e011030.PDF

GENERAL INTEREST

Microprocessor

Controlled

Light Dimmer

with pushbutton control

Design by P. Staelens

This easy to construct light dimmer is controlled by

a PIC controller and can handle a maximum cur-

rent of 6 A. Three pushbuttons ensure extremely

user-friendly operation.

The light dimmer described here can

be split into two parts. The light

dimmer proper follows the familiar

recipe. It is an analogue circuit,

which uses a triac for phase control

of the mains voltage. This is hardly a

surprise. A little more striking is the

fact that this dimmer circuit is not

controlled with a simple poten-

tiometer but with a programmed

PIC16F84 processor. The processor is

operated with the aid of three push-

buttons: a combined on/off button,

an up and a down button.

Usage Instructions

The microprocessor is programmed

in such a way that the dimmer will

be in the ‘off’ position when first

powered up, while at the same time

the control circuit starts off in the

Elektor Electronics

1/2001

GENERAL INTEREST

100n

MCLR

RB0

RB1

RB2

RB3

RB4

RB5

RB6

RB7

330 Ω

470 Ω

BC337

IC1

RA0

RA1

PIC16

RA2

RA3

IC2

F84

ZERO

RA4

47n

400V

TRI1

OSC2

OSC1

S202S11

R12

2k7

TIC226D

10n

400V

BC337

27p

4MHz

27p

IC4 = 74HCT14

IC4c

IC4d

IC4e

IC4f

R13

470k

IC4b

470n

400V

IC4a

R11

230V

IC3

7 805

1N4007

R10

100k

1N4007

IC5

IC4

22µ

16V

9V1

500mW

100n

4N25

000117 - 11

Figure 1. The classic triac light dimmer is driven from a programmed PIC16F84 processor.

‘fully open’ position. Pushing the

on/off button activates the dimmer,

only then will current be supplied to

the attached lamp. However, it is

also possible to adjust the control

circuit before the dimmer is acti-

vated; in other words, it makes no

difference whether the dimmer is

activated or not.

The processor circuit is a text-

book example and requires no fur-

ther explanation. The three input

pushbuttons are connected to RA0,

RA1 and RA2, while the triac circuit

is driven from RB1 and RB7. The con-

nection that has been drawn

between RB7 and RA4 is now actu-

ally superfluous. Initially, the soft-

ware used this connection to sense

if the dimmer was on or off. Later on

this was changed to enable the

processor to keep track if this itself.

Now to the triac circuit . This is

also a textbook example, with the

exception that the triac (TRI1) is dri-

ven by a triac-driver (IC2). This pro-

vides separation between the low

voltage and the high voltage cir-

cuitry. This is required because

ground of the 5 V power supply has

a different potential with respect to

the high voltage circuit. T1 and T2 control the

triac driver. These two transistors together

perform an AND-function. The drive signal is

applied only when both transistors conduct

(RB1 and RB7 high). Snubber network R9/C5

protects the triac from inductive loads.

In order to determine the exact trigger

point, a zero-crossing detector is indispens-

able. IC5 (optocoupler 4N25) is the most

important part in this sub-circuit. A separate

bridge rectiﬁer drives the light source in the

optocoupler. This is because the other bridge

rectifier does not provide phase information

of the mains, due to the presence of C3. The

100 Hz voltage applied to the input of IC5

causes the transistor in the optocoupler to

block briefly at every zero crossing of the

mains voltage. As a consequence, at every

zero crossing there is a short 5-V pulse at the

collector of this transistor. This signal is

applied to the processor via the double

Schmitt-trigger IC4a/IC4b. This makes it pos-

Operation

The dimmer schematic is shown in

Figure 1 . It is immediately obvious

that the amount of hardware

required is quite small. The various

functional blocks will be easily

recognised by most of you. At top

centre is the PIC-processor, to its

right is the triac circuit, at bottom

left is the zero-crossing detector and

at bottom right is the power supply.

