• on Miscellaneous Electronics
  • Published in issue 12/2003 on page 0
About the article

Secure On/Off Pushbutton

Secure On/Off Pushbutton
The author used to employ a switching central that allows mains appliances among his PC equipment to be individually switched on and off by pressing pushbuttons. One of these pushbuttons switches the PC on and off. Good economics, but not perfect, because awkward situations can arise when a wrong pushbutton is pressed, causing the PC to be switched off instead of, say, powering up the printer.A solution was found in the use of relay which is switched on and off only if the relevant pushbutton is pressed twice within a certain period. The first press causes a LED inside the pushbutton to flash for about 7 seconds. If you press again within this period, the relay is energised and the LED lights permanently. The switch-off procedure is identical. In this way, the switch is given a level of security.
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Gerber file

CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor GREEN and GOLD members can exclusively download these files for free as part of their membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service.

Elektor recommends its business partner Eurocircuits as the company of choice for its own prototypes and volume production in selected cases.

Component list
R1,R5 = 10kOhm
R2 = 1kOhm
R3 = 56kOhm
R4 = 2kOhm2
R6 = 100kOhm
R7 = 5kOhm6
C1 = 10nF 5mm lead pitch
C2 = 1000µ F 16V radial
C2 = 100µ F 16V radial
C2 = 2µ F2 16V radial
C5-C8 = 100nF
D1 = 1N4148
D2 = LED, red, low current
T1 = BC547
IC1 = 4027
IC2,IC3 = 4093
K1, K2 = solder pin
Re1 = relay, 12V (e.g. Siemens V23057-12V)
S1 = pushbutton, 1 make contact
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