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Sunday, August 09, 2009

5 to 30 Minute Timer


Simple to build, simple to make, nothing too complicated here. However you must use the CMOS type 555 timer designated the 7555, a normal 555 timer will not work here due to the resistor values. Also a low leakage type capacitor must be used for C1, and I would strongly suggest a Tantalum Bead type. Switch 3 adds an extra resistor in series to the timing chain with each rotation, the timing period us defined as :-

Timing = 1.1 C1 x R1








Note that R1 has a value of 8.2M with S3 at position "a" and 49.2M at position "f". This equates to just short of 300 seconds for each position of S3. C1 and R1 through R6 may be changed for different timing periods. The output current from Pin 3 of the timer, is amplified by Q1 and used to drive a relay.

Parts
Relay 9 volt coil with c/o contact (1)
S1 On/Off (1)
S2 Start (1)
S3 Range (1)
IC1 7555 (1)
B1 9V (1)
C1 33uF CAP (1)
Q1 BC109C NPN (1)
D1 1N4004 DIODE (1)
C2 100n CAP (1)
R6,R5,R4,R3,R2,R1 8.2M RESISTOR (6)
R8 100k RESISTOR (1)
R7 4.7k RESISTOR (1)

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