I have this circuit and all I need is to somehow change S1 so that it is a normally closed switch and as soon it opens it triggers the 555. Anybody have any ideas? I have the timing correct in terms of lighting my LED. I have 12vdc running through S1. Thanks.

 

Swap the positions of S1 and R1. You may need to reduce R1 to, say, 2k2.

 

As said,,, increase R2 to say 100k and swap R1 and S1...

 

You could use an External switch or Sw1 to be the input switch, an External switch can be a switch from other circuit or relay, and the Sw1 can be a switch on board or an independent switch.

 

You could use an External switch or Sw1 to be the input switch, an External switch can be a switch from other circuit or relay, and the Sw1 can be a switch on board or an independent switch.

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As said,,, increase R2 to say 100k and swap R1 and S1...

I tried switching R1 and S1 and that worked, but I need to keep one side of the switch connected to ground. I haven't tried the suggestion by ScottWang yet. Would prefer to do it without additional components if possible, but that may not be possible. Do you guys think that using a transistor is the only simple way ? Thanks for your help.

 

If the switch must be grounded then you need something to invert the signal and a transistor is the simplest way to do that.

 

You could use an External switch or Sw1 to be the input switch, an External switch can be a switch from other circuit or relay, and the Sw1 can be a switch on board or an independent switch.

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I'll be running 12vdc through the circuit and probably 8vdc to the switch. I have some Mosfets and BJT's. I don't think I have any that will trigger at that voltage. Does anybody know a particular model # for the transistor that might work ? Thanks.

 

I'll be running 12vdc through the circuit and probably 8vdc to the switch. I have some Mosfets and BJT's. I don't think I have any that will trigger at that voltage. Does anybody know a particular model # for the transistor that might work ? Thanks.

What do you trying to connects the 8vdc to the switch?
If you could upload the schematic then it might help.

 

Any NPN transistor will work.

 

What do you trying to connects the 8vdc to the switch?
If you could upload the schematic then it might help.

The voltage to the external switch needs to be about 7.5vdc and from 9-12vdc at the 555 output. I'm powering it with a 12.4vdc power supply. I tried the smaller npn's 2n3904 & 2n2222, but didn't seem to switch. I'll try a TIP 31 and see if that helps. I've tried using the resistors listed, but so far it isn't working. I'll keep trying. The circuit I posted at the beginning is the schematic I started with and the only thing that I needed to do differently was have the switch go to ground like the external switch ScottWang posted. Thanks

 

I'll be running 12vdc through the circuit and probably 8vdc to the switch.

The switch (Scott Wang cct.) connects across the base and emitter of the transistor. It does not need, and shouldn't have, any other voltage applied to it. When the switch is open, the voltage across it will be the base/emitter voltage of the transistor - about 0.7V,

 

Can you post a picture of the switch you're using or a model number,

never known a switch to require a dc supply for it to work?

 

So maybe this setup won't work ? I need 9-12vdc to the output of pin 3 on the 555 and I need about 7.5vdc going to the external switch and the switch needs to be nc. So basically I'm using the switch to control voltage through pin 3 when the switch opens. I'm trying to control the duration of the voltage to pin 3 with the 555 monostable using the capacitor/resistor timing function. The switch could be any nc switch, toggle etc. I just need 7.5vdc going to the switch and one side to ground so that when the switch opens power will flow through pin 3.

 

So draw the schematic is more easy to understand.

 

I'm using the diagram in Post #4. I have tried NPN Mosfet irf 510, 540, PNP Mosfet, 2n2222, 2n3904 and others. Can't get the circuit to light the bulb when the switch opens. In fact only once did I get the switch to light the bulb when I closed it (I need it to do just the opposite, lights when open). I really need 7.5 vdc running to the switch.

The circuit in Post #1 does what I need when R1 is swapped with S1, but I need S1 connected to ground as in Post #4.

Can anybody suggest a transistor that might work in this situation ? Or a modification to the above schematics. Thanks

 

I really need 7.5 vdc running to the switch.

I don't really understand what does it means?
One pin is connects to 7.5v and another pin is connects to ground?

 

The 2N2222 or 2N3904 should work just fine in the #4 circuit.
This need for 7.5V running to the switch bothers me.
Why do you need that?
Do you have 7.5V connected to the switch somewhere?

 

If you have a multimeter can you measure the base and collector voltages of the transistor when the switch is closed and when it is open, please.
(Using either 2N2222 or 2N3904)

 

If you have a multimeter can you measure the base and collector voltages of the transistor when the switch is closed and when it is open, please.
(Using either 2N2222 or 2N3904)

 

The voltage when switch is open is .74v at the base to collector and 0v when switch is closed.