronic Touch Switch Circuit Diagram

The output of IC1.b is therefore a ‘0’. If the two touch contacts are bridged with a finger, the voltage at the non-inverting input will become low enough to cause the comparator to toggle state. The moistness of the skin results in a resistance of 1 to 10 MR. If this circuit is used in the vicinity of equipment that’s connected to the mains, then it can be sufficient to touch only the upper contact to operate the switch, provided that the circuit has been earthed. The body then acts as an antenna which receives the 50 Hz (or 60 Hz) from the mains. This is enough to toggle IC1.a at the same 50 Hz. C1/R3 prevent this 50 Hz from reaching the input of IC1b and provide a useable ‘pulse’ of about 10 s at the output of IC1.b. Note that a fly walking across the touch switch conducts enough to generate a switching signal. So do not operate important things with this circuit (such as the heating system or the garage door). Do not make the wires between the touch contacts and the circuit too long to prevent picking up interference. The power supply voltage for the circuit is not very critical. Any regulated DC voltage in the range from 6 to 20 V can be used.

Author: Heino Peters - Copyright: Elektor Electronics Magazine