Circuit : Andy
Collinson
Description:
A switched timer for intervals of 5 to 30 minutes
incremented in 5 minute steps.
Notes:
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 List:
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)
