Article : Andy Collinson
A switched timer for intervals of 5 to 30 minutes incremented in 5 minute steps.
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 is defined as :-
Timing = 1.1 C1 x R1
Note that R1 has a value of 8.2M with S3 at position "a". Each turn of the rotary switch adds additional resistance so
that at position "f" the series resistance is 49.2Meg. The timing period is 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. The relay contacts give physical
and electrical separation and may be used to switch circuits using higher current or voltage. Alternatively if an audible or
visual output is desired then the relay may be substituted for a buzzer or LED.
As a first realisation I made a layout on breadboard. The rotary switch, relay and driver transistor were omitted, the parts being replaced
by a single red LED and 2.7k resistor (see below).
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)
C2: 100n CAP (1)
C3: 100n CAP (1)
Q1: BC109C NPN (1)
D1: 1N4004 DIODE (1)
R1,R2,R3,R4,R5,R6 : 8.2M RESISTOR (6)
R7: 4.7k RESISTOR (1)
R8: 100k RESISTOR (1)