# Desperate help needed with circuit diagram

#### Manghameister

Joined Jan 9, 2009
3
I am a complete electronics amateur, literally, i can barely even rewire a plug, but I desperately need help with this circuit. Its a counter circuit that simply adds one on with every pulse from a clock, however when i added in an SPST switch all of the counters and decoders blew up saying that the voltage had gone 6V over the limit. Any ideas would be very welcome.

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#### mik3

Joined Feb 4, 2008
4,843
If I am right 74HCXXX logic family needs 5V for the supply voltage. Thus you can't apply more than 5V at their inputs.

#### Alberto

Joined Nov 7, 2008
169
Mik3 is right, you should use 5 volts not 6Volts! Use a 7805 and a 9 volts battery.

Then you must add a 10K resistor and 100 nF capacitor from the common of every switches to ground, to avoid to mess up every thing when you use the switches.

Have you grounded all the unused inputs of 74hc00 ? (From your schematic there must be two unused)

pin 5 of 74hc4511 must be connected to ground

Pin 5 of 74hc190 must be pulled up with a 10k resistor

Never leave TTL inputs floating!

Alberto

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#### mik3

Joined Feb 4, 2008
4,843
Was the circuit working before adding the switch?
Which switch is it?

#### SgtWookie

Joined Jul 17, 2007
22,230
Actually, it's never leave CMOS inputs open.

Tie unused inputs to either +V or GND.

#### bertus

Joined Apr 5, 2008
22,138
Hello,

The 74HC family may work from 2 to 6 Volts.

74HC and 74HCT family characteristics:
• 74HC Supply: 2 to 6V, small fluctuations are tolerated.
• 74HCT Supply: 5V ±0.5V, a regulated supply is best.
• Inputs have very high impedance (resistance), this is good because it means they will not affect the part of the circuit where they are connected. However, it also means that unconnected inputs can easily pick up electrical noise and rapidly change between high and low states in an unpredictable way. This is likely to make the IC behave erratically and it will significantly increase the supply current. To prevent problems all unused inputs MUST be connected to the supply (either +Vs or 0V), this applies even if that part of the IC is not being used in the circuit!
Note that 74HC inputs cannot be reliably driven by 74LS outputs because the voltage ranges used for logic 0 are not quite compatible. For reliability use 74HCT if the system includes some 74LS ICs.
• Outputs can sink and source about 4mA if you wish to maintain the correct output voltage to drive logic inputs, but if there is no need to drive any inputs the maximum current is about 20mA. To switch larger currents you can connect a transistor.
• Fan-out: one output can drive many inputs (50+), except 74LS inputs because these require a higher current and only 10 can be driven.
• Gate propagation time: about 10ns for a signal to travel through a gate.
• Frequency: up to 25MHz.
• Power consumption (of the IC itself) is very low, a few µW. It is much greater at high frequencies, a few mW at 1MHz for example.
http://www.kpsec.freeuk.com/components/74series.htm

Greetings,
Bertus

#### Manghameister

Joined Jan 9, 2009
3
@mik3

The problem occurs when I place an SPST switch in the left hand corner next to the power supply. I've attached a zip file that has a JPEG picture of the circuit that has slightly changed and the original file. Unfortunately it's in the format of a crocodile technology file so apologies in advance.

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#### SgtWookie

Joined Jul 17, 2007
22,230
I've extracted the schematic and attached it.
If you can, please use .PNG format files, as .JPG is a "lossy" format while .PNG is not. .PNG images are much easier to view as a result.

I don't know what that item is to the left of IC6b (NAND gate, upper left).
It looks like it's shorting out the battery the way it's placed.

Also, in logic circuits, it's more typical to have a power switch in the + supply rail rather than the ground rail.

You still have a lot of inputs that are not connected to anything. Connect them to ground or +V.

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#### mik3

Joined Feb 4, 2008
4,843
I can say the same Sgt. The battery will blow when you close the switch. What is the problem Manghameister?