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Parts explanation for Count-down timer




Counter (74HC192)


It is using for the counter in the minute and the second.
This counter can be used for either of the count-up and the count-down. This time, it is doing the way of using the count-down exclusive use.




7 segment LED driver (74LS247)


This is the IC which changes from the figure information which is shown with the BCD(Binary Coded Decimal) into the digital display of the 7 segment LED. Because the grounding part becomes common, the LED must use the anode common type.




The clock generator (NE555)


This is the oscillator to make the 1-Hz clock signal.




2 input NAND gate (74HC00)


This is the NAND gate where there are two input terminals. The four gate circuits are housed in the one package.
Only when the two input terminals are the High level (+5V), the output becomes the Low level (0V).




4 input NAND gate (74HC20)


This is the NAND gate where there are four input terminals. The two gate circuits are housed in the one package.
The output becomes the L level only when all four input terminals are the H level.




Inverter (74HC04)


This is the inverter which outputs the condition contrary to the input. The six gate circuits are housed in the one package.




7 segment LED

This is the LED in the 7 segments which display the minute and the second.
I used the a little biggish one for the minute display.
For the minute display, it is SL-8139 which is made by the SANYO and for the second display, it is the name SL-1199.
Each size is shown below.
    For the minute display : 33mm(H) x 22.7mm(W) x 9mm(D)
    For the second display : 18.8mm(H) x 12mm(W) x 8mm(D)
The depth is 1 mm different. Install the LED for the minute display in the printed board earlier. Then, install the LED for the second display from behind to become the height which is the same as the minute display LED.



Variable resistor (For the frequency adjustment)

This is the variable resistor which adjusts the clock frequency to do the count-down.
The frequency becomes high when turning clockwise. (The count down time becomes short)
The measured result is shown below.
    1 k-ohm : 0.98Hz(59 pulses/minute)
    (Full turn to the left)

    0 k-ohm : 1.13Hz(68 pulses/minute)
    (Full turn to the right)
When setting the counter to the 99 minutes, each time-out time is shown below.(By the calculation)
    1 k-ohm : 101 minutes (+1.6%)
    (Full turn to the left)

    0 k-ohm : 87 minutes (-13%)
    (Full turn to the right)
The 10 graduations are carved on the variable resistor. In case of the setting in the 99 minutes, it is possible to do the adjustment in the about 1.5 minutes at the one graduation.
As for the resistance value, in the smaller one, the adjustment range becomes narrower and the adjustment is easy.(It is easy to adjust correctly). However, it is necessary to make it be possible to set to the 1 second in the adjustment range. The obtaining of the fixed resistance (R4) is difficult.



Relay


It closes the voltageless point of contact while the timer works to control the equipment outside.
Because the relay is small, the electric current which can pour into the point of contact isn't so big.(Hundreds mA?)
Because it is, you must prepare the relay which is appropriate to the outside to control the equipment which needs the big electric current.



Diode


With the rapid change when the electric current of the coil of the relay flows or broken, the high voltage occurs to the coil. This diode is put to protect the transistor from the voltage. Even if it doesn't put, the transistor doesn't break immediately.



Transistor (2SC1815)


This is the thing to drive the relay by the output of the NAND gate.
It may are not 2SC1815. I used because there was this at hand.



Thumbwheel switch (The BCD switch)


This is the BCD switch to set in the minute. The switch is the thin cylinder and can see the setting figure from the front. The circular cylinder has the small nail. You can set it by raising and lowering it with the finger.
The one which I used this time is made by the JAE company. It is composed of following components. You can use the necessary components by combining. This time, it is the 2-digit composition but you can compose the switch of a lot of digits by combining them.

NameTypeUse
Body of
the thumbwheel switch		31J02MThe BCD switch (1 digit/piece)
End assemblyE 31 SM The switch cover in both sides
(Either side set)
The one is necessary in one piece
of the switch composition.
SpacerS 31 SM It uses when wanting to expand the interval
of the installation of the switch.

This time, I used the two bodies of the switch, a couple of the end assembly and one spacer.
The size in this case is 24mm(H) x 33mm(W) x 30mm(D).



Connector

The control unit and the display unit of the timer are separated. It connects the interval of it with the cable. It is convenient for the maintenance when installing the connector in the side of the cable. This time, it made the side of the control unit connect with the connector.
    For the display : 18 pins
    For the BCD switch : 10 pins
    For the power supply : 5 pins



Cable

I used the numerous strand screened cable for the connection between the control unit and the display unit and the connection between the control unit and the BCD switch.
    For the display : 24 wires (It is OK with the 16 wires.)
    For the BCD switch : 10 wires
The screen part connects with the grounding.
I protected the end of the cable with the tube(Type which shrinks when applying heat).




Terminal


It is using to connect the power supply line, the start switch and the relay output.
Because the about 40 pins are connected, you cut and use the necessary number.



Acrylic sheet (The purple)


You can see the figure by the clearance when you see the LED through the purple acrylic sheet.
It becomes beautiful like the design, too, because the body of the LED doesn't seem to be direct.



Display unit installation screw


The display unit is installed in the hole of the case of the equipment which installs the timer.
First, you put the acrylic sheet outside the case and fix the acrylic sheet on the case with the installation screw and the nut. Next, you install the display unit with the installation screw from inside the case. You adjust the interval of the unit and the case in the position of the nut.





Metallic stud


It uses to install the printed board to the case.
It is OK rather than the one which is made from the metal, too.
It doesn't care in the plastic one.



Bypass capacitor


The digital circuit handles the square wave. As for the square wave, like the frequency, the high frequency component is contained. The voltage changes rapidly for the high frequency component to be contained. The high frequency component sometimes has an influence which is bad for the circuit to work. Because it is, the capacitor with the good high frequency characteristic can be put to the power circuit to lead the high frequency component to the ground (the 0-V potential).

The ceramic capacitor