Handy Dandy Little Circuits #22

Download # 22 in PDF


Updated 23 Feb. 2012

These circuit projects require advanced electronic experience . No tuition is available.

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)

Circuit description

i 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) .

Circuit for four buttons

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 ouputs .
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 .

Important note

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. IC's

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 .