Thanks a lot. It's working. Is there a way to measure current that the 555 timer take while waiting for trigger? It doesn't seem right (0.1uA). I want to calculate the battery life.

Can we discuss about design #2, using astable mode of 555 timer? My aim for this design is that, until the button is pressed, there's no battery loss to the circuit. Completely shut down circuit. That's why I connect the button to the gate of the mosfet, thinking that the button will trigger the first mosfet, then power the second mosfet and run the 555 timer. Like I said, I never use mosfet before, that's what I think it should work but obviously it's not and I have no idea why.

The CMOS version of the 555 draws very little quiescent current, typically 65μA, maximum 200μA. Is it possible that you meant you measured .1mA instead of .1μA? My Fluke meter won't measure less than 1mA.

I have not studied design #2. but the simplest way to prevent any battery drain is to use an on/off switch for the entire circuit.

 

Mine shows the measurement is 0. Change to uA, I got 0.1uA sometimes and other times is 0.
The on/off switch you mention, does it mean we turn on and turn off when we're done?
I thought the mosfet can act as the power supply for the 555 when it's switched on and charged.

 

Forgot to ask, what if I decide to use P-channel Mosfet instead of N-channel for design 1? I read on one of the source that use P channel or NPN at out put of 555 timer. Using N-channel won't work because pin3 of the timer doesn't rise high enough to turn off the transistor/mosfet. Obviously, N-channel works. And this is confused me.

 

Mine shows the measurement is 0. Change to uA, I got 0.1uA sometimes and other times is 0.
The on/off switch you mention, does it mean we turn on and turn off when we're done?
I thought the mosfet can act as the power supply for the 555 when it's switched on and charged.

An N channel MOSFET is turned on with a positive gate voltage. In your design 2, the first MOSFET is turned on by the pushbutton, but it does not supply a positive gate voltage to the second MOSFET or a positive supply voltage to the 555. The 555 never turns on. N channel MOSFETs are used for "low side" switching, not "high side." You should consult an electronics reference book for a better explanation.

 

Below is my design #2, I added a resistor between the Drain of 1st mosfet and 12VDC. So when the button is pushed, it turned on the 1st mosfet and power the 2nd mosfet.

Tried it, button didn't do anything. I got Vgs = 12V, Vgd = 12V, Vds=0. I've tried to go through the theoretical logic of this design; it makes sense to me but can't track the problem spot.

 

View attachment 56380Below is my design #2, I added a resistor between the Drain of 1st mosfet and 12VDC. So when the button is pushed, it turned on the 1st mosfet and power the 2nd mosfet.

Tried it, button didn't do anything. I got Vgs = 12V, Vgd = 12V, Vds=0. I've tried to go through the theoretical logic of this design; it makes sense to me but can't track the problem spot.

You need to study the operation of N channel MOSFETs. They are used for low side switching.

 

The first MOSFET is OK. The reason why the switch didn't work is that you have a capacitor between the switch and the MOSFET. Remove it and the MOSFET should work as planned. You can also remove the second 10K resistor.

The second MOSFET is not connected correctly. There is no current between the gate and either the drain or the source. In order to work the way you want, you need to connect the drain (or is it the source) to +12 V and the source (or is it the drain) to what the drain is connected to in your diagram. To get enough voltage on the gate, exchange places of the 10K and 2K resistors so you get ~10V on the gate.

 

The first MOSFET is OK. The reason why the switch didn't work is that you have a capacitor between the switch and the MOSFET. Remove it and the MOSFET should work as planned. You can also remove the second 10K resistor.

In the configuration you describe, the MOSFET will be turned on as soon as power is applied, and pressing the NO pushbutton switch will turn it off.

 

In the configuration you describe, the MOSFET will be turned on as soon as power is applied, and pressing the NO pushbutton switch will turn it off.

Actually, it depends on whether it is a depletion mode or enhancement mode MOSFET. The way he drew it, it should be a depletion mode MOSFET, which is on unless a (negative ?) gate voltage is applied. In either case, I guess it doesn't work the way he wants it to.

 

Actually, it depends on whether it is a depletion mode or enhancement mode MOSFET. The way he drew it, it should be a depletion mode MOSFET, which is on unless a (negative ?) gate voltage is applied. In either case, I guess it doesn't work the way he wants it to.

He is using an IRF510.

 

I believe it is possible to use a pair of MOSFETs (one N-channel and one P-channel) to do what you want. However, the MOSFETs will have leakage current that will pass through them when they are off, so there will still be a drain on your batteries. The current will be very small, but about the same as the idle current on the TS555CN, so you would not be saving much, if any power.

Or, you could use a relay and there would be no power consumption when the circuit is idle, but the power used by the coil of the relay when the circuit is active would be a significant added drain on the batteries.

Or, you could add an on/off switch to the circuit. This would eliminate any circuit drain on the batteries, but it would require the user to turn the power on before pressing the play button.

Do the calculation on how much the TS555CN idle current draw would affect the life of your batteries. I bet it will be negligible.

Here's a link to using MOSFETs as switches that helps me.

http://www.electronics-tutorials.ws/transistor/tran_7.html

 

I believe it is possible to use a pair of MOSFETs (one N-channel and one P-channel) to do what you want. However, the MOSFETs will have leakage current that will pass through them when they are off, so there will still be a drain on your batteries. The current will be very small, but about the same as the idle current on the TS555CN, so you would not be saving much, if any power.

Do the calculation on how much the TS555CN idle current draw would affect the life of your batteries. I bet it will be negligible.

I originally designed circuit 2 using one N-channel and one P-channel, but my P-channel came as surface mount parts . Radio shack doesn't have P-channel. So I decide to try two N-channel to see how it turns out. Oh well...

In design 1, on top of 555 timer idle current draw is the mosfet current when they're off. What I'm seeing is that both design possibly have similar amount of current draw when it's in idling mode. Is that correct?

And because I like to have solid proof, I need to measure or find the current that those two parts draw

 

I originally designed circuit 2 using one N-channel and one P-channel, but my P-channel came as surface mount parts . Radio shack doesn't have P-channel. So I decide to try two N-channel to see how it turns out. Oh well...

In design 1, on top of 555 timer idle current draw is the mosfet current when they're off. What I'm seeing is that both design possibly have similar amount of current draw when it's in idling mode. Is that correct?

And because I like to have solid proof, I need to measure or find the current that those two parts draw

You could use small MOSFETs for the front end pair, which should have much lower off leakage current than power MOSFETs, probably less than 1μA each. Of course, you would still need a power MOSFET on the output of the TS555CN for your display. The IRF510 has a power off drain of about 50μA.

 

I don't know the difference between small Mosfets and power Mosfets. Can you give me some hints?

In design 1, I couldn't measure the current goes through the LED. I saw the voltage is changing when button is pushed but not enough to turn on the LED. My multimeter show 0mA although I could measure the current before 0.95mA and 75mA depends on the value of the resistor.