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015

LED Flasher

+12V ACCU

R1

8

4

R

7

DIS

D2

R2

IC1

2x

3

OUT

1N4148

D3

R3

6

555

THR

R

2

T R

CV

+12V CONTACT

1

5

D1

C2

C1

220n

100n

014084 - 11

cuit begin to flash. You may then ask what the purpose of

D3 is. Well, this has been added in order to prevent the var-

ious loads in the car from being connected in parallel with

C2 — because that was not the intention, of course.

A minuscule printed circuit board has been designed for

the circuit, that, when fully built-up will fit exactly in an old

fluorescent tube starter. The latter also immediately solves

the problem of finding a suitable enclosure.

(014084-1)

A flashing LED based on a 555 is not likely to win the first

prize for originality, but such a circuit continues to be use-

ful for all kinds of applications. The thing that makes this

version special is the very low current consumption,

because the LED flashes only very briefly each time (duty-

cycle of only 10%) and because it was specifically

designed to serve as a (fake) alarm indicator in a car. The

circuit has been configured such that the flashing only

starts when the ignition is switched off.

The latter could have been achieved in the usual way

with the aid of two resistors and a transistor connected to

the reset line. But by using a clever trick instead, this has

become even simpler. The +12 V connection from ‘behind’

the ignition key is connected to the threshold input via

diode D3. The astable multi-vibrator is held in the inactive

state whenever this point is at +12 V. Only when the igni-

tion is switched off, is the clamp removed and can the cir-

R1

D2

R2

014084

IC1

D3

R3

COMPONENTS LIST

C2 = 220nF

Resistors:

R1 = 470k Ω

R2 = 1M

Semiconductors:

D1 = LED

D2,D3 = 1N4149

IC1 = 555

Capacitors:

C1 = 100nF

12/2001

Elektor Electronics

41

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