Elektor Team
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On/Off Timer

On/Off Timer
If you need an adjustable ‘on’ or ‘off’ time for some application, then this is the circuit you have been looking for. A problem that often occurs when adjusting timers is that the individual times affect each other. This circuit completely solves this problem because the time defining elements — both R and C — are switched over. That means that there is an RC pair (P1 + R3 and C1) for the ‘off’ time and another pair (P2 + R4 and C2) for the ‘on’ time.
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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 = 4MOhm 7
R2 = 100kOhm
R3,R4 = 10kOhm
P1,P2 = 1MOhm preset
C1,C2,C4 = 100nF
C3 = 100pF
C5 = 10µ F 25V radial
D1 = 1N4001
T1 = BC517 (Darlington)
IC1 = 4053
IC2 = 4060
Re1 = E-card relay, 12V coil, V23057 B0002 A201 (Schrack)
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