How to Design Schematic Circuit of On Off Touch Relay

 AO-powered appliances, such as lamps, small home appliances, tools and

other items, can be controlled by your fingertips using this circuit. Also, test

equipment and experiments can be controlled by this circuit.You can turn on

any load by first touching time sensor X1, then turn it otf by touching Xl

again. There is no shock hazard, as the control circuit is completely isolated

from the AC power line.

The control circuit is powered from four or eight AA cells, or, if you prefer, a

power supply ranging from 6 lo 12 volts according to the relay coil.

Don't use transformerless power supplies because they are not isolated from

the AC power line and can cause severe shocks and shorts.

The schematic diagram of theTouch Onffouch Off Relay is given in Figure 1.

The circuit uses two lOs. The 555 timer works as a monostable, producing

one control pulse when the sensor is touched. The 4013 is a flip-flop that

controls the relay by a transistor.

Components placement on a homemade printed-circuit board is as shown in

Figure 2. Component layout should be altered if the relay used in the project

is an equivalent of the type indicated in parts list.

Relay coil voltage is determined by the power supply voltage and the load

current requirement. You can use a mini 1A DPDT (Radio Shack 275-2491

12V,280 ohm, 43 mA, and wire it as shown in the figure, or use another type

of relay, depending on the load requirements.

A 10A SPDT mini-relay (Radio Shack 275-278) is suitable for heavy duty

appliances.

As a simple rule, you can use a 6- or 12-volt relay with coil currents ranging

from 10 to 100 mA and contacts up to 10A, according to the task you have in

mind.

Positions of the polarized components, such as the diode, electrolyticapacitors and transistor, should be observed.

Sensor are the Same as the'Touch Switch" pQect described in this book.

Don't use metallic box to house the device, as there are parts connected

directly to the AC power line. Be sure that there are no power line contacts

with the low voltage circuit to avoid shocks and dangerous shorts.

The load is connected as shown in Figure 3, to be ON when the relay coil is

energized. You can also use the normally closed contacts to turn otf a load

when the relay coil is energized.

Remember that current requirements are high when the relay coil is energized and low when the transistor Q2 is otf (relay coil nonenergized). That is

an important factor to consider if you are using batteries to power the unit.

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After you are sure that you have wired all parts of the circuit correctly, plug the

power cord into the wall socket. You can now test the circuit.

Connect aload to the output (relay contacts) and touch the sensor. The load

should be powered ON if all is OK.

Parts List -Touch On/Touch Off Relay

lC1 - 555 Integrated circuit - timer

lC2 - 40138 CMOS integrated circuit - dual flip-flop

Q1, 2 - 8C548 General-purpose NPN silicon transistors

D1 - 1N914 General-purposeilicon diode

K1 - Relay - see text

R1, 2, 5 - 100,000 ohm, 114W,5% resistors

R3, 4 - 47,OOO ohm, 114W,57o resistors

RO - 1,000 ohm, 114W,5% resistor

Cl - 1 uF, 25 WVDC electrolyticapacitor

C2 - 0.1 uF ceramic or metal film capacitor

Cg - 4.7 uF, 12 WVDC electrolyticapacitor

C4 - 100 uF, 16 WVDC electrolyticapacitor

S1 - SPST toggle or slide switch

81 - 6 or 12V (according to relay) - see text

X1,2 - sensor - see text