# 2 Grounds in a circuit (555 Timer)

Discussion in 'General Electronics Chat' started by Xufyan, May 9, 2011.

1. ### Xufyan Thread Starter Member

Aug 3, 2010
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0
Hello Everyone,

i have circuit with two ICs , AND Gate and a 555 Timer

The output of the AND GATE IC is the input Voltage of 555 Timer

i am using timer to blink the LED connected at the pin '3',

Here is the circuit ,

AND gate IC 14 pin is connected to VCC and 7 pin to Ground,
since the input voltage of Timer is the Output of AND GATE ic ,
what should be the ground of Timer ???

I so confused

when i am connecting Timer ground to the Same ground as AND Gate ic then the LED connected at the output terminal of Timer is not blinking it is remain in 'ON' state.

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2. ### DerStrom8 Well-Known Member

Feb 20, 2011
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All grounds should be connected to the (-) terminal of your power supply. The reason it is in a constant high state is because you have it connected directly to +V. Connect the LED (via a resistor) directly to the output of the 555. DO NOT connect pin 3 to (+).
Der Strom

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3. ### Xufyan Thread Starter Member

Aug 3, 2010
114
0
Pin '3' is not connected to (+) :S ?? it is just connect to LED

Apr 5, 2008
16,965
2,960
Hello,

Arre you trying to power an 555 with the output of a logic IC?
The output of the logic IC will never be powerfull enough to power the 555.

What are you trying to do with your circuit?

Bertus

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5. ### tom66 Senior Member

May 9, 2009
2,613
215
Connect Vcc permanently to the power source.

Connect RESET to the logic gate output.

Then you will not be powering the whole timer using the logic gate.

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6. ### Wendy Moderator

Mar 24, 2008
20,993
2,731
The ground of a circuit is the more important of the two, but both Vcc and ground is important.

Ground is the point all other voltages are referenced too, including the logic voltages.

This project shows how one 555 might turn another one on/off. I needed to invert the signal, so I had to make a simple transistor inverter.

CMOS 555 Long Duration LED Flyback Flasher

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7. ### DerStrom8 Well-Known Member

Feb 20, 2011
2,428
1,333
Then what is the "2.5v" all about? It would also help if you showed us your entire circuit schematic.

This is also a very good point. At the very least, use a transistor. There are actually a few things wrong with this circuit. A few more details about what you are trying to do will help.
Der Strom

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8. ### Xufyan Thread Starter Member

Aug 3, 2010
114
0
You've said that LOgic IC cannot power the Timer , but if we first amplify the logic output and then power the 555 timer , will it work ??

Why the circuit is working if i am connecting reset pin to the timer output ??

9. ### Wendy Moderator

Mar 24, 2008
20,993
2,731
I've seen it done, and didn't like it. You can use a logic level to turn a transistor on/off which powers another circuit. You must pay extreme attention to voltages though, a TTL and a 555 both do not go to Vcc, but miss it by around 1.3V less than Vcc.

In turning a 555 on and off it is much better to use the reset line (pin 4), that is what it is for.