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Timer circuit needed

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jeremybb

New Member
Hi folks, just discovered this place and it looks great. I would like to build a circuit with the following characteristics, and if someone could help out it would be appreciated.

The circuit needs to switch 12VDC (<1A). Normally the output should be high. When a trigger arrives the circuit should wait ~1 second (I would like to be able to fine tune this parameter +/- 50% or so) and then put the output at ground. The output should stay at ground until the circuit is reset by the user.

The trigger is a 12 VDC sine wave at a few KHz. This trigger is going to a cheap buzzer and I would like the buzzer to still function with the circuit attached. If this adds too much complication, I may also have access to a 12V constant trigger.

When the output goes high (at power up or after reset) the buzzer/trigger will go off; this must be ignored, possibly by holding down the reset button?

Finally an LED should be on when the output is high.

Thanks!
-----------------------
In case you are wondering or if it may be relevant: the purpose is to turn off a laser jamming device (http://www.blinder-laser-jammer.com/m_twin.html)
after it has activated. If it matters to you, these are legal where I live.
 

Roff

Well-Known Member
jeremybb said:
Hi folks, just discovered this place and it looks great. I would like to build a circuit with the following characteristics, and if someone could help out it would be appreciated.

The circuit needs to switch 12VDC (<1A). Normally the output should be high. When a trigger arrives the circuit should wait ~1 second (I would like to be able to fine tune this parameter +/- 50% or so) and then put the output at ground. The output should stay at ground until the circuit is reset by the user.
Does the output have to be forced to ground (i.e., by a transistor) or can it be allowed to float? Floating would still turn off the
jammer.
The trigger is a 12 VDC sine wave at a few KHz.
What is a 12 VDC sine wave?
This trigger is going to a cheap buzzer and I would like the buzzer to still function with the circuit attached. If this adds too much complication, I may also have access to a 12V constant trigger.

When the output goes high (at power up or after reset) the buzzer/trigger will go off; this must be ignored, possibly by holding down the reset button?
What is the polarity and level of the reset?

Finally an LED should be on when the output is high.

Thanks!
-----------------------
In case you are wondering or if it may be relevant: the purpose is to turn off a laser jamming device (http://www.blinder-laser-jammer.com/m_twin.html)
after it has activated. If it matters to you, these are legal where I live.
 

jeremybb

New Member
Ron H said:
Does the output have to be forced to ground (i.e., by a transistor) or can it be allowed to float? Floating would still turn off the
jammer.
I've never understood the concept of floating, but I believe this would be fine. The purpose is only to turn it off.

What is a 12 VDC sine wave?
I believe this is the signal that is sent to the buzzer. What I meant was that the amplitude is 12V and the frequency is in the low KHz range. I'm not actually sure it's a sine wave, just some oscillating signal. I imagined the details wouldn't be important, picturing running it through a filter to extract a signal for the trigger. I can hook it up to an oscilloscope if that's important.

What is the polarity and level of the reset?
This can be anything. It will be implemented as a switch activated by the user, so it can hold something high or low or whatever is needed.
 

Roff

Well-Known Member
I haven't breadboarded this. I haven't even simulated it.
If the buzzer input does not start out at GND and transition to +12v, we will have to design an interface circuit.
The p-channel MOSFET I found at Digikey. You can use just about any one that has less than about 0.5 ohms Rds, and can handle your load current.
R7 and C5 provide a long power-on reset. You can change C5 if you need to.
The polarized caps should be rated at 20 volts or more.
The reset button turns the jammer back on (I hope).
If you build this, be sure to let us know how it works (or not).
 

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jeremybb

New Member
Wow! Thanks.

I will definetely let you know how it turns out. I am too much of a poser to understand the circuit but for the most part I can follow it enough to build it. A couple of quick questions though:

When you say I can change C5, my guess is that it is the draining of this cap through R8 that causes the reset. This suggests to me that I should increase C5 if the circuit is still catching the tail end of the power-on buzz.

I'm new to MOSFETs. Looking at the digikey product sheet for the one you have given it's not clear to me which lead will be which. It appears that two fo the four pins are tied together and this is the drain. Will the other leads be marked or is there a convention?

Are there any other constraints on the type of capacitor other than the voltage ratings? Is a typical electrolytic OK?

Thanks again, I'll be making my digikey order ASAP.
 

Roff

Well-Known Member
Re: Wow! Thanks.

jeremybb said:
I will definetely let you know how it turns out. I am too much of a poser to understand the circuit but for the most part I can follow it enough to build it. A couple of quick questions though:

When you say I can change C5, my guess is that it is the draining of this cap through R8 that causes the reset. This suggests to me that I should increase C5 if the circuit is still catching the tail end of the power-on buzz.

I'm new to MOSFETs. Looking at the digikey product sheet for the one you have given it's not clear to me which lead will be which. It appears that two fo the four pins are tied together and this is the drain. Will the other leads be marked or is there a convention?

Are there any other constraints on the type of capacitor other than the voltage ratings? Is a typical electrolytic OK?

Thanks again, I'll be making my digikey order ASAP.
With power off, C5 starts out with zero volts across it. When you turn on power, the junction of C5 and R7 jumps to +12V. C5 slowly charges through R7, causing the reset node to eventually reach GND potential. This process holds the two flip-flops reset for several seconds. The reset switch will quickly discharge the cap, pulling reset back up to +12V. Yes, making C5 larger will hold the flip-flops reset for a longer period of time.
If you expect the power to be cycling rapidly (<1 minute ON or OFF), you should connect a 1N4001 diode across R7, anode to GND.

Here is the package info. Source and drain are indicated on the datasheet. On my schematic, the top terminal is the source, the bottom terminal is the drain, and, of course, the left terminal is the gate.
I think Digikey has a minimum order. Mouser has a lot of Zetex parts that should work, but they all seem to be surface mount (which may be OK with you). The last time I checked, Mouser didn't have a minimum order. If you want further help picking a MOSFET, let me know.
 
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