555 timer pulse voltage

Thread Starter

evanwidloski

Joined Oct 8, 2011
9
I read somewhere that the output pulse of a 555 timer was 1.7 volts less than it's power. So does this mean that with a 9volt it will give an output pulse with 7.3 volts?

If so, will a 2n7000 MOSFET be fully activated with this pulse?
 

SgtWookie

Joined Jul 17, 2007
22,220
The output of a BJT (transistorized) 555 timer will be ~ 1.3v less than Vcc under a light load. You can make it go higher by using a pull-up resistor, but that will increase the current drain on your battery.

You can use a CMOS 555 timer. It works very similarly to the bjt 555. It's output goes nearly to the supply rail; but it's current is more limited than the bjt version.

Since you're using a 9v battery, you really should use the CMOS version instead. It will make your battery last longer.
 

SgtWookie

Joined Jul 17, 2007
22,220
The CMOS version is Complementary Metal Oxide Semiconductor. It uses much less current than the bjt version. It is very different inside, but it functions similarly to the bjt version.

It would help if you would post a schematic of what you're considering presently.
 

Thread Starter

evanwidloski

Joined Oct 8, 2011
9
This is a monostable circuit


The unlabeled capacitor and resistor values are 50k ohm and 100 uf.

The part labeled "device" will control the MOSFET which will start and stop a stopwatch.
 

SgtWookie

Joined Jul 17, 2007
22,220
Sure, that should work fine.

Just so you know, 50k is not a standard value of resistance, but you could use 47k or 51k, or two 100k resistors in parallel to get 50k.
 

SgtWookie

Joined Jul 17, 2007
22,220
Is there anything wrong with adding an led ( with a resistor of course) in parallel with the MOSFET, so I can see the pulse?
Do you mean connected to the output of the 555?

I will suggest that you don't do that, as a CMOS 555 timer has weak current sourcing ability (10mA).

Connect the cathode of the LED to the 2N7000's drain instead, and connect it's anode to a resistor, the other end of the resistor to +V.
 

Wendy

Joined Mar 24, 2008
22,178
If you really want to you can use the drivers shown in this tutorial. A CMOS could do it by itself in theory, but the drive from it is unpredictable. Of course, one of the things I suggest often is to try it and see. This is how we learn, and LEDs do not need the maximum current they are rated for.

CMOS 555 Long Duration Red LED Flasher
 

Thread Starter

evanwidloski

Joined Oct 8, 2011
9
If you really want to you can use the drivers shown in this tutorial. A CMOS could do it by itself in theory, but the drive from it is unpredictable.
Could this damage the ic?

Connect the cathode of the LED to the 2N7000's drain instead, and connect it's anode to a resistor, the other end of the resistor to +V.
The device being switched by the MOSFET is a battery powered stopwatch, so I have to use a different v+ for the led. Can the stopwatch and the led share the same drain but have different power supplies?

Also should I connect the ground of the stopwatch and the 555 circuit?
 

SgtWookie

Joined Jul 17, 2007
22,220
The device being switched by the MOSFET is a battery powered stopwatch, so I have to use a different v+ for the led. Can the stopwatch and the led share the same drain but have different power supplies?
I missed this somehow.
I have no idea of the power requirements of your stopwatch. Is this a substitute to closing a switch on the stopwatch? What is the voltage present if the switch is not pressed?

Also should I connect the ground of the stopwatch and the 555 circuit?
You need to tell us what the voltage is across the stopwatch connections.

The more negative side will need to be connected to the MOSFET source terminal; and the MOSFET source terminal will need to be connected to the 555 ground terminal.

The more positive watch terminal will need to be connected to the MOSFET drain.

If you really want to see the LED light up, you will either need to settle for a lower brightness for the LED by using a larger resistor which will let less current flow through the LED, or you will need to use another 2N7000 to sink current from the LED.
 

Thread Starter

evanwidloski

Joined Oct 8, 2011
9
Im on vacation so I can't test the stopwatch.

But assuming that the button is connected to ground, can the led share drain but be connected to it's own power supply? Will I need a diode?
 

KMoffett

Joined Dec 19, 2007
2,911
...The device being switched by the MOSFET is a battery powered stopwatch, so I have to use a different v+ for the led. Can the stopwatch and the led share the same drain but have different power supplies?

Also should I connect the ground of the stopwatch and the 555 circuit?
Many years ago I triggered an array of battery stopwatches using CD4066 analog switches wired across the stopwatches' start/stop button pads.

ken
 
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