Updated 23 Feb. 2012
These circuit projects require advanced electronic experience . No tuition is available.
Circuit for four buttons
I had a few requests for a circuit for a first-response discriminator where the first to push a button would activate
a lamp, horn or bell or all of them and also prevent others from activation. A complete set-up for four position is shown with its suggested layout and actual size PCB that can be copied
The next two circuits provide for a total of six or eight stations ( buttons ) . I have not provided a PCB for these designs because of the complexity required as only the braves and pro will venture to design the required PCBs. (There is no copyright restriction)
When one of the push-to-close switches is closed say #1 to input pin 4 of the CD4042 IC ,the output at pin 2 goes hight and activates the LED or relay through the CD4050 IC output pin 2 .
Simultaneously pin 3 of the CD4042 IC goes low and is fed to one of the four inputs of IC CD4012(A) or CD4023 which in turn send out a high input to the clock ( pin 5 ) of the CD4042 and disables all the other inputs . At this point the System is locked and the LED or relay or both will stay on until the system is reactivated by the reset - button switch .
The inputs 1megohms resistors are used to clamp down the inputs to ground to prevent any spurious voltage noise to activate the inputs while off .
The 1K ohms resistors connected in series from the outputs of CD4050 are used to limit the current to the LED's and transistors . The 1K value is used for a 9 Vdc supply for approx 9 mA and should be adjusted to match the voltage source to the load (exp. relay,horn) .
Graphic Layout . The PCB is actual size
Display Setting , screen resolution 1600x900
Circuits for six and eight buttons
Studying the set -up for the six positions a CD4023 IC was used as it provides for the three needed inputs instead of the CD4012 used for the eight positions
set-up which you will notice is similar to the set-up for four buttons but to which one each of the CD4042 and the CD4050 was added to provide for eight
By adding steering diodes to the outputs of the two CD4042 we isolate all the signals from each output and save ourself the addition of another CD4012 . Pin 5 of both CD4042 are tied together to receive the clock signal from pin 13 of the CD4012 disabling all but one of the lines as in the four buttons set-up .
Any unused logic pins wether input or output must be connected to Positive or negative bus through a 1K resistance . This will insure that the unused logic will not be left floating and be either pulled up or down in addtion to eliminate any noise , another avantage to this practice is to allow in case needed to access these inputs or output without disturbing any PCB connections .
Relay = 6 to 9 volts miniature low current , normally open contacts.
A word of caution , CMOS IC's can easily be damaged by static voltage, IC sockets should be used and all connections made
and supply buss tested before inserting the IC's.
You can use a CD4049 IC instead of the CD4050 ( same pins connection ), the diodes must be reversed in polarity and connected to 9VDC and PNP transistors must be used with supply polarity reversed.
Note: CMOS is the general type (MOS) designation of logic ICs , CD: (CMOS) is Motorola designation and is identical to any IC bearting the same number designation.
CD4012 = CMOS ( Dual 4-input "NAND" gate) 14 pins . Pin 6 is not connected to the IC ,Note = used for four or eight buttons .
CD4023 = CMOS ( Triple 3-input "NAND" gate) 14 pins . Note = used for six buttons .
CD4042 = CMOS ( Quad D Latch) 16 pins
CD4050 = CMOS ( Hex Buffer) 16 pins . Note = Pins 13 and 16 have no connection to the IC .