Price - Stock No 220
Check Your Kit
- To print the Board Refer Printing
- Inspect the tracks for fine breaks and test the
continuity of each track with an electronic circuit tester or ohm-meter.
- Identify the resistors by their colour bands and an Ohm-meter.
Bend the legs so they fit the hole spacings, push the legs through from the
plain side of the PCB and bend the legs to hold them in place. They may be
soldered now if you like.
- C1 and C2 will push into their
locations, but they are electrolytic capacitors; polarity has to
be observed. Look on the cylindrical
body for an arrow pointing down one leg. This leg is the -ve (K)
leg. Make sure the capacitors go the right way round.
- The eight pin socket can be soldered in place ready to receive the
- The PCBoard layout allows for
either a Trimpot or a Potentiometer. Check
and find which one is in your kit . For the trimpot, bend one of the
outside legs up under the body and push the remaining two legs (centre and one
outside legs) through the holes and solder off. If
using a potentiometer either solder its three legs to the donuts on the Board,
or use wires to lead it off to its position in your case.
- The two LEDs can be mounted on the Board. Check their polarity - the
flat on the flange below the globe is on the K side. The red LED
is a "pilot" that lights when the unit is switched on and
timing.The green LED is in parallel with the buzzer and will light
when the set time expires. The buzzer has red
and black leads. Make sure that you solder the red lead to the A donut, and
the black lead to the K donut.
- The switch is a Push-on/Push-off type. Bare the ends of wires,
tin them with solder, push them into the holes and screw
the screws down firmly. The "ON" position can be found by
connecting a continuity tester or a multi-meter and operating the
switch. The green LED with its 390R metering resistor can be connected
to indicate that the timer is counting. Work out the
connections you will need to make so the LED will glow when the switch is on.
An interesting variation is to use a mercury switch and design a case
that pivots like an hour glass. To switch it ON you will
need only to pivot it round, and back to the upright position to switch OFF.
- The battery snap also has red (+) and black (-) wires. Solder them
as the drawing indicates.
- Finally take the 555 IC. Look at the top surface to find a dot
locating mark. This dot must be at the end that the
drawing shows. Place the IC in an IC Inserting tool and push it
into its socket. If no inserting tool is available, turn the IC on its side
with its legs on a flat surface and roll the IC so the
legs are bent slightly inwards. Turn it over and repeat with the other
set of legs. This will line the legs up to fit into the socket.
Make sure the "DOT" is right and push it into place.
- To test, turn the potentiometer anti-clockwise,
connect a 9Volt battery and switch on. The
buzzer will sound after a few
seconds. If not then check all components for values,
locations and polarity, and check the dot on the IC is correctly positioned.
Very few things can go wrong so the problem will be found in one of the above.
- To calibrate the timer, put a dot of correcting fluid on the
knob and proceed to put matching marks on your case to
indicate the times you require - hard boiled - soft boiled etc., using a
clock with sweep second hand or a digital timer.
- Note that the timer circuit is a typical time delay
circuit. The time factor is variable in that the
capacitor C2 will charge at different rates depending
on the resistance R2 and VR1. The voltage at the capacitor will rise
until it reaches 6 Volts viz. two-thirds of the supply voltage
(9V). This voltage applied to pins 6 & 7 then outputs to pin 3 and
the buzzer sounds.
- NOTE also that R1 and C1 work as an
automatic trigger, applied to pin 2. Pin 1 of the 555 goes to 0
Volts, and pins 4 & 8 are connected to 9 Volts.
This gives a Monostable configuration for the 555 viz. it "flips" when turned
on and remains in that state until 2/3 the input voltage is
applied at 3 & 6 when it "flops", and remains in that state until
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