1/2001

Elektor Electronics

GENERAL INTEREST

sible for the software to synchronise itself to

the mains.

Finally, the power supply . As can be

observed, the required 5 V supply voltage is

derived directly from the mains, without the

use of a transformer. ‘AC resistor’ C3 reduces

the mains voltage to an acceptable level. This

reduced voltage is rectiﬁed by a bridge recti-

fier (D4-D7) and regulated by a 7805. Diode

D1 prevents the input voltage of IC3 from

exceeding safe limits. Two 100 nF capacitors

are used for power supply decoupling.

Because a 5 V power supply appears

harmless, we would like to stress that this

voltage in this instance is everything but

safe! The reason is that this power supply is

not electrically isolated from the mains. Be

very careful!

GAT42

OUT

TRI1

IC1

R12

R11

GAT

000117-1

Construction

Figure 2 depicts the layout for the PCB

designed for the project. The construction of

the light dimmer is a relatively simple task,

because the number of components required

is very modest, and the PCB and pro-

grammed PIC are available from the Elektor

Electronics Readers Services (the source code

ﬁles as well, for that matter).

We again urge all prospective constructors

to take the utmost caution. The entire circuit

is directly connected to the mains! Despite

the presence of an optical triac driver and an

optocoupler, the entire low voltage circuit is

not electrically isolated from the mains. Keep

this in mind while testing and making mea-

surements on this circuit.

Another important practical matter is that

for C3, C4 and C5, Class-X2 capacitors must

be used. These are short-circuit proof and are

guaranteed never to cause a short circuit in

the event that they develop a defect. If you

intend to fit the circuit into a typical enclo-

sure, it may be better to mount voltage regu-

lator IC3 horizontally. This can be achieved by

bending IC3 over the top of IC4; do not trim

the leads too short, otherwise this will not be

possible.

Because of the compact dimensions of the

PCB, ﬁnding a suitable enclosure should not

be an issue. Use plastic screws and stand-

offs to fasten the PCB, in order to maintain

the required insulation spacing between

mains voltage and parts that may be

touched. Our prototype was built into a plas-

tic case from Bopla. The PCB ﬁtted nicely, but

it was necessary to shorten the internal

stand-offs by some 3 mm for the cover to be

ﬁtted properly.

Figure 2. The PCB has everything clearly arranged and is compact. Keep in mind that

the entire circuit is connected to the mains!

COMPONENTS LIST

D2-D9 = 1N4007

T1,T2 = BC337

Tri1 = TIC226D

IC1 = PIC16F84-04, programmed,

order code 000117-41 (see

Readers Services page)

IC2 = MOC3021

IC3 = 7805

IC4 = 74HCT14

IC5 = 4N25

Resistors:

R1,R2,R12 = 33 k

Ω

R3 = 120 Ω

R4 = 2k

R5,R13 = 470 k Ω

R6 = 47

Ω

R7 = 330 Ω

R8 = 470

Ω

R9 = 39 Ω

R10 = 100 k

Miscellaneous:

S1,S2,S3 = pushbutton with make

contact, safety Class 2

X1 =4 MHz quartz crystal

K1,K2 = 2-way PCB terminal block,

lead pitch 7.5 mm

Case: e.g. Bopla (Conrad-Electronics

order code 52 22 52-11)

PCB, order code 000117-1 (see

Readers Services page)

Project disk (PIC source code ﬁles),

order code 000117-11

Ω

1 W

R11 = 10 k Ω

Capacitors:

C1,C2 = 27 pF

C3 = 0.47

F 400 V (Class X2)

C4 = 47 nF 400 V (Class X2)

C5 =10 nF 400 V (Class X2)

C6,C7= 100 nF

C8 = 22 µF 16 V radial

Semiconductors:

D1 = zener diode 9.1V 500mW

(000117-1)

Elektor Electronics

1/2001

